| Message ID | 20240327022903.776-1-justinjiang@vivo.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | dmabuf: fix dmabuf file poll uaf issue | expand |
On Tue, Mar 26, 2024 at 7:29 PM Zhiguo Jiang <justinjiang@vivo.com> wrote: > > The issue is a UAF issue of dmabuf file fd. Throght debugging, we found > that the dmabuf file fd is added to the epoll event listener list, and > when it is released, it is not removed from the epoll list, which leads > to the UAF(Use-After-Free) issue. > > The UAF issue can be solved by checking dmabuf file->f_count value and > skipping the poll operation for the closed dmabuf file in the > dma_buf_poll(). We have tested this solved patch multiple times and > have not reproduced the uaf issue. > Hi Zhiguo, What is the most recent kernel version you've seen the bug on? You are closing the dmabuf fd from another thread while it is still part of the epoll interest list? Thanks, T.J.
Hi T.J., > What is the most recent kernel version you've seen the bug on? The latest kernel version of the issue we discovered is kernel-6.1.25, and kernel-5.15 also reported this issue. > You are closing the dmabuf fd from another thread while it is still > part of the epoll interest list? Yes, we found that threads related to android.display modules performed an operation to close this dmabuf fd while it was still part of the epoll interest list, but the specific threads were random. So I think this is also issue with the logic of kernel dmabuf code. Thanks, Zhiguo 在 2024/3/30 7:36, T.J. Mercier 写道: > [你通常不会收到来自 tjmercier@google.com 的电子邮件。请访问 https://aka.ms/LearnAboutSenderIdentification，以了解这一点为什么很重要] > > On Tue, Mar 26, 2024 at 7:29 PM Zhiguo Jiang <justinjiang@vivo.com> wrote: >> The issue is a UAF issue of dmabuf file fd. Throght debugging, we found >> that the dmabuf file fd is added to the epoll event listener list, and >> when it is released, it is not removed from the epoll list, which leads >> to the UAF(Use-After-Free) issue. >> >> The UAF issue can be solved by checking dmabuf file->f_count value and >> skipping the poll operation for the closed dmabuf file in the >> dma_buf_poll(). We have tested this solved patch multiple times and >> have not reproduced the uaf issue. >> > Hi Zhiguo, > > What is the most recent kernel version you've seen the bug on? > > You are closing the dmabuf fd from another thread while it is still > part of the epoll interest list? > > Thanks, > T.J.
Am 27.03.24 um 03:29 schrieb Zhiguo Jiang: > The issue is a UAF issue of dmabuf file fd. Throght debugging, we found > that the dmabuf file fd is added to the epoll event listener list, and > when it is released, it is not removed from the epoll list, which leads > to the UAF(Use-After-Free) issue. As far as I can see that's not because of the DMA-buf code, but because you are somehow using this interface incorrectly. When dma_buf_poll() is called it is mandatory for the caller to hold a reference to the file descriptor on which the poll operation is executed. So adding code like "if (!file_count(file))" in the beginning of dma_buf_poll() is certainly broken. My best guess is that you have some unbalanced dma_buf_get()/dma_buf_put() somewhere instead. Regards, Christian. > > The UAF issue can be solved by checking dmabuf file->f_count value and > skipping the poll operation for the closed dmabuf file in the > dma_buf_poll(). We have tested this solved patch multiple times and > have not reproduced the uaf issue. > > crash dump: > list_del corruption, ffffff8a6f143a90->next is LIST_POISON1 > (dead000000000100) > ------------[ cut here ]------------ > kernel BUG at lib/list_debug.c:55! > Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP > pc : __list_del_entry_valid+0x98/0xd4 > lr : __list_del_entry_valid+0x98/0xd4 > sp : ffffffc01d413d00 > x29: ffffffc01d413d00 x28: 00000000000000c0 x27: 0000000000000020 > x26: 0000000000000000 x25: 0000000000000000 x24: 0000000000080007 > x23: ffffff8b22e5dcc0 x22: ffffff88a6be12d0 x21: ffffff8b22e572b0 > x20: ffffff80254ed0a0 x19: ffffff8a6f143a00 x18: ffffffda5efed3c0 > x17: 6165642820314e4f x16: 53494f505f545349 x15: 4c20736920747865 > x14: 6e3e2d3039613334 x13: 2930303130303030 x12: 0000000000000018 > x11: ffffff8b6c188000 x10: 00000000ffffffff x9 : 6c8413a194897b00 > x8 : 6c8413a194897b00 x7 : 74707572726f6320 x6 : 6c65645f7473696c > x5 : ffffff8b6c3b2a3e x4 : ffffff8b6c3b2a40 x3 : ffff103000001005 > x2 : 0000000000000001 x1 : 00000000000000c0 x0 : 000000000000004e > Call trace: > __list_del_entry_valid+0x98/0xd4 > dma_buf_file_release+0x48/0x90 > __fput+0xf4/0x280 > ____fput+0x10/0x20 > task_work_run+0xcc/0xf4 > do_notify_resume+0x2a0/0x33c > el0_svc+0x5c/0xa4 > el0t_64_sync_handler+0x68/0xb4 > el0t_64_sync+0x1a0/0x1a4 > Code: d0006fe0 912c5000 f2fbd5a2 94231a01 (d4210000) > ---[ end trace 0000000000000000 ]--- > Kernel panic - not syncing: Oops - BUG: Fatal exception > SMP: stopping secondary CPUs > > Signed-off-by: Zhiguo Jiang <justinjiang@vivo.com> > --- > drivers/dma-buf/dma-buf.c | 28 ++++++++++++++++++++++++---- > 1 file changed, 24 insertions(+), 4 deletions(-) > mode change 100644 => 100755 drivers/dma-buf/dma-buf.c > > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c > index 8fe5aa67b167..e469dd9288cc > --- a/drivers/dma-buf/dma-buf.c > +++ b/drivers/dma-buf/dma-buf.c > @@ -240,6 +240,10 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll) > struct dma_resv *resv; > __poll_t events; > > + /* Check if the file exists */ > + if (!file_count(file)) > + return EPOLLERR; > + > dmabuf = file->private_data; > if (!dmabuf || !dmabuf->resv) > return EPOLLERR; > @@ -266,8 +270,15 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll) > spin_unlock_irq(&dmabuf->poll.lock); > > if (events & EPOLLOUT) { > - /* Paired with fput in dma_buf_poll_cb */ > - get_file(dmabuf->file); > + /* > + * Paired with fput in dma_buf_poll_cb, > + * Skip poll for the closed file. > + */ > + if (!get_file_rcu(&dmabuf->file)) { > + events &= ~EPOLLOUT; > + dcb->active = 0; > + goto clear_out_event; > + } > > if (!dma_buf_poll_add_cb(resv, true, dcb)) > /* No callback queued, wake up any other waiters */ > @@ -277,6 +288,7 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll) > } > } > > +clear_out_event: > if (events & EPOLLIN) { > struct dma_buf_poll_cb_t *dcb = &dmabuf->cb_in; > > @@ -289,8 +301,15 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll) > spin_unlock_irq(&dmabuf->poll.lock); > > if (events & EPOLLIN) { > - /* Paired with fput in dma_buf_poll_cb */ > - get_file(dmabuf->file); > + /* > + * Paired with fput in dma_buf_poll_cb, > + * Skip poll for the closed file. > + */ > + if (!get_file_rcu(&dmabuf->file)) { > + events &= ~EPOLLIN; > + dcb->active = 0; > + goto clear_in_event; > + } > > if (!dma_buf_poll_add_cb(resv, false, dcb)) > /* No callback queued, wake up any other waiters */ > @@ -300,6 +319,7 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll) > } > } > > +clear_in_event: > dma_resv_unlock(resv); > return events; > }
> As far as I can see that's not because of the DMA-buf code, but > because you are somehow using this interface incorrectly. > > When dma_buf_poll() is called it is mandatory for the caller to hold a > reference to the file descriptor on which the poll operation is executed. > > So adding code like "if (!file_count(file))" in the beginning of > dma_buf_poll() is certainly broken. > > My best guess is that you have some unbalanced > dma_buf_get()/dma_buf_put() somewhere instead. > > Hi Christian, The kernel dma_buf_poll() code shound not cause system crashes due to the user mode usage logical issues ? Thanks 在 2024/4/1 20:22, Christian König 写道: > Am 27.03.24 um 03:29 schrieb Zhiguo Jiang: >> The issue is a UAF issue of dmabuf file fd. Throght debugging, we found >> that the dmabuf file fd is added to the epoll event listener list, and >> when it is released, it is not removed from the epoll list, which leads >> to the UAF(Use-After-Free) issue. > > As far as I can see that's not because of the DMA-buf code, but > because you are somehow using this interface incorrectly. > > When dma_buf_poll() is called it is mandatory for the caller to hold a > reference to the file descriptor on which the poll operation is executed. > > So adding code like "if (!file_count(file))" in the beginning of > dma_buf_poll() is certainly broken. > > My best guess is that you have some unbalanced > dma_buf_get()/dma_buf_put() somewhere instead. > > Regards, > Christian. > >> >> The UAF issue can be solved by checking dmabuf file->f_count value and >> skipping the poll operation for the closed dmabuf file in the >> dma_buf_poll(). We have tested this solved patch multiple times and >> have not reproduced the uaf issue. >> >> crash dump: >> list_del corruption, ffffff8a6f143a90->next is LIST_POISON1 >> (dead000000000100) >> ------------[ cut here ]------------ >> kernel BUG at lib/list_debug.c:55! >> Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP >> pc : __list_del_entry_valid+0x98/0xd4 >> lr : __list_del_entry_valid+0x98/0xd4 >> sp : ffffffc01d413d00 >> x29: ffffffc01d413d00 x28: 00000000000000c0 x27: 0000000000000020 >> x26: 0000000000000000 x25: 0000000000000000 x24: 0000000000080007 >> x23: ffffff8b22e5dcc0 x22: ffffff88a6be12d0 x21: ffffff8b22e572b0 >> x20: ffffff80254ed0a0 x19: ffffff8a6f143a00 x18: ffffffda5efed3c0 >> x17: 6165642820314e4f x16: 53494f505f545349 x15: 4c20736920747865 >> x14: 6e3e2d3039613334 x13: 2930303130303030 x12: 0000000000000018 >> x11: ffffff8b6c188000 x10: 00000000ffffffff x9 : 6c8413a194897b00 >> x8 : 6c8413a194897b00 x7 : 74707572726f6320 x6 : 6c65645f7473696c >> x5 : ffffff8b6c3b2a3e x4 : ffffff8b6c3b2a40 x3 : ffff103000001005 >> x2 : 0000000000000001 x1 : 00000000000000c0 x0 : 000000000000004e >> Call trace: >> __list_del_entry_valid+0x98/0xd4 >> dma_buf_file_release+0x48/0x90 >> __fput+0xf4/0x280 >> ____fput+0x10/0x20 >> task_work_run+0xcc/0xf4 >> do_notify_resume+0x2a0/0x33c >> el0_svc+0x5c/0xa4 >> el0t_64_sync_handler+0x68/0xb4 >> el0t_64_sync+0x1a0/0x1a4 >> Code: d0006fe0 912c5000 f2fbd5a2 94231a01 (d4210000) >> ---[ end trace 0000000000000000 ]--- >> Kernel panic - not syncing: Oops - BUG: Fatal exception >> SMP: stopping secondary CPUs >> >> Signed-off-by: Zhiguo Jiang <justinjiang@vivo.com> >> --- >> drivers/dma-buf/dma-buf.c | 28 ++++++++++++++++++++++++---- >> 1 file changed, 24 insertions(+), 4 deletions(-) >> mode change 100644 => 100755 drivers/dma-buf/dma-buf.c >> >> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c >> index 8fe5aa67b167..e469dd9288cc >> --- a/drivers/dma-buf/dma-buf.c >> +++ b/drivers/dma-buf/dma-buf.c >> @@ -240,6 +240,10 @@ static __poll_t dma_buf_poll(struct file *file, >> poll_table *poll) >> struct dma_resv *resv; >> __poll_t events; >> + /* Check if the file exists */ >> + if (!file_count(file)) >> + return EPOLLERR; >> + >> dmabuf = file->private_data; >> if (!dmabuf || !dmabuf->resv) >> return EPOLLERR; >> @@ -266,8 +270,15 @@ static __poll_t dma_buf_poll(struct file *file, >> poll_table *poll) >> spin_unlock_irq(&dmabuf->poll.lock); >> if (events & EPOLLOUT) { >> - /* Paired with fput in dma_buf_poll_cb */ >> - get_file(dmabuf->file); >> + /* >> + * Paired with fput in dma_buf_poll_cb, >> + * Skip poll for the closed file. >> + */ >> + if (!get_file_rcu(&dmabuf->file)) { >> + events &= ~EPOLLOUT; >> + dcb->active = 0; >> + goto clear_out_event; >> + } >> if (!dma_buf_poll_add_cb(resv, true, dcb)) >> /* No callback queued, wake up any other waiters */ >> @@ -277,6 +288,7 @@ static __poll_t dma_buf_poll(struct file *file, >> poll_table *poll) >> } >> } >> +clear_out_event: >> if (events & EPOLLIN) { >> struct dma_buf_poll_cb_t *dcb = &dmabuf->cb_in; >> @@ -289,8 +301,15 @@ static __poll_t dma_buf_poll(struct file >> *file, poll_table *poll) >> spin_unlock_irq(&dmabuf->poll.lock); >> if (events & EPOLLIN) { >> - /* Paired with fput in dma_buf_poll_cb */ >> - get_file(dmabuf->file); >> + /* >> + * Paired with fput in dma_buf_poll_cb, >> + * Skip poll for the closed file. >> + */ >> + if (!get_file_rcu(&dmabuf->file)) { >> + events &= ~EPOLLIN; >> + dcb->active = 0; >> + goto clear_in_event; >> + } >> if (!dma_buf_poll_add_cb(resv, false, dcb)) >> /* No callback queued, wake up any other waiters */ >> @@ -300,6 +319,7 @@ static __poll_t dma_buf_poll(struct file *file, >> poll_table *poll) >> } >> } >> +clear_in_event: >> dma_resv_unlock(resv); >> return events; >> } >
Am 02.04.24 um 08:49 schrieb zhiguojiang: >> As far as I can see that's not because of the DMA-buf code, but >> because you are somehow using this interface incorrectly. >> >> When dma_buf_poll() is called it is mandatory for the caller to hold >> a reference to the file descriptor on which the poll operation is >> executed. >> >> So adding code like "if (!file_count(file))" in the beginning of >> dma_buf_poll() is certainly broken. >> >> My best guess is that you have some unbalanced >> dma_buf_get()/dma_buf_put() somewhere instead. >> >> > Hi Christian, > > The kernel dma_buf_poll() code shound not cause system crashes due to > the user mode usage logical issues ? What user mode logical issues are you talking about? Closing a file while polling on it is perfectly valid. dma_buf_poll() is called by the filesystem layer and it's the filesystem layer which should make sure that a file can't be closed while polling for an event. If that doesn't work then you have stumbled over a massive bug in the fs layer. And I have some doubts that this is actually the case. What is more likely is that some driver messes up the reference count and because of this you see an UAF. Anyway as far as I can see the DMA-buf code is correct regarding this. Regards, Christian. > > Thanks > > > 在 2024/4/1 20:22, Christian König 写道: >> Am 27.03.24 um 03:29 schrieb Zhiguo Jiang: >>> The issue is a UAF issue of dmabuf file fd. Throght debugging, we found >>> that the dmabuf file fd is added to the epoll event listener list, and >>> when it is released, it is not removed from the epoll list, which leads >>> to the UAF(Use-After-Free) issue. >> >> As far as I can see that's not because of the DMA-buf code, but >> because you are somehow using this interface incorrectly. >> >> When dma_buf_poll() is called it is mandatory for the caller to hold >> a reference to the file descriptor on which the poll operation is >> executed. >> >> So adding code like "if (!file_count(file))" in the beginning of >> dma_buf_poll() is certainly broken. >> >> My best guess is that you have some unbalanced >> dma_buf_get()/dma_buf_put() somewhere instead. >> >> Regards, >> Christian. >> >>> >>> The UAF issue can be solved by checking dmabuf file->f_count value and >>> skipping the poll operation for the closed dmabuf file in the >>> dma_buf_poll(). We have tested this solved patch multiple times and >>> have not reproduced the uaf issue. >>> >>> crash dump: >>> list_del corruption, ffffff8a6f143a90->next is LIST_POISON1 >>> (dead000000000100) >>> ------------[ cut here ]------------ >>> kernel BUG at lib/list_debug.c:55! >>> Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP >>> pc : __list_del_entry_valid+0x98/0xd4 >>> lr : __list_del_entry_valid+0x98/0xd4 >>> sp : ffffffc01d413d00 >>> x29: ffffffc01d413d00 x28: 00000000000000c0 x27: 0000000000000020 >>> x26: 0000000000000000 x25: 0000000000000000 x24: 0000000000080007 >>> x23: ffffff8b22e5dcc0 x22: ffffff88a6be12d0 x21: ffffff8b22e572b0 >>> x20: ffffff80254ed0a0 x19: ffffff8a6f143a00 x18: ffffffda5efed3c0 >>> x17: 6165642820314e4f x16: 53494f505f545349 x15: 4c20736920747865 >>> x14: 6e3e2d3039613334 x13: 2930303130303030 x12: 0000000000000018 >>> x11: ffffff8b6c188000 x10: 00000000ffffffff x9 : 6c8413a194897b00 >>> x8 : 6c8413a194897b00 x7 : 74707572726f6320 x6 : 6c65645f7473696c >>> x5 : ffffff8b6c3b2a3e x4 : ffffff8b6c3b2a40 x3 : ffff103000001005 >>> x2 : 0000000000000001 x1 : 00000000000000c0 x0 : 000000000000004e >>> Call trace: >>> __list_del_entry_valid+0x98/0xd4 >>> dma_buf_file_release+0x48/0x90 >>> __fput+0xf4/0x280 >>> ____fput+0x10/0x20 >>> task_work_run+0xcc/0xf4 >>> do_notify_resume+0x2a0/0x33c >>> el0_svc+0x5c/0xa4 >>> el0t_64_sync_handler+0x68/0xb4 >>> el0t_64_sync+0x1a0/0x1a4 >>> Code: d0006fe0 912c5000 f2fbd5a2 94231a01 (d4210000) >>> ---[ end trace 0000000000000000 ]--- >>> Kernel panic - not syncing: Oops - BUG: Fatal exception >>> SMP: stopping secondary CPUs >>> >>> Signed-off-by: Zhiguo Jiang <justinjiang@vivo.com> >>> --- >>> drivers/dma-buf/dma-buf.c | 28 ++++++++++++++++++++++++---- >>> 1 file changed, 24 insertions(+), 4 deletions(-) >>> mode change 100644 => 100755 drivers/dma-buf/dma-buf.c >>> >>> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c >>> index 8fe5aa67b167..e469dd9288cc >>> --- a/drivers/dma-buf/dma-buf.c >>> +++ b/drivers/dma-buf/dma-buf.c >>> @@ -240,6 +240,10 @@ static __poll_t dma_buf_poll(struct file *file, >>> poll_table *poll) >>> struct dma_resv *resv; >>> __poll_t events; >>> + /* Check if the file exists */ >>> + if (!file_count(file)) >>> + return EPOLLERR; >>> + >>> dmabuf = file->private_data; >>> if (!dmabuf || !dmabuf->resv) >>> return EPOLLERR; >>> @@ -266,8 +270,15 @@ static __poll_t dma_buf_poll(struct file *file, >>> poll_table *poll) >>> spin_unlock_irq(&dmabuf->poll.lock); >>> if (events & EPOLLOUT) { >>> - /* Paired with fput in dma_buf_poll_cb */ >>> - get_file(dmabuf->file); >>> + /* >>> + * Paired with fput in dma_buf_poll_cb, >>> + * Skip poll for the closed file. >>> + */ >>> + if (!get_file_rcu(&dmabuf->file)) { >>> + events &= ~EPOLLOUT; >>> + dcb->active = 0; >>> + goto clear_out_event; >>> + } >>> if (!dma_buf_poll_add_cb(resv, true, dcb)) >>> /* No callback queued, wake up any other waiters */ >>> @@ -277,6 +288,7 @@ static __poll_t dma_buf_poll(struct file *file, >>> poll_table *poll) >>> } >>> } >>> +clear_out_event: >>> if (events & EPOLLIN) { >>> struct dma_buf_poll_cb_t *dcb = &dmabuf->cb_in; >>> @@ -289,8 +301,15 @@ static __poll_t dma_buf_poll(struct file >>> *file, poll_table *poll) >>> spin_unlock_irq(&dmabuf->poll.lock); >>> if (events & EPOLLIN) { >>> - /* Paired with fput in dma_buf_poll_cb */ >>> - get_file(dmabuf->file); >>> + /* >>> + * Paired with fput in dma_buf_poll_cb, >>> + * Skip poll for the closed file. >>> + */ >>> + if (!get_file_rcu(&dmabuf->file)) { >>> + events &= ~EPOLLIN; >>> + dcb->active = 0; >>> + goto clear_in_event; >>> + } >>> if (!dma_buf_poll_add_cb(resv, false, dcb)) >>> /* No callback queued, wake up any other waiters */ >>> @@ -300,6 +319,7 @@ static __poll_t dma_buf_poll(struct file *file, >>> poll_table *poll) >>> } >>> } >>> +clear_in_event: >>> dma_resv_unlock(resv); >>> return events; >>> } >> >
On Tue, Apr 2, 2024 at 1:08 AM Christian König <christian.koenig@amd.com> wrote: > > Am 02.04.24 um 08:49 schrieb zhiguojiang: > >> As far as I can see that's not because of the DMA-buf code, but > >> because you are somehow using this interface incorrectly. > >> > >> When dma_buf_poll() is called it is mandatory for the caller to hold > >> a reference to the file descriptor on which the poll operation is > >> executed. > >> > >> So adding code like "if (!file_count(file))" in the beginning of > >> dma_buf_poll() is certainly broken. > >> > >> My best guess is that you have some unbalanced > >> dma_buf_get()/dma_buf_put() somewhere instead. > >> > >> > > Hi Christian, > > > > The kernel dma_buf_poll() code shound not cause system crashes due to > > the user mode usage logical issues ? > > What user mode logical issues are you talking about? Closing a file > while polling on it is perfectly valid. > > dma_buf_poll() is called by the filesystem layer and it's the filesystem > layer which should make sure that a file can't be closed while polling > for an event. > > If that doesn't work then you have stumbled over a massive bug in the fs > layer. And I have some doubts that this is actually the case. > > What is more likely is that some driver messes up the reference count > and because of this you see an UAF. > > Anyway as far as I can see the DMA-buf code is correct regarding this. > > Regards, > Christian. I tried to reproduce this problem but I couldn't. What I see is a ref get taken when poll is first called. So subsequently closing the fd in userspace while it's being polled doesn't take the refcount all the way to 0. That happens when dma_buf_poll_cb fires, either due to an event or when the fd is closed upon timeout. I don't really see how this could be triggered from userspace so I am also suspicious of dma_buf_get/put.
在 2024/4/3 2:22, T.J. Mercier 写道: > [你通常不会收到来自 tjmercier@google.com 的电子邮件。请访问 https://aka.ms/LearnAboutSenderIdentification，以了解这一点为什么很重要] > > On Tue, Apr 2, 2024 at 1:08 AM Christian König <christian.koenig@amd.com> wrote: >> Am 02.04.24 um 08:49 schrieb zhiguojiang: >>>> As far as I can see that's not because of the DMA-buf code, but >>>> because you are somehow using this interface incorrectly. >>>> >>>> When dma_buf_poll() is called it is mandatory for the caller to hold >>>> a reference to the file descriptor on which the poll operation is >>>> executed. >>>> >>>> So adding code like "if (!file_count(file))" in the beginning of >>>> dma_buf_poll() is certainly broken. >>>> >>>> My best guess is that you have some unbalanced >>>> dma_buf_get()/dma_buf_put() somewhere instead. >>>> >>>> >>> Hi Christian, >>> >>> The kernel dma_buf_poll() code shound not cause system crashes due to >>> the user mode usage logical issues ? >> What user mode logical issues are you talking about? Closing a file >> while polling on it is perfectly valid. >> >> dma_buf_poll() is called by the filesystem layer and it's the filesystem >> layer which should make sure that a file can't be closed while polling >> for an event. >> >> If that doesn't work then you have stumbled over a massive bug in the fs >> layer. And I have some doubts that this is actually the case. >> >> What is more likely is that some driver messes up the reference count >> and because of this you see an UAF. >> >> Anyway as far as I can see the DMA-buf code is correct regarding this. >> >> Regards, >> Christian. > I tried to reproduce this problem but I couldn't. What I see is a ref > get taken when poll is first called. So subsequently closing the fd in > userspace while it's being polled doesn't take the refcount all the > way to 0. That happens when dma_buf_poll_cb fires, either due to an > event or when the fd is closed upon timeout. > > I don't really see how this could be triggered from userspace so I am > also suspicious of dma_buf_get/put. Hi, Panic signature: > list_del corruption, ffffff8a6f143a90->next is LIST_POISON1 > (dead000000000100) > ------------[ cut here ]------------ > kernel BUG at lib/list_debug.c:55! > Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP > pc : __list_del_entry_valid+0x98/0xd4 > lr : __list_del_entry_valid+0x98/0xd4 > sp : ffffffc01d413d00 > x29: ffffffc01d413d00 x28: 00000000000000c0 x27: 0000000000000020 > x26: 0000000000000000 x25: 0000000000000000 x24: 0000000000080007 > x23: ffffff8b22e5dcc0 x22: ffffff88a6be12d0 x21: ffffff8b22e572b0 > x20: ffffff80254ed0a0 x19: ffffff8a6f143a00 x18: ffffffda5efed3c0 > x17: 6165642820314e4f x16: 53494f505f545349 x15: 4c20736920747865 > x14: 6e3e2d3039613334 x13: 2930303130303030 x12: 0000000000000018 > x11: ffffff8b6c188000 x10: 00000000ffffffff x9 : 6c8413a194897b00 > x8 : 6c8413a194897b00 x7 : 74707572726f6320 x6 : 6c65645f7473696c > x5 : ffffff8b6c3b2a3e x4 : ffffff8b6c3b2a40 x3 : ffff103000001005 > x2 : 0000000000000001 x1 : 00000000000000c0 x0 : 000000000000004e > Call trace: > __list_del_entry_valid+0x98/0xd4 > dma_buf_file_release+0x48/0x90 > __fput+0xf4/0x280 > ____fput+0x10/0x20 > task_work_run+0xcc/0xf4 > do_notify_resume+0x2a0/0x33c > el0_svc+0x5c/0xa4 > el0t_64_sync_handler+0x68/0xb4 > el0t_64_sync+0x1a0/0x1a4 It is caused that the same dma buf file released twice in short time, so it should be race issue of dma buf release. Below ftrace log shows the procedure how dma buf release twice: Line 715473: android.display-2220 [006] 22160.660738: bprint: __fget_files __fget: [file: 0xffffff8ab9e57b80, dmabuf: 0xffffff8b1baa6a00, count: 0x3] call:(('do_epoll_ctl', 356)ffffffd4ad46411c<-('__arm64_sys_epoll_ctl', 112)ffffffd4ad463dd8<-('invoke_syscall', 96)ffffffd4acebe00c<-('el0_svc_common', 140)ffffffd4acebdf40<-('do_el0_svc', 40)ffffffd4acebde44<-('el0_svc', 36)ffffffd4ae57ffcc<-('el0t_64_sync_handler', 136)ffffffd4ae57ff44) Line 715475: android.display-2220 [006] 22160.660739: bprint: get_file get_file[file: 0xffffff8ab9e57b80, dmabuf: 0xffffff8b1baa6a00, f_count: 0x4] call:(('dma_buf_poll', 760)ffffffd4adb685c8<-('ep_insert', 1120)ffffffd4ad464bcc<-('do_epoll_ctl', 1196)ffffffd4ad464464<-('__arm64_sys_epoll_ctl', 112)ffffffd4ad463dd8<-('invoke_syscall', 96)ffffffd4acebe00c<-('el0_svc_common', 140)ffffffd4acebdf40<-('do_el0_svc', 40)ffffffd4acebde44) Line 715477: android.display-2220 [006] 22160.660740: bprint: fput_many fput for dma buf file[file: 0xffffff8ab9e57b80, dmabuf: 0xffffff8b1baa6a00, count: 0x4] call:(('dma_buf_poll', 1104)ffffffd4adb68720<-('ep_insert', 1120)ffffffd4ad464bcc<-('do_epoll_ctl', 1196)ffffffd4ad464464<-('__arm64_sys_epoll_ctl', 112)ffffffd4ad463dd8<-('invoke_syscall', 96)ffffffd4acebe00c<-('el0_svc_common', 140)ffffffd4acebdf40<-('do_el0_svc', 40)ffffffd4acebde44) Line 715479: android.display-2220 [006] 22160.660741: bprint: fput_many fput for dma buf file[file: 0xffffff8ab9e57b80, dmabuf: 0xffffff8b1baa6a00, count: 0x3] call:(('do_epoll_ctl', 652)ffffffd4ad464244<-('__arm64_sys_epoll_ctl', 112)ffffffd4ad463dd8<-('invoke_syscall', 96)ffffffd4acebe00c<-('el0_svc_common', 140)ffffffd4acebdf40<-('do_el0_svc', 40)ffffffd4acebde44<-('el0_svc', 36)ffffffd4ae57ffcc<-('el0t_64_sync_handler', 136)ffffffd4ae57ff44) -> Here task 2220 do epoll for dma_buf file twice, and the fget/fput match. After this the file refcount is 2. Line 716521: RenderThread-3470 [005] 22160.664236: bprint: fput_many fput for dma buf file[file: 0xffffff8ab9e57b80, dmabuf: 0xffffff8b1baa6a00, count: 0x2] call:(('vm_area_free_no_check', 140)ffffffd4acf5021c<-('__do_munmap', 1572)ffffffd4ad306eb8<-('__vm_munmap', 216)ffffffd4ad3095b4<-('__arm64_sys_munmap', 68)ffffffd4ad3094c4<-('invoke_syscall', 96)ffffffd4acebe00c<-('el0_svc_common', 140)ffffffd4acebdf40<-('do_el0_svc', 40)ffffffd4acebde44) Line 716525: RenderThread-3470 [005] 22160.664243: bprint: fput_many fput for dma buf file[file: 0xffffff8ab9e57b80, dmabuf: 0xffffff8b1baa6a00, count: 0x1] call:(('close_fd', 376)ffffffd4ad40b404<-('__arm64_sys_close', 24)ffffffd4ad3bfb1c<-('invoke_syscall', 96)ffffffd4acebe00c<-('el0_svc_common', 140)ffffffd4acebdf40<-('do_el0_svc', 40)ffffffd4acebde44<-('el0_svc', 36)ffffffd4ae57ffcc<-('el0t_64_sync_handler', 136)ffffffd4ae57ff44) Line 716527: RenderThread-3470 [005] 22160.664244: bprint: fput_many fput for dma buf file ret: 0, [file: 0xffffff8ab9e57b80, dmabuf: 0xffffff8b1baa6a00] start to free -> Here task3470 do unmap and close(fd) then decrease the file count to zero. Then start to free file buf. Line 716566: android.display-2220 [006] 22160.664424: bprint: get_file get_file[file: 0xffffff8ab9e57b80, dmabuf: 0xffffff8b1baa6a00, f_count: 0x1] call:(('dma_buf_poll', 760)ffffffd4adb685c8<-('do_epoll_wait', 1020)ffffffd4ad46229c<-('do_epoll_pwait', 84)ffffffd4ad463b70<-('__arm64_sys_epoll_pwait', 276)ffffffd4ad463d1c<-('invoke_syscall', 96)ffffffd4acebe00c<-('el0_svc_common', 140)ffffffd4acebdf40<-('do_el0_svc', 40)ffffffd4acebde44) Line 716568: android.display-2220 [006] 22160.664425: bprint: fput_many fput for dma buf file[file: 0xffffff8ab9e57b80, dmabuf: 0xffffff8b1baa6a00, count: 0x1] call:(('dma_buf_poll', 1104)ffffffd4adb68720<-('do_epoll_wait', 1020)ffffffd4ad46229c<-('do_epoll_pwait', 84)ffffffd4ad463b70<-('__arm64_sys_epoll_pwait', 276)ffffffd4ad463d1c<-('invoke_syscall', 96)ffffffd4acebe00c<-('el0_svc_common', 140)ffffffd4acebdf40<-('do_el0_svc', 40)ffffffd4acebde44) Line 716570: android.display-2220 [006] 22160.664427: bprint: fput_many fput for dma buf file ret: 0, [file: 0xffffff8ab9e57b80, dmabuf: 0xffffff8b1baa6a00] start to free -> Here task 2220 do epoll again where internally it will get/put then start to free twice and lead to final crash. Here is the basic flow: 1. Thread A install the dma_buf_fd via dma_buf_export, the fd refcount is 1 2. Thread A add the fd to epoll list via epoll_ctl(EPOLL_CTL_ADD) 3. After use the dma buf, Thread A close the fd, then here fd refcount is 0, and it will run __fput finally to release the file 4. Here Thread A not do epoll_ctl(EPOLL_CTL_DEL) manunally, so it still resides in one epoll_list. Although __fput will call eventpoll_release to remove the file from binded epoll list, but it has small time window where Thread B jumps in. 5. During the small window, Thread B do the poll action for dma_buf_fd, where it will fget/fput for the file, this means the fd refcount will be 0 -> 1 -> 0, and it will call __fput again. This will lead to __fput twice for the same file. 6. So the potenial fix is use get_file_rcu which to check if file refcount already zero which means under free. If so, we just return and no need to do the dma_buf_poll. Here is the race condition: Thread A Thread B dma_buf_export fd_refcount is 1 epoll_ctl(EPOLL_ADD) add dma_buf_fd to epoll list close(dma_buf_fd) fd_refcount is 0 __fput dma_buf_poll fget fput fd_refcount is zero again Thanks
Am 12.04.24 um 08:19 schrieb zhiguojiang: > [SNIP] > -> Here task 2220 do epoll again where internally it will get/put then > start to free twice and lead to final crash. > > Here is the basic flow: > > 1. Thread A install the dma_buf_fd via dma_buf_export, the fd refcount > is 1 > > 2. Thread A add the fd to epoll list via epoll_ctl(EPOLL_CTL_ADD) > > 3. After use the dma buf, Thread A close the fd, then here fd refcount > is 0, > and it will run __fput finally to release the file Stop, that isn't correct. The fs layer which calls dma_buf_poll() should make sure that the file can't go away until the function returns. Then inside dma_buf_poll() we add another reference to the file while installing the callback: /* Paired with fput in dma_buf_poll_cb */ get_file(dmabuf->file); This reference is only dropped after the callback is completed in dma_buf_poll_cb(): /* Paired with get_file in dma_buf_poll */ fput(dmabuf->file); So your explanation for the issue just seems to be incorrect. > > 4. Here Thread A not do epoll_ctl(EPOLL_CTL_DEL) manunally, so it > still resides in one epoll_list. > Although __fput will call eventpoll_release to remove the file from > binded epoll list, > but it has small time window where Thread B jumps in. Well if that is really the case then that would then be a bug in epoll_ctl(). > > 5. During the small window, Thread B do the poll action for > dma_buf_fd, where it will fget/fput for the file, > this means the fd refcount will be 0 -> 1 -> 0, and it will call > __fput again. > This will lead to __fput twice for the same file. > > 6. So the potenial fix is use get_file_rcu which to check if file > refcount already zero which means under free. > If so, we just return and no need to do the dma_buf_poll. Well to say it bluntly as far as I can see this suggestion is completely nonsense. When the reference to the file goes away while dma_buf_poll() is executed then that's a massive bug in the caller of that function. Regards, Christian. > > Here is the race condition: > > Thread A Thread B > dma_buf_export > fd_refcount is 1 > epoll_ctl(EPOLL_ADD) > add dma_buf_fd to epoll list > close(dma_buf_fd) > fd_refcount is 0 > __fput > dma_buf_poll > fget > fput > fd_refcount is zero again > > Thanks >
在 2024/4/12 14:39, Christian König 写道: > [Some people who received this message don't often get email from > christian.koenig@amd.com. Learn why this is important at > https://aka.ms/LearnAboutSenderIdentification ] > > Am 12.04.24 um 08:19 schrieb zhiguojiang: >> [SNIP] >> -> Here task 2220 do epoll again where internally it will get/put then >> start to free twice and lead to final crash. >> >> Here is the basic flow: >> >> 1. Thread A install the dma_buf_fd via dma_buf_export, the fd refcount >> is 1 >> >> 2. Thread A add the fd to epoll list via epoll_ctl(EPOLL_CTL_ADD) >> >> 3. After use the dma buf, Thread A close the fd, then here fd refcount >> is 0, >> and it will run __fput finally to release the file > > Stop, that isn't correct. > > The fs layer which calls dma_buf_poll() should make sure that the file > can't go away until the function returns. > > Then inside dma_buf_poll() we add another reference to the file while > installing the callback: > > /* Paired with fput in dma_buf_poll_cb */ > get_file(dmabuf->file); Hi, The problem may just occurred here. Is it possible file reference count already decreased to 0 and fput already being in progressing just before calling get_file(dmabuf->file) in dma_buf_poll()? > > This reference is only dropped after the callback is completed in > dma_buf_poll_cb(): > > /* Paired with get_file in dma_buf_poll */ > fput(dmabuf->file); > > So your explanation for the issue just seems to be incorrect. > >> >> 4. Here Thread A not do epoll_ctl(EPOLL_CTL_DEL) manunally, so it >> still resides in one epoll_list. >> Although __fput will call eventpoll_release to remove the file from >> binded epoll list, >> but it has small time window where Thread B jumps in. > > Well if that is really the case then that would then be a bug in > epoll_ctl(). > >> >> 5. During the small window, Thread B do the poll action for >> dma_buf_fd, where it will fget/fput for the file, >> this means the fd refcount will be 0 -> 1 -> 0, and it will call >> __fput again. >> This will lead to __fput twice for the same file. >> >> 6. So the potenial fix is use get_file_rcu which to check if file >> refcount already zero which means under free. >> If so, we just return and no need to do the dma_buf_poll. > > Well to say it bluntly as far as I can see this suggestion is completely > nonsense. > > When the reference to the file goes away while dma_buf_poll() is > executed then that's a massive bug in the caller of that function. > > Regards, > Christian. > >> >> Here is the race condition: >> >> Thread A Thread B >> dma_buf_export >> fd_refcount is 1 >> epoll_ctl(EPOLL_ADD) >> add dma_buf_fd to epoll list >> close(dma_buf_fd) >> fd_refcount is 0 >> __fput >> dma_buf_poll >> fget >> fput >> fd_refcount is zero again >> >> Thanks >> >
Am 15.04.24 um 12:35 schrieb zhiguojiang: > 在 2024/4/12 14:39, Christian König 写道: >> [Some people who received this message don't often get email from >> christian.koenig@amd.com. Learn why this is important at >> https://aka.ms/LearnAboutSenderIdentification ] >> >> Am 12.04.24 um 08:19 schrieb zhiguojiang: >>> [SNIP] >>> -> Here task 2220 do epoll again where internally it will get/put then >>> start to free twice and lead to final crash. >>> >>> Here is the basic flow: >>> >>> 1. Thread A install the dma_buf_fd via dma_buf_export, the fd refcount >>> is 1 >>> >>> 2. Thread A add the fd to epoll list via epoll_ctl(EPOLL_CTL_ADD) >>> >>> 3. After use the dma buf, Thread A close the fd, then here fd refcount >>> is 0, >>> and it will run __fput finally to release the file >> >> Stop, that isn't correct. >> >> The fs layer which calls dma_buf_poll() should make sure that the file >> can't go away until the function returns. >> >> Then inside dma_buf_poll() we add another reference to the file while >> installing the callback: >> >> /* Paired with fput in dma_buf_poll_cb */ >> get_file(dmabuf->file); > Hi, > > The problem may just occurred here. > > Is it possible file reference count already decreased to 0 and fput > already being in progressing just before calling > get_file(dmabuf->file) in dma_buf_poll()? No, exactly that isn't possible. If a function gets a dma_buf pointer or even more general any reference counted pointer which has already decreased to 0 then that is a major bug in the caller of that function. BTW: It's completely illegal to work around such issues by using file_count() or RCU functions. So when you suggest stuff like that it will immediately face rejection. Regards, Christian. > >> >> This reference is only dropped after the callback is completed in >> dma_buf_poll_cb(): >> >> /* Paired with get_file in dma_buf_poll */ >> fput(dmabuf->file); >> >> So your explanation for the issue just seems to be incorrect. >> >>> >>> 4. Here Thread A not do epoll_ctl(EPOLL_CTL_DEL) manunally, so it >>> still resides in one epoll_list. >>> Although __fput will call eventpoll_release to remove the file from >>> binded epoll list, >>> but it has small time window where Thread B jumps in. >> >> Well if that is really the case then that would then be a bug in >> epoll_ctl(). >> >>> >>> 5. During the small window, Thread B do the poll action for >>> dma_buf_fd, where it will fget/fput for the file, >>> this means the fd refcount will be 0 -> 1 -> 0, and it will call >>> __fput again. >>> This will lead to __fput twice for the same file. >>> >>> 6. So the potenial fix is use get_file_rcu which to check if file >>> refcount already zero which means under free. >>> If so, we just return and no need to do the dma_buf_poll. >> >> Well to say it bluntly as far as I can see this suggestion is completely >> nonsense. >> >> When the reference to the file goes away while dma_buf_poll() is >> executed then that's a massive bug in the caller of that function. >> >> Regards, >> Christian. >> >>> >>> Here is the race condition: >>> >>> Thread A Thread B >>> dma_buf_export >>> fd_refcount is 1 >>> epoll_ctl(EPOLL_ADD) >>> add dma_buf_fd to epoll list >>> close(dma_buf_fd) >>> fd_refcount is 0 >>> __fput >>> dma_buf_poll >>> fget >>> fput >>> fd_refcount is zero again >>> >>> Thanks >>> >> >
在 2024/4/15 19:57, Christian König 写道: > [Some people who received this message don't often get email from > christian.koenig@amd.com. Learn why this is important at > https://aka.ms/LearnAboutSenderIdentification ] > > Am 15.04.24 um 12:35 schrieb zhiguojiang: >> 在 2024/4/12 14:39, Christian König 写道: >>> [Some people who received this message don't often get email from >>> christian.koenig@amd.com. Learn why this is important at >>> https://aka.ms/LearnAboutSenderIdentification ] >>> >>> Am 12.04.24 um 08:19 schrieb zhiguojiang: >>>> [SNIP] >>>> -> Here task 2220 do epoll again where internally it will get/put then >>>> start to free twice and lead to final crash. >>>> >>>> Here is the basic flow: >>>> >>>> 1. Thread A install the dma_buf_fd via dma_buf_export, the fd refcount >>>> is 1 >>>> >>>> 2. Thread A add the fd to epoll list via epoll_ctl(EPOLL_CTL_ADD) >>>> >>>> 3. After use the dma buf, Thread A close the fd, then here fd refcount >>>> is 0, >>>> and it will run __fput finally to release the file >>> >>> Stop, that isn't correct. >>> >>> The fs layer which calls dma_buf_poll() should make sure that the file >>> can't go away until the function returns. >>> >>> Then inside dma_buf_poll() we add another reference to the file while >>> installing the callback: >>> >>> /* Paired with fput in dma_buf_poll_cb */ >>> get_file(dmabuf->file); >> Hi, >> >> The problem may just occurred here. >> >> Is it possible file reference count already decreased to 0 and fput >> already being in progressing just before calling >> get_file(dmabuf->file) in dma_buf_poll()? > > No, exactly that isn't possible. > > If a function gets a dma_buf pointer or even more general any reference > counted pointer which has already decreased to 0 then that is a major > bug in the caller of that function. > > BTW: It's completely illegal to work around such issues by using > file_count() or RCU functions. So when you suggest stuff like that it > will immediately face rejection. > > Regards, > Christian. Hi, Thanks for your kind comment. 'If a function gets a dma_buf pointer or even more general any reference counted pointer which has already decreased to 0 then that is a major bug in the caller of that function.' According to the issue log, we can see the dma buf file close and epoll operation running in parallel. Apparently at the moment caller calls epoll, although another task caller already called close on the same fd, but this fd was still in progress to close, so fd is still valid, thus no EBADF returned to caller. Then the two task contexts operate on same dma buf fd(one is close, another is epoll) in kernel space. Please kindly help to comment whether above scenario is possible. If there is some bug in the caller logic, Can u help to point it out? :) Regards, Zhiguo > >> >>> >>> This reference is only dropped after the callback is completed in >>> dma_buf_poll_cb(): >>> >>> /* Paired with get_file in dma_buf_poll */ >>> fput(dmabuf->file); >>> >>> So your explanation for the issue just seems to be incorrect. >>> >>>> >>>> 4. Here Thread A not do epoll_ctl(EPOLL_CTL_DEL) manunally, so it >>>> still resides in one epoll_list. >>>> Although __fput will call eventpoll_release to remove the file from >>>> binded epoll list, >>>> but it has small time window where Thread B jumps in. >>> >>> Well if that is really the case then that would then be a bug in >>> epoll_ctl(). >>> >>>> >>>> 5. During the small window, Thread B do the poll action for >>>> dma_buf_fd, where it will fget/fput for the file, >>>> this means the fd refcount will be 0 -> 1 -> 0, and it will call >>>> __fput again. >>>> This will lead to __fput twice for the same file. >>>> >>>> 6. So the potenial fix is use get_file_rcu which to check if file >>>> refcount already zero which means under free. >>>> If so, we just return and no need to do the dma_buf_poll. >>> >>> Well to say it bluntly as far as I can see this suggestion is >>> completely >>> nonsense. >>> >>> When the reference to the file goes away while dma_buf_poll() is >>> executed then that's a massive bug in the caller of that function. >>> >>> Regards, >>> Christian. >>> >>>> >>>> Here is the race condition: >>>> >>>> Thread A Thread B >>>> dma_buf_export >>>> fd_refcount is 1 >>>> epoll_ctl(EPOLL_ADD) >>>> add dma_buf_fd to epoll list >>>> close(dma_buf_fd) >>>> fd_refcount is 0 >>>> __fput >>>> dma_buf_poll >>>> fget >>>> fput >>>> fd_refcount is zero again >>>> >>>> Thanks >>>> >>> >> >
Am 18.04.24 um 03:33 schrieb zhiguojiang: > 在 2024/4/15 19:57, Christian König 写道: >> [Some people who received this message don't often get email from >> christian.koenig@amd.com. Learn why this is important at >> https://aka.ms/LearnAboutSenderIdentification ] >> >> Am 15.04.24 um 12:35 schrieb zhiguojiang: >>> 在 2024/4/12 14:39, Christian König 写道: >>>> [Some people who received this message don't often get email from >>>> christian.koenig@amd.com. Learn why this is important at >>>> https://aka.ms/LearnAboutSenderIdentification ] >>>> >>>> Am 12.04.24 um 08:19 schrieb zhiguojiang: >>>>> [SNIP] >>>>> -> Here task 2220 do epoll again where internally it will get/put >>>>> then >>>>> start to free twice and lead to final crash. >>>>> >>>>> Here is the basic flow: >>>>> >>>>> 1. Thread A install the dma_buf_fd via dma_buf_export, the fd >>>>> refcount >>>>> is 1 >>>>> >>>>> 2. Thread A add the fd to epoll list via epoll_ctl(EPOLL_CTL_ADD) >>>>> >>>>> 3. After use the dma buf, Thread A close the fd, then here fd >>>>> refcount >>>>> is 0, >>>>> and it will run __fput finally to release the file >>>> >>>> Stop, that isn't correct. >>>> >>>> The fs layer which calls dma_buf_poll() should make sure that the file >>>> can't go away until the function returns. >>>> >>>> Then inside dma_buf_poll() we add another reference to the file while >>>> installing the callback: >>>> >>>> /* Paired with fput in dma_buf_poll_cb */ >>>> get_file(dmabuf->file); >>> Hi, >>> >>> The problem may just occurred here. >>> >>> Is it possible file reference count already decreased to 0 and fput >>> already being in progressing just before calling >>> get_file(dmabuf->file) in dma_buf_poll()? >> >> No, exactly that isn't possible. >> >> If a function gets a dma_buf pointer or even more general any reference >> counted pointer which has already decreased to 0 then that is a major >> bug in the caller of that function. >> >> BTW: It's completely illegal to work around such issues by using >> file_count() or RCU functions. So when you suggest stuff like that it >> will immediately face rejection. >> >> Regards, >> Christian. > Hi, > > Thanks for your kind comment. > > 'If a function gets a dma_buf pointer or even more general any reference > > counted pointer which has already decreased to 0 then that is a major > > bug in the caller of that function.' > > According to the issue log, we can see the dma buf file close and > epoll operation running in parallel. > > Apparently at the moment caller calls epoll, although another task > caller already called close on the same fd, but this fd was still in > progress to close, so fd is still valid, thus no EBADF returned to > caller. No, exactly that can't happen either. As far as I can see the EPOLL holds a reference to the files it contains. So it is perfectly valid to add the file descriptor to EPOLL and then close it. In this case the file won't be closed, but be kept alive by it's reference in the EPOLL file descriptor. > > Then the two task contexts operate on same dma buf fd(one is close, > another is epoll) in kernel space. > > Please kindly help to comment whether above scenario is possible. > > If there is some bug in the caller logic, Can u help to point it out? :) Well what could be is that the EPOLL implementation is broken somehow, but I have really doubts on that. As I said before the most likely explanation is that you have a broken device driver which messes up the DMA-buf file reference count somehow. Regards, Christian. > > Regards, > Zhiguo >> >>> >>>> >>>> This reference is only dropped after the callback is completed in >>>> dma_buf_poll_cb(): >>>> >>>> /* Paired with get_file in dma_buf_poll */ >>>> fput(dmabuf->file); >>>> >>>> So your explanation for the issue just seems to be incorrect. >>>> >>>>> >>>>> 4. Here Thread A not do epoll_ctl(EPOLL_CTL_DEL) manunally, so it >>>>> still resides in one epoll_list. >>>>> Although __fput will call eventpoll_release to remove the file from >>>>> binded epoll list, >>>>> but it has small time window where Thread B jumps in. >>>> >>>> Well if that is really the case then that would then be a bug in >>>> epoll_ctl(). >>>> >>>>> >>>>> 5. During the small window, Thread B do the poll action for >>>>> dma_buf_fd, where it will fget/fput for the file, >>>>> this means the fd refcount will be 0 -> 1 -> 0, and it will call >>>>> __fput again. >>>>> This will lead to __fput twice for the same file. >>>>> >>>>> 6. So the potenial fix is use get_file_rcu which to check if file >>>>> refcount already zero which means under free. >>>>> If so, we just return and no need to do the dma_buf_poll. >>>> >>>> Well to say it bluntly as far as I can see this suggestion is >>>> completely >>>> nonsense. >>>> >>>> When the reference to the file goes away while dma_buf_poll() is >>>> executed then that's a massive bug in the caller of that function. >>>> >>>> Regards, >>>> Christian. >>>> >>>>> >>>>> Here is the race condition: >>>>> >>>>> Thread A Thread B >>>>> dma_buf_export >>>>> fd_refcount is 1 >>>>> epoll_ctl(EPOLL_ADD) >>>>> add dma_buf_fd to epoll list >>>>> close(dma_buf_fd) >>>>> fd_refcount is 0 >>>>> __fput >>>>> dma_buf_poll >>>>> fget >>>>> fput >>>>> fd_refcount is zero again >>>>> >>>>> Thanks >>>>> >>>> >>> >> >
Thanks Christian/TJ for the inputs!! On 4/18/2024 12:16 PM, Christian König wrote: > As far as I can see the EPOLL holds a reference to the files it > contains. So it is perfectly valid to add the file descriptor to EPOLL > and then close it. > > In this case the file won't be closed, but be kept alive by it's > reference in the EPOLL file descriptor. I am not seeing that adding a 'fd' into the epoll monitoring list will increase its refcount. Confirmed by writing a testcase that just do open + EPOLL_CTL_ADD and see the /proc/../fdinfo of epoll fd(Added file_count() info to the output) # cat /proc/136/fdinfo/3 pos: 0 flags: 02 mnt_id: 14 ino: 1041 tfd: 4 events: 19 data: 4 pos:0 ino:81 sdev:5 refcount: 1<-- The fd added to the epoll monitoring is still 1(same as open file refcount) From the code too, I don't see a file added in the epoll monitoring list will have an extra refcount but momentarily (where it increases the refcount of target file, add it to the rbtree of the epoll context and then decreasing the file refcount): do_epoll_ctl(): f = fdget(epfd); tf = fdget(fd); epoll_mutex_lock(&ep->mtx) EPOLL_CTL_ADD: ep_insert(ep, epds, tf.file, fd, full_check); // Added to the epoll monitoring rb tree list as epitem. mutex_unlock(&ep->mtx); fdput(tf); fdput(f); Not sure If i am completely mistaken what you're saying here. > The fs layer which calls dma_buf_poll() should make sure that the file > can't go away until the function returns. > > Then inside dma_buf_poll() we add another reference to the file while > installing the callback: > > /* Paired with fput in dma_buf_poll_cb */ > get_file(dmabuf->file); No, exactly that can't > happen either. > I am not quite comfortable with epoll internals but I think below race is possible where "The 'file' passed to dma_buf_poll() is proper but ->f_count == 0, which is indicating that a parallel freeing is happening". So, we should check the file->f_count value before taking the refcount. (A 'fd' registered for the epoll monitoring list is maintained as 'epitem(epi)' in the rbtree of 'eventpoll(ep, called as epoll context). epoll_wait() __fput()(as file->f_count=0) ------------------------------------------------------------------------ a) ep_poll_callback(): Is the waitqueue function called when signaled on the wait_queue_head_t of the registered poll() function. 1) It links the 'epi' to the ready list of 'ep': if (!ep_is_linked(epi)) list_add_tail_lockless(&epi->rdllink, &ep->rdllist) 2) wake_up(&ep->wq); Wake up the process waiting on epoll_wait() that endup in ep_poll. b) ep_poll(): 1) while (1) { eavail = ep_events_available(ep); (checks ep->rdlist) ep_send_events(ep); (notify the events to user) } (epoll_wait() calling process gets woken up from a.2 and process the events raised added to rdlist in a.1) 2) ep_send_events(): mutex_lock(&ep->mtx); (** The sync lock is taken **) list_for_each_entry_safe(epi, tmp, &txlist, rdllink) { list_del_init(&epi->rdllink); revents = ep_item_poll(epi, &pt, 1) (vfs_poll()-->...f_op->poll(=dma_buf_poll) } mutex_unlock(&ep->mtx) (**release the lock.**) (As part of procession of events, each epitem is removed from rdlist and call the f_op->poll() of a file which will endup in dma_buf_poll()) 3) dma_buf_poll(): get_file(dmabuf->file); (** f_count changed from 0->1 **) dma_buf_poll_add_cb(resv, true, dcb): (registers dma_buf_poll_cb() against fence) c) eventpoll_release_file(): mutex_lock(&ep->mtx); __ep_remove(ep, epi, true): mutex_unlock(&ep->mtx); (__ep_remove() will remove the 'epi' from rbtree and if present from rdlist as well) d) file_free(file), free the 'file'. e) dma_buf_poll_cb: /* Paired with get_file in dma_buf_poll */ fput(dmabuf->file); (f_count changed from 1->0, where we try to free the 'file' again which is UAF/double free). In the above race, If c) gets called first, then the 'epi' is removed from both rbtree and 'rdlink' under ep->mtx lock thus b.2 don't end up in calling the ->poll() as it don't see this event in the rdlist. Race only exist If b.2 executes first, where it will call dma_buf_poll with __valid 'struct file' under ep->mtx but its refcount is already could have been zero__. Later When e) is executed, it turns into double free of the 'file' structure. If you're convinced with the above race, should the fix here will be this simple check: diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c index 8fe5aa67b167..e469dd9288cc --- a/drivers/dma-buf/dma-buf.c +++ b/drivers/dma-buf/dma-buf.c @@ -240,6 +240,10 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll) struct dma_resv *resv; __poll_t events; + /* Check parallel freeing of file */ + if (!file_count(file)) + return 0; + Thanks, Charan
On Fri, May 3, 2024 at 6:40 AM Charan Teja Kalla <quic_charante@quicinc.com> wrote: > > Thanks Christian/TJ for the inputs!! > > On 4/18/2024 12:16 PM, Christian König wrote: > > As far as I can see the EPOLL holds a reference to the files it > > contains. So it is perfectly valid to add the file descriptor to EPOLL > > and then close it. > > > > In this case the file won't be closed, but be kept alive by it's > > reference in the EPOLL file descriptor. > > I am not seeing that adding a 'fd' into the epoll monitoring list will > increase its refcount. Confirmed by writing a testcase that just do > open + EPOLL_CTL_ADD and see the /proc/../fdinfo of epoll fd(Added > file_count() info to the output) > # cat /proc/136/fdinfo/3 > pos: 0 > flags: 02 > mnt_id: 14 > ino: 1041 > tfd: 4 events: 19 data: 4 pos:0 ino:81 sdev:5 > refcount: 1<-- The fd added to the epoll monitoring is still 1(same as > open file refcount) > > From the code too, I don't see a file added in the epoll monitoring list > will have an extra refcount but momentarily (where it increases the > refcount of target file, add it to the rbtree of the epoll context and > then decreasing the file refcount): > do_epoll_ctl(): > f = fdget(epfd); > tf = fdget(fd); > epoll_mutex_lock(&ep->mtx) > EPOLL_CTL_ADD: > ep_insert(ep, epds, tf.file, fd, full_check); // Added to the epoll > monitoring rb tree list as epitem. > mutex_unlock(&ep->mtx); > fdput(tf); > fdput(f); > > > Not sure If i am completely mistaken what you're saying here. > > > The fs layer which calls dma_buf_poll() should make sure that the file > > can't go away until the function returns. > > > > Then inside dma_buf_poll() we add another reference to the file while > > installing the callback: > > > > /* Paired with fput in dma_buf_poll_cb */ > > get_file(dmabuf->file); No, exactly that can't > > happen either. > > > > I am not quite comfortable with epoll internals but I think below race > is possible where "The 'file' passed to dma_buf_poll() is proper but > ->f_count == 0, which is indicating that a parallel freeing is > happening". So, we should check the file->f_count value before taking > the refcount. > > (A 'fd' registered for the epoll monitoring list is maintained as > 'epitem(epi)' in the rbtree of 'eventpoll(ep, called as epoll context). > > epoll_wait() __fput()(as file->f_count=0) > ------------------------------------------------------------------------ > a) ep_poll_callback(): > Is the waitqueue function > called when signaled on the > wait_queue_head_t of the registered > poll() function. > > 1) It links the 'epi' to the ready list > of 'ep': > if (!ep_is_linked(epi)) > list_add_tail_lockless(&epi->rdllink, > &ep->rdllist) > > 2) wake_up(&ep->wq); > Wake up the process waiting > on epoll_wait() that endup > in ep_poll. > > > b) ep_poll(): > 1) while (1) { > eavail = ep_events_available(ep); > (checks ep->rdlist) > ep_send_events(ep); > (notify the events to user) > } > (epoll_wait() calling process gets > woken up from a.2 and process the > events raised added to rdlist in a.1) > > 2) ep_send_events(): > mutex_lock(&ep->mtx); > (** The sync lock is taken **) > list_for_each_entry_safe(epi, tmp, > &txlist, rdllink) { > list_del_init(&epi->rdllink); > revents = ep_item_poll(epi, &pt, 1) > (vfs_poll()-->...f_op->poll(=dma_buf_poll) > } > mutex_unlock(&ep->mtx) > (**release the lock.**) > > (As part of procession of events, > each epitem is removed from rdlist > and call the f_op->poll() of a file > which will endup in dma_buf_poll()) > > 3) dma_buf_poll(): > get_file(dmabuf->file); > (** f_count changed from 0->1 **) > dma_buf_poll_add_cb(resv, true, dcb): > (registers dma_buf_poll_cb() against fence) > > > c) eventpoll_release_file(): > mutex_lock(&ep->mtx); > __ep_remove(ep, epi, true): > mutex_unlock(&ep->mtx); > (__ep_remove() will remove the > 'epi' from rbtree and if present > from rdlist as well) > > d) file_free(file), free the 'file'. > > e) dma_buf_poll_cb: > /* Paired with get_file in dma_buf_poll */ > fput(dmabuf->file); > (f_count changed from 1->0, where > we try to free the 'file' again > which is UAF/double free). > > > > In the above race, If c) gets called first, then the 'epi' is removed > from both rbtree and 'rdlink' under ep->mtx lock thus b.2 don't end up > in calling the ->poll() as it don't see this event in the rdlist. > > Race only exist If b.2 executes first, where it will call dma_buf_poll > with __valid 'struct file' under ep->mtx but its refcount is already > could have been zero__. Later When e) is executed, it turns into double > free of the 'file' structure. > > If you're convinced with the above race, should the fix here will be > this simple check: > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c > index 8fe5aa67b167..e469dd9288cc > --- a/drivers/dma-buf/dma-buf.c > +++ b/drivers/dma-buf/dma-buf.c > @@ -240,6 +240,10 @@ static __poll_t dma_buf_poll(struct file *file, > poll_table *poll) > struct dma_resv *resv; > __poll_t events; > > + /* Check parallel freeing of file */ > + if (!file_count(file)) > + return 0; > + > > Thanks, > Charan Hi Charan, It looks like a similar conclusion about epoll was reached at: https://lore.kernel.org/all/a87d7ef8-2c59-4dc5-ba0a-b821d1effc72@amd.com/ I agree with Christian that it should not be possible for the file to be freed while inside dma_buf_poll. Aside from causing problems in dma_buf_poll, ep_item_poll itself would have issues dereferencing the freed file pointer. Christian's patch there makes me wonder about what the epoll man page says about closing files. "Will closing a file descriptor cause it to be removed from all epoll interest lists?" Answer: Yes https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/man/man7/epoll.7#n442 It looks like eventpoll_release_file is responsible for that, but if the epitem is changed to hold a reference then that can't be true anymore because __fput will never call eventpoll_release_file (until EPOLL_CTL_DEL). But how do you call EPOLL_CTL_DEL if you've closed all your references to the file in userspace? So I think epoll needs a slightly more complicated fix.
Thanks T.J for the reply!! On 5/4/2024 4:43 AM, T.J. Mercier wrote: > It looks like a similar conclusion about epoll was reached at: > https://lore.kernel.org/all/a87d7ef8-2c59-4dc5-ba0a-b821d1effc72@amd.com/ > I am unaware of this discussion. Thanks... > I agree with Christian that it should not be possible for the file to > be freed while inside dma_buf_poll. Aside from causing problems in > dma_buf_poll, ep_item_poll itself would have issues dereferencing the > freed file pointer. > Not sure about my understanding: ep_item_poll() always call the ->poll() interface with a stable 'struct file' because of ep->mtx. This lock ensures that: a) If eventpoll_release_file() get the ep->mtx first, ->poll() corresponds to the epitem(target file) will never be called, because it is removed from the rdlist. b) If ep_send_events() get the ep->mtx() first, ->poll() will get called with a stable 'struct file', __but the refcount(->f_count) of a file can be zero__. I am saying that this is stable because the 'struct file' contents are still valid till we are in ->poll(). Can you/Christian help me with what I am missing here to say that ->poll() is receiving stale 'struct file*', please? And, If you are convinced with above, I think, It should have been the responsibility of ->poll() implementation to have taken refcount on a file that is going to be still valid even after ->poll() exits. Incase of dma_buf_poll() implementation, it took the refcount on a file that is not going to be valid once the dma_buf_poll() exits(because of mentioned race with the freeing of the 'struct file*'). So, in dma_buf_poll(), Should we be using atomic_long_inc_not_zero() based implementation to take the refcount on a file? Thanks, Charan
Hi TJ, Seems I have got answers from [1], where it is agreed upon epoll() is the source of issue. Thanks a lot for the discussion. [1] https://lore.kernel.org/lkml/0000000000002d631f0615918f1e@google.com/ Thanks Charan On 5/5/2024 9:50 PM, Charan Teja Kalla wrote: > Thanks T.J for the reply!! > > On 5/4/2024 4:43 AM, T.J. Mercier wrote: >> It looks like a similar conclusion about epoll was reached at: >> https://lore.kernel.org/all/a87d7ef8-2c59-4dc5-ba0a-b821d1effc72@amd.com/ >> > I am unaware of this discussion. Thanks... > >> I agree with Christian that it should not be possible for the file to >> be freed while inside dma_buf_poll. Aside from causing problems in >> dma_buf_poll, ep_item_poll itself would have issues dereferencing the >> freed file pointer. >> > Not sure about my understanding: ep_item_poll() always call the ->poll() > interface with a stable 'struct file' because of ep->mtx. This lock > ensures that: > a) If eventpoll_release_file() get the ep->mtx first, ->poll() > corresponds to the epitem(target file) will never be called, because it > is removed from the rdlist. > > b) If ep_send_events() get the ep->mtx() first, ->poll() will get > called with a stable 'struct file', __but the refcount(->f_count) of a > file can be zero__. I am saying that this is stable because the 'struct > file' contents are still valid till we are in ->poll(). > > Can you/Christian help me with what I am missing here to say that > ->poll() is receiving stale 'struct file*', please? > > And, If you are convinced with above, I think, It should have been the > responsibility of ->poll() implementation to have taken refcount on a > file that is going to be still valid even after ->poll() exits. Incase > of dma_buf_poll() implementation, it took the refcount on a file that is > not going to be valid once the dma_buf_poll() exits(because of mentioned > race with the freeing of the 'struct file*'). > > So, in dma_buf_poll(), Should we be using atomic_long_inc_not_zero() > based implementation to take the refcount on a file? > > Thanks, > Charan
On Mon, May 6, 2024 at 2:30 AM Charan Teja Kalla <quic_charante@quicinc.com> wrote: > > Hi TJ, > > Seems I have got answers from [1], where it is agreed upon epoll() is > the source of issue. > > Thanks a lot for the discussion. > > [1] https://lore.kernel.org/lkml/0000000000002d631f0615918f1e@google.com/ > > Thanks > Charan Oh man, quite a set of threads on this over the weekend. Thanks for the link.
Am 06.05.24 um 21:04 schrieb T.J. Mercier: > On Mon, May 6, 2024 at 2:30 AM Charan Teja Kalla > <quic_charante@quicinc.com> wrote: >> Hi TJ, >> >> Seems I have got answers from [1], where it is agreed upon epoll() is >> the source of issue. >> >> Thanks a lot for the discussion. >> >> [1] https://lore.kernel.org/lkml/0000000000002d631f0615918f1e@google.com/ >> >> Thanks >> Charan > Oh man, quite a set of threads on this over the weekend. Thanks for the link. Yeah and it also has some interesting side conclusion: We should probably tell people to stop using DMA-buf with epoll. The background is that the mutex approach epoll uses to make files disappear from the interest list on close results in the fact that each file can only be part of a single epoll at a time. Now since DMA-buf is build around the idea that we share the buffer representation as file between processes it means that only one process at a time can use epoll with each DMA-buf. So for example if a window manager uses epoll everything is fine. If a client is using epoll everything is fine as well. But if *both* use epoll at the same time it won't work. This can lead to rather funny and hard to debug combinations of failures and I think we need to document this limitation and explicitly point it out. Regards, Christian.
On Tue, May 07, 2024 at 12:10:07PM +0200, Christian König wrote: > Am 06.05.24 um 21:04 schrieb T.J. Mercier: > > On Mon, May 6, 2024 at 2:30 AM Charan Teja Kalla > > <quic_charante@quicinc.com> wrote: > > > Hi TJ, > > > > > > Seems I have got answers from [1], where it is agreed upon epoll() is > > > the source of issue. > > > > > > Thanks a lot for the discussion. > > > > > > [1] https://lore.kernel.org/lkml/0000000000002d631f0615918f1e@google.com/ > > > > > > Thanks > > > Charan > > Oh man, quite a set of threads on this over the weekend. Thanks for the link. > > Yeah and it also has some interesting side conclusion: We should probably > tell people to stop using DMA-buf with epoll. > > The background is that the mutex approach epoll uses to make files disappear > from the interest list on close results in the fact that each file can only > be part of a single epoll at a time. > > Now since DMA-buf is build around the idea that we share the buffer > representation as file between processes it means that only one process at a > time can use epoll with each DMA-buf. > > So for example if a window manager uses epoll everything is fine. If a > client is using epoll everything is fine as well. But if *both* use epoll at > the same time it won't work. > > This can lead to rather funny and hard to debug combinations of failures and > I think we need to document this limitation and explicitly point it out. Ok, I tested this with a small C program, and you're mixing things up. Here's what I got - You cannot add a file twice to the same epoll file/fd. So that part is correct, and also my understanding from reading the kernel code. - You can add the same file to two different epoll file instaces. Which means it's totally fine to use epoll on a dma_buf in different processes like both in the compositor and in clients. - Substantially more entertaining, you can nest epoll instances, and e.g. add a 2nd epoll file as an event to the first one. That way you can add the same file to both epoll fds, and so end up with the same file essentially being added twice to the top-level epoll file. So even within one application there's no real issue when e.g. different userspace drivers all want to use epoll on the same fd, because you can just throw in another level of epoll and it's fine again and you won't get an EEXISTS on EPOLL_CTL_ADD. But I also don't think we have this issue right now anywhere, since it's kinda a general epoll issue that happens with any duplicated file. So I don't think there's any reasons to recommend against using epoll on dma-buf fd (or sync_file or drm_syncobj or any of the sharing primitives we have really). Cheers, Sima
Am 07.05.24 um 15:39 schrieb Daniel Vetter: > On Tue, May 07, 2024 at 12:10:07PM +0200, Christian König wrote: >> Am 06.05.24 um 21:04 schrieb T.J. Mercier: >>> On Mon, May 6, 2024 at 2:30 AM Charan Teja Kalla >>> <quic_charante@quicinc.com> wrote: >>>> Hi TJ, >>>> >>>> Seems I have got answers from [1], where it is agreed upon epoll() is >>>> the source of issue. >>>> >>>> Thanks a lot for the discussion. >>>> >>>> [1] https://lore.kernel.org/lkml/0000000000002d631f0615918f1e@google.com/ >>>> >>>> Thanks >>>> Charan >>> Oh man, quite a set of threads on this over the weekend. Thanks for the link. >> Yeah and it also has some interesting side conclusion: We should probably >> tell people to stop using DMA-buf with epoll. >> >> The background is that the mutex approach epoll uses to make files disappear >> from the interest list on close results in the fact that each file can only >> be part of a single epoll at a time. >> >> Now since DMA-buf is build around the idea that we share the buffer >> representation as file between processes it means that only one process at a >> time can use epoll with each DMA-buf. >> >> So for example if a window manager uses epoll everything is fine. If a >> client is using epoll everything is fine as well. But if *both* use epoll at >> the same time it won't work. >> >> This can lead to rather funny and hard to debug combinations of failures and >> I think we need to document this limitation and explicitly point it out. > Ok, I tested this with a small C program, and you're mixing things up. > Here's what I got > > - You cannot add a file twice to the same epoll file/fd. So that part is > correct, and also my understanding from reading the kernel code. > > - You can add the same file to two different epoll file instaces. Which > means it's totally fine to use epoll on a dma_buf in different processes > like both in the compositor and in clients. Ah! Than I misunderstood that comment in the discussion. Thanks for clarifying that. > > - Substantially more entertaining, you can nest epoll instances, and e.g. > add a 2nd epoll file as an event to the first one. That way you can add > the same file to both epoll fds, and so end up with the same file > essentially being added twice to the top-level epoll file. So even > within one application there's no real issue when e.g. different > userspace drivers all want to use epoll on the same fd, because you can > just throw in another level of epoll and it's fine again and you won't > get an EEXISTS on EPOLL_CTL_ADD. > > But I also don't think we have this issue right now anywhere, since it's > kinda a general epoll issue that happens with any duplicated file. I actually have been telling people to (ab)use the epoll behavior to check if two file descriptors point to the same underlying file when KCMP isn't available. Some environments (Android?) disable KCMP because they see it as security problem. > So I don't think there's any reasons to recommend against using epoll on > dma-buf fd (or sync_file or drm_syncobj or any of the sharing primitives > we have really). No, that indeed seems to be fine then. Thanks, Christian. > > Cheers, Sima
On Tue, May 7, 2024 at 7:04 AM Christian König <christian.koenig@amd.com> wrote: > > Am 07.05.24 um 15:39 schrieb Daniel Vetter: > > On Tue, May 07, 2024 at 12:10:07PM +0200, Christian König wrote: > >> Am 06.05.24 um 21:04 schrieb T.J. Mercier: > >>> On Mon, May 6, 2024 at 2:30 AM Charan Teja Kalla > >>> <quic_charante@quicinc.com> wrote: > >>>> Hi TJ, > >>>> > >>>> Seems I have got answers from [1], where it is agreed upon epoll() is > >>>> the source of issue. > >>>> > >>>> Thanks a lot for the discussion. > >>>> > >>>> [1] https://lore.kernel.org/lkml/0000000000002d631f0615918f1e@google.com/ > >>>> > >>>> Thanks > >>>> Charan > >>> Oh man, quite a set of threads on this over the weekend. Thanks for the link. > >> Yeah and it also has some interesting side conclusion: We should probably > >> tell people to stop using DMA-buf with epoll. > >> > >> The background is that the mutex approach epoll uses to make files disappear > >> from the interest list on close results in the fact that each file can only > >> be part of a single epoll at a time. > >> > >> Now since DMA-buf is build around the idea that we share the buffer > >> representation as file between processes it means that only one process at a > >> time can use epoll with each DMA-buf. > >> > >> So for example if a window manager uses epoll everything is fine. If a > >> client is using epoll everything is fine as well. But if *both* use epoll at > >> the same time it won't work. > >> > >> This can lead to rather funny and hard to debug combinations of failures and > >> I think we need to document this limitation and explicitly point it out. > > Ok, I tested this with a small C program, and you're mixing things up. > > Here's what I got > > > > - You cannot add a file twice to the same epoll file/fd. So that part is > > correct, and also my understanding from reading the kernel code. > > > > - You can add the same file to two different epoll file instaces. Which > > means it's totally fine to use epoll on a dma_buf in different processes > > like both in the compositor and in clients. > > Ah! Than I misunderstood that comment in the discussion. Thanks for > clarifying that. > > > > > - Substantially more entertaining, you can nest epoll instances, and e.g. > > add a 2nd epoll file as an event to the first one. That way you can add > > the same file to both epoll fds, and so end up with the same file > > essentially being added twice to the top-level epoll file. So even > > within one application there's no real issue when e.g. different > > userspace drivers all want to use epoll on the same fd, because you can > > just throw in another level of epoll and it's fine again and you won't > > get an EEXISTS on EPOLL_CTL_ADD. > > > > But I also don't think we have this issue right now anywhere, since it's > > kinda a general epoll issue that happens with any duplicated file. > > I actually have been telling people to (ab)use the epoll behavior to > check if two file descriptors point to the same underlying file when > KCMP isn't available. > > Some environments (Android?) disable KCMP because they see it as > security problem. > Didn't know about this so I checked. Our kernel has CONFIG_KCMP, but seccomp does look like it's blocking kcmp for apps. https://cs.android.com/android/platform/superproject/main/+/main:bionic/libc/SYSCALLS.TXT
On Tue, May 7, 2024 at 11:17 AM T.J. Mercier <tjmercier@google.com> wrote: > > On Tue, May 7, 2024 at 7:04 AM Christian König <christian.koenig@amd.com> wrote: > > > > Am 07.05.24 um 15:39 schrieb Daniel Vetter: > > > On Tue, May 07, 2024 at 12:10:07PM +0200, Christian König wrote: > > >> Am 06.05.24 um 21:04 schrieb T.J. Mercier: > > >>> On Mon, May 6, 2024 at 2:30 AM Charan Teja Kalla > > >>> <quic_charante@quicinc.com> wrote: > > >>>> Hi TJ, > > >>>> > > >>>> Seems I have got answers from [1], where it is agreed upon epoll() is > > >>>> the source of issue. > > >>>> > > >>>> Thanks a lot for the discussion. > > >>>> > > >>>> [1] https://lore.kernel.org/lkml/0000000000002d631f0615918f1e@google.com/ > > >>>> > > >>>> Thanks > > >>>> Charan > > >>> Oh man, quite a set of threads on this over the weekend. Thanks for the link. > > >> Yeah and it also has some interesting side conclusion: We should probably > > >> tell people to stop using DMA-buf with epoll. > > >> > > >> The background is that the mutex approach epoll uses to make files disappear > > >> from the interest list on close results in the fact that each file can only > > >> be part of a single epoll at a time. > > >> > > >> Now since DMA-buf is build around the idea that we share the buffer > > >> representation as file between processes it means that only one process at a > > >> time can use epoll with each DMA-buf. > > >> > > >> So for example if a window manager uses epoll everything is fine. If a > > >> client is using epoll everything is fine as well. But if *both* use epoll at > > >> the same time it won't work. > > >> > > >> This can lead to rather funny and hard to debug combinations of failures and > > >> I think we need to document this limitation and explicitly point it out. > > > Ok, I tested this with a small C program, and you're mixing things up. > > > Here's what I got > > > > > > - You cannot add a file twice to the same epoll file/fd. So that part is > > > correct, and also my understanding from reading the kernel code. > > > > > > - You can add the same file to two different epoll file instaces. Which > > > means it's totally fine to use epoll on a dma_buf in different processes > > > like both in the compositor and in clients. > > > > Ah! Than I misunderstood that comment in the discussion. Thanks for > > clarifying that. > > > > > > > > - Substantially more entertaining, you can nest epoll instances, and e.g. > > > add a 2nd epoll file as an event to the first one. That way you can add > > > the same file to both epoll fds, and so end up with the same file > > > essentially being added twice to the top-level epoll file. So even > > > within one application there's no real issue when e.g. different > > > userspace drivers all want to use epoll on the same fd, because you can > > > just throw in another level of epoll and it's fine again and you won't > > > get an EEXISTS on EPOLL_CTL_ADD. > > > > > > But I also don't think we have this issue right now anywhere, since it's > > > kinda a general epoll issue that happens with any duplicated file. > > > > I actually have been telling people to (ab)use the epoll behavior to > > check if two file descriptors point to the same underlying file when > > KCMP isn't available. > > > > Some environments (Android?) disable KCMP because they see it as > > security problem. > > > Didn't know about this so I checked. Our kernel has CONFIG_KCMP, but > seccomp does look like it's blocking kcmp for apps. > https://cs.android.com/android/platform/superproject/main/+/main:bionic/libc/SYSCALLS.TXT Getting a bit off the original topic, but fwiw this is what CrOS did for CONFIG_KCMP: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/3501193 Ie. allow the kcmp syscall, but block type == KCMP_SYSVSEM, which was the more specific thing that android wanted to block. BR, -R
From: Christian König > Sent: 07 May 2024 15:05 ... > I actually have been telling people to (ab)use the epoll behavior to > check if two file descriptors point to the same underlying file when > KCMP isn't available. In what way? You can add both fd to the same epoll fd. Relying on the implicit EPOLL_CTL_DEL not happening until both fd are closed is a recipe for disaster. (And I can't see an obvious way of testing it.) Q6/A6 on epoll(7) should always have had a caveat that it is an 'implementation detail' and shouldn't be relied on. (it is written as a 'beware of' ...) The other point is that there are two ways to get multiple fd that reference the same underlying file. dup() fork() etc share the file offset, but open("/dev/fd/n") adds a reference count later and has a separate file offset. I don't know which structure epoll is using, but I suspect it is the former. So it may not tell you what you want to know. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c index 8fe5aa67b167..e469dd9288cc --- a/drivers/dma-buf/dma-buf.c +++ b/drivers/dma-buf/dma-buf.c @@ -240,6 +240,10 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll) struct dma_resv *resv; __poll_t events; + /* Check if the file exists */ + if (!file_count(file)) + return EPOLLERR; + dmabuf = file->private_data; if (!dmabuf || !dmabuf->resv) return EPOLLERR; @@ -266,8 +270,15 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll) spin_unlock_irq(&dmabuf->poll.lock); if (events & EPOLLOUT) { - /* Paired with fput in dma_buf_poll_cb */ - get_file(dmabuf->file); + /* + * Paired with fput in dma_buf_poll_cb, + * Skip poll for the closed file. + */ + if (!get_file_rcu(&dmabuf->file)) { + events &= ~EPOLLOUT; + dcb->active = 0; + goto clear_out_event; + } if (!dma_buf_poll_add_cb(resv, true, dcb)) /* No callback queued, wake up any other waiters */ @@ -277,6 +288,7 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll) } } +clear_out_event: if (events & EPOLLIN) { struct dma_buf_poll_cb_t *dcb = &dmabuf->cb_in; @@ -289,8 +301,15 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll) spin_unlock_irq(&dmabuf->poll.lock); if (events & EPOLLIN) { - /* Paired with fput in dma_buf_poll_cb */ - get_file(dmabuf->file); + /* + * Paired with fput in dma_buf_poll_cb, + * Skip poll for the closed file. + */ + if (!get_file_rcu(&dmabuf->file)) { + events &= ~EPOLLIN; + dcb->active = 0; + goto clear_in_event; + } if (!dma_buf_poll_add_cb(resv, false, dcb)) /* No callback queued, wake up any other waiters */ @@ -300,6 +319,7 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll) } } +clear_in_event: dma_resv_unlock(resv); return events; }
The issue is a UAF issue of dmabuf file fd. Throght debugging, we found that the dmabuf file fd is added to the epoll event listener list, and when it is released, it is not removed from the epoll list, which leads to the UAF(Use-After-Free) issue. The UAF issue can be solved by checking dmabuf file->f_count value and skipping the poll operation for the closed dmabuf file in the dma_buf_poll(). We have tested this solved patch multiple times and have not reproduced the uaf issue. crash dump: list_del corruption, ffffff8a6f143a90->next is LIST_POISON1 (dead000000000100) ------------[ cut here ]------------ kernel BUG at lib/list_debug.c:55! Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP pc : __list_del_entry_valid+0x98/0xd4 lr : __list_del_entry_valid+0x98/0xd4 sp : ffffffc01d413d00 x29: ffffffc01d413d00 x28: 00000000000000c0 x27: 0000000000000020 x26: 0000000000000000 x25: 0000000000000000 x24: 0000000000080007 x23: ffffff8b22e5dcc0 x22: ffffff88a6be12d0 x21: ffffff8b22e572b0 x20: ffffff80254ed0a0 x19: ffffff8a6f143a00 x18: ffffffda5efed3c0 x17: 6165642820314e4f x16: 53494f505f545349 x15: 4c20736920747865 x14: 6e3e2d3039613334 x13: 2930303130303030 x12: 0000000000000018 x11: ffffff8b6c188000 x10: 00000000ffffffff x9 : 6c8413a194897b00 x8 : 6c8413a194897b00 x7 : 74707572726f6320 x6 : 6c65645f7473696c x5 : ffffff8b6c3b2a3e x4 : ffffff8b6c3b2a40 x3 : ffff103000001005 x2 : 0000000000000001 x1 : 00000000000000c0 x0 : 000000000000004e Call trace: __list_del_entry_valid+0x98/0xd4 dma_buf_file_release+0x48/0x90 __fput+0xf4/0x280 ____fput+0x10/0x20 task_work_run+0xcc/0xf4 do_notify_resume+0x2a0/0x33c el0_svc+0x5c/0xa4 el0t_64_sync_handler+0x68/0xb4 el0t_64_sync+0x1a0/0x1a4 Code: d0006fe0 912c5000 f2fbd5a2 94231a01 (d4210000) ---[ end trace 0000000000000000 ]--- Kernel panic - not syncing: Oops - BUG: Fatal exception SMP: stopping secondary CPUs Signed-off-by: Zhiguo Jiang <justinjiang@vivo.com> --- drivers/dma-buf/dma-buf.c | 28 ++++++++++++++++++++++++---- 1 file changed, 24 insertions(+), 4 deletions(-) mode change 100644 => 100755 drivers/dma-buf/dma-buf.c