| Message ID | 20230901202137.214666-1-john.fastabend@gmail.com (mailing list archive) |
|---|---|
| State | Accepted |
| Commit | a454d84ee20baf7bd7be90721b9821f73c7d23d9 |
| Delegated to: | BPF |
| Headers | show |
| Series | [bpf] bpf: sockmap, fix skb refcnt race after locking changes | expand |
On Fri, Sep 01, 2023 at 01:21:37PM -0700, John Fastabend wrote: > There is a race where skb's from the sk_psock_backlog can be referenced > after userspace side has already skb_consumed() the sk_buff and its > refcnt dropped to zer0 causing use after free. > > The flow is the following, > > while ((skb = skb_peek(&psock->ingress_skb)) > sk_psock_handle_Skb(psock, skb, ..., ingress) > if (!ingress) ... > sk_psock_skb_ingress > sk_psock_skb_ingress_enqueue(skb) > msg->skb = skb > sk_psock_queue_msg(psock, msg) > skb_dequeue(&psock->ingress_skb) > > The sk_psock_queue_msg() puts the msg on the ingress_msg queue. This is > what the application reads when recvmsg() is called. An application can > read this anytime after the msg is placed on the queue. The recvmsg > hook will also read msg->skb and then after user space reads the msg > will call consume_skb(skb) on it effectively free'ing it. > > But, the race is in above where backlog queue still has a reference to > the skb and calls skb_dequeue(). If the skb_dequeue happens after the > user reads and free's the skb we have a use after free. > > The !ingress case does not suffer from this problem because it uses > sendmsg_*(sk, msg) which does not pass the sk_buff further down the > stack. > > The following splat was observed with 'test_progs -t sockmap_listen': > > [ 1022.710250][ T2556] general protection fault, ... > ... > [ 1022.712830][ T2556] Workqueue: events sk_psock_backlog > [ 1022.713262][ T2556] RIP: 0010:skb_dequeue+0x4c/0x80 > [ 1022.713653][ T2556] Code: ... > ... > [ 1022.720699][ T2556] Call Trace: > [ 1022.720984][ T2556] <TASK> > [ 1022.721254][ T2556] ? die_addr+0x32/0x80^M > [ 1022.721589][ T2556] ? exc_general_protection+0x25a/0x4b0 > [ 1022.722026][ T2556] ? asm_exc_general_protection+0x22/0x30 > [ 1022.722489][ T2556] ? skb_dequeue+0x4c/0x80 > [ 1022.722854][ T2556] sk_psock_backlog+0x27a/0x300 > [ 1022.723243][ T2556] process_one_work+0x2a7/0x5b0 > [ 1022.723633][ T2556] worker_thread+0x4f/0x3a0 > [ 1022.723998][ T2556] ? __pfx_worker_thread+0x10/0x10 > [ 1022.724386][ T2556] kthread+0xfd/0x130 > [ 1022.724709][ T2556] ? __pfx_kthread+0x10/0x10 > [ 1022.725066][ T2556] ret_from_fork+0x2d/0x50 > [ 1022.725409][ T2556] ? __pfx_kthread+0x10/0x10 > [ 1022.725799][ T2556] ret_from_fork_asm+0x1b/0x30 > [ 1022.726201][ T2556] </TASK> > > To fix we add an skb_get() before passing the skb to be enqueued in > the engress queue. This bumps the skb->users refcnt so that consume_skb > and kfree_skb will not immediately free the sk_buff. With this we can > be sure the skb is still around when we do the dequeue. Then we just > need to decrement the refcnt or free the skb in the backlog case which > we do by calling kfree_skb() on the ingress case as well as the sendmsg > case. > > Before locking change from fixes tag we had the sock locked so we > couldn't race with user and there was no issue here. > > Fixes: 799aa7f98d53e (skmsg: Avoid lock_sock() in sk_psock_backlog()) > Reported-by: Jiri Olsa <jolsa@kernel.org> > Signed-off-by: John Fastabend <john.fastabend@gmail.com> > --- > net/core/skmsg.c | 12 ++++++++---- > 1 file changed, 8 insertions(+), 4 deletions(-) > > diff --git a/net/core/skmsg.c b/net/core/skmsg.c > index a0659fc29bcc..6c31eefbd777 100644 > --- a/net/core/skmsg.c > +++ b/net/core/skmsg.c > @@ -612,12 +612,18 @@ static int sk_psock_skb_ingress_self(struct sk_psock *psock, struct sk_buff *skb > static int sk_psock_handle_skb(struct sk_psock *psock, struct sk_buff *skb, > u32 off, u32 len, bool ingress) > { > + int err = 0; > + > if (!ingress) { > if (!sock_writeable(psock->sk)) > return -EAGAIN; > return skb_send_sock(psock->sk, skb, off, len); > } > - return sk_psock_skb_ingress(psock, skb, off, len); > + skb_get(skb); > + err = sk_psock_skb_ingress(psock, skb, off, len); > + if (err < 0) > + kfree_skb(skb); > + return err; > } > > static void sk_psock_skb_state(struct sk_psock *psock, > @@ -685,9 +691,7 @@ static void sk_psock_backlog(struct work_struct *work) > } while (len); > > skb = skb_dequeue(&psock->ingress_skb); > - if (!ingress) { > - kfree_skb(skb); > - } > + kfree_skb(skb); > } > end: > mutex_unlock(&psock->work_mutex); > -- > 2.33.0 > there's no crash wit with fix, but I noticed I occasionally get FAIL #212/78 sockmap_listen/sockmap Unix test_unix_redir:OK ./test_progs:vsock_unix_redir_connectible:1501: ingress: write: Transport endpoint is not connected vsock_unix_redir_connectible:FAIL:1501 ./test_progs:vsock_unix_redir_connectible:1501: egress: write: Transport endpoint is not connected vsock_unix_redir_connectible:FAIL:1501 #212/79 sockmap_listen/sockmap VSOCK test_vsock_redir:FAIL #212/80 sockmap_listen/sockhash IPv4 TCP test_insert_invalid:OK no idea if it's related jirka
On Fri, 2023-09-01 at 23:20 +0200, Jiri Olsa wrote: > On Fri, Sep 01, 2023 at 01:21:37PM -0700, John Fastabend wrote: > > There is a race where skb's from the sk_psock_backlog can be referenced > > after userspace side has already skb_consumed() the sk_buff and its > > refcnt dropped to zer0 causing use after free. > > > > The flow is the following, > > > > while ((skb = skb_peek(&psock->ingress_skb)) > > sk_psock_handle_Skb(psock, skb, ..., ingress) > > if (!ingress) ... > > sk_psock_skb_ingress > > sk_psock_skb_ingress_enqueue(skb) > > msg->skb = skb > > sk_psock_queue_msg(psock, msg) > > skb_dequeue(&psock->ingress_skb) > > > > The sk_psock_queue_msg() puts the msg on the ingress_msg queue. This is > > what the application reads when recvmsg() is called. An application can > > read this anytime after the msg is placed on the queue. The recvmsg > > hook will also read msg->skb and then after user space reads the msg > > will call consume_skb(skb) on it effectively free'ing it. > > > > But, the race is in above where backlog queue still has a reference to > > the skb and calls skb_dequeue(). If the skb_dequeue happens after the > > user reads and free's the skb we have a use after free. > > > > The !ingress case does not suffer from this problem because it uses > > sendmsg_*(sk, msg) which does not pass the sk_buff further down the > > stack. > > > > The following splat was observed with 'test_progs -t sockmap_listen': > > > > [ 1022.710250][ T2556] general protection fault, ... > > ... > > [ 1022.712830][ T2556] Workqueue: events sk_psock_backlog > > [ 1022.713262][ T2556] RIP: 0010:skb_dequeue+0x4c/0x80 > > [ 1022.713653][ T2556] Code: ... > > ... > > [ 1022.720699][ T2556] Call Trace: > > [ 1022.720984][ T2556] <TASK> > > [ 1022.721254][ T2556] ? die_addr+0x32/0x80^M > > [ 1022.721589][ T2556] ? exc_general_protection+0x25a/0x4b0 > > [ 1022.722026][ T2556] ? asm_exc_general_protection+0x22/0x30 > > [ 1022.722489][ T2556] ? skb_dequeue+0x4c/0x80 > > [ 1022.722854][ T2556] sk_psock_backlog+0x27a/0x300 > > [ 1022.723243][ T2556] process_one_work+0x2a7/0x5b0 > > [ 1022.723633][ T2556] worker_thread+0x4f/0x3a0 > > [ 1022.723998][ T2556] ? __pfx_worker_thread+0x10/0x10 > > [ 1022.724386][ T2556] kthread+0xfd/0x130 > > [ 1022.724709][ T2556] ? __pfx_kthread+0x10/0x10 > > [ 1022.725066][ T2556] ret_from_fork+0x2d/0x50 > > [ 1022.725409][ T2556] ? __pfx_kthread+0x10/0x10 > > [ 1022.725799][ T2556] ret_from_fork_asm+0x1b/0x30 > > [ 1022.726201][ T2556] </TASK> > > > > To fix we add an skb_get() before passing the skb to be enqueued in > > the engress queue. This bumps the skb->users refcnt so that consume_skb > > and kfree_skb will not immediately free the sk_buff. With this we can > > be sure the skb is still around when we do the dequeue. Then we just > > need to decrement the refcnt or free the skb in the backlog case which > > we do by calling kfree_skb() on the ingress case as well as the sendmsg > > case. > > > > Before locking change from fixes tag we had the sock locked so we > > couldn't race with user and there was no issue here. > > > > Fixes: 799aa7f98d53e (skmsg: Avoid lock_sock() in sk_psock_backlog()) > > Reported-by: Jiri Olsa <jolsa@kernel.org> > > Signed-off-by: John Fastabend <john.fastabend@gmail.com> > > --- > > net/core/skmsg.c | 12 ++++++++---- > > 1 file changed, 8 insertions(+), 4 deletions(-) > > > > diff --git a/net/core/skmsg.c b/net/core/skmsg.c > > index a0659fc29bcc..6c31eefbd777 100644 > > --- a/net/core/skmsg.c > > +++ b/net/core/skmsg.c > > @@ -612,12 +612,18 @@ static int sk_psock_skb_ingress_self(struct sk_psock *psock, struct sk_buff *skb > > static int sk_psock_handle_skb(struct sk_psock *psock, struct sk_buff *skb, > > u32 off, u32 len, bool ingress) > > { > > + int err = 0; > > + > > if (!ingress) { > > if (!sock_writeable(psock->sk)) > > return -EAGAIN; > > return skb_send_sock(psock->sk, skb, off, len); > > } > > - return sk_psock_skb_ingress(psock, skb, off, len); > > + skb_get(skb); > > + err = sk_psock_skb_ingress(psock, skb, off, len); > > + if (err < 0) > > + kfree_skb(skb); > > + return err; > > } > > > > static void sk_psock_skb_state(struct sk_psock *psock, > > @@ -685,9 +691,7 @@ static void sk_psock_backlog(struct work_struct *work) > > } while (len); > > > > skb = skb_dequeue(&psock->ingress_skb); > > - if (!ingress) { > > - kfree_skb(skb); > > - } > > + kfree_skb(skb); > > } > > end: > > mutex_unlock(&psock->work_mutex); > > -- > > 2.33.0 > > > > there's no crash wit with fix, but I noticed I occasionally get FAIL > Please note this patch: https://lore.kernel.org/bpf/20230901031037.3314007-1-xukuohai@huaweicloud.com/ Which should fix the test in question. > #212/78 sockmap_listen/sockmap Unix test_unix_redir:OK > ./test_progs:vsock_unix_redir_connectible:1501: ingress: write: Transport endpoint is not connected > vsock_unix_redir_connectible:FAIL:1501 > ./test_progs:vsock_unix_redir_connectible:1501: egress: write: Transport endpoint is not connected > vsock_unix_redir_connectible:FAIL:1501 > #212/79 sockmap_listen/sockmap VSOCK test_vsock_redir:FAIL > #212/80 sockmap_listen/sockhash IPv4 TCP test_insert_invalid:OK > > no idea if it's related > > jirka
On 9/2/2023 4:21 AM, John Fastabend wrote: > There is a race where skb's from the sk_psock_backlog can be referenced > after userspace side has already skb_consumed() the sk_buff and its > refcnt dropped to zer0 causing use after free. > > The flow is the following, > > while ((skb = skb_peek(&psock->ingress_skb)) > sk_psock_handle_Skb(psock, skb, ..., ingress) > if (!ingress) ... > sk_psock_skb_ingress > sk_psock_skb_ingress_enqueue(skb) > msg->skb = skb > sk_psock_queue_msg(psock, msg) > skb_dequeue(&psock->ingress_skb) > > The sk_psock_queue_msg() puts the msg on the ingress_msg queue. This is > what the application reads when recvmsg() is called. An application can > read this anytime after the msg is placed on the queue. The recvmsg > hook will also read msg->skb and then after user space reads the msg > will call consume_skb(skb) on it effectively free'ing it. > > But, the race is in above where backlog queue still has a reference to > the skb and calls skb_dequeue(). If the skb_dequeue happens after the > user reads and free's the skb we have a use after free. > > The !ingress case does not suffer from this problem because it uses > sendmsg_*(sk, msg) which does not pass the sk_buff further down the > stack. > > The following splat was observed with 'test_progs -t sockmap_listen': > > [ 1022.710250][ T2556] general protection fault, ... > ... > [ 1022.712830][ T2556] Workqueue: events sk_psock_backlog > [ 1022.713262][ T2556] RIP: 0010:skb_dequeue+0x4c/0x80 > [ 1022.713653][ T2556] Code: ... > ... > [ 1022.720699][ T2556] Call Trace: > [ 1022.720984][ T2556] <TASK> > [ 1022.721254][ T2556] ? die_addr+0x32/0x80^M > [ 1022.721589][ T2556] ? exc_general_protection+0x25a/0x4b0 > [ 1022.722026][ T2556] ? asm_exc_general_protection+0x22/0x30 > [ 1022.722489][ T2556] ? skb_dequeue+0x4c/0x80 > [ 1022.722854][ T2556] sk_psock_backlog+0x27a/0x300 > [ 1022.723243][ T2556] process_one_work+0x2a7/0x5b0 > [ 1022.723633][ T2556] worker_thread+0x4f/0x3a0 > [ 1022.723998][ T2556] ? __pfx_worker_thread+0x10/0x10 > [ 1022.724386][ T2556] kthread+0xfd/0x130 > [ 1022.724709][ T2556] ? __pfx_kthread+0x10/0x10 > [ 1022.725066][ T2556] ret_from_fork+0x2d/0x50 > [ 1022.725409][ T2556] ? __pfx_kthread+0x10/0x10 > [ 1022.725799][ T2556] ret_from_fork_asm+0x1b/0x30 > [ 1022.726201][ T2556] </TASK> > > To fix we add an skb_get() before passing the skb to be enqueued in > the engress queue. This bumps the skb->users refcnt so that consume_skb > and kfree_skb will not immediately free the sk_buff. With this we can > be sure the skb is still around when we do the dequeue. Then we just > need to decrement the refcnt or free the skb in the backlog case which > we do by calling kfree_skb() on the ingress case as well as the sendmsg > case. > > Before locking change from fixes tag we had the sock locked so we > couldn't race with user and there was no issue here. > > Fixes: 799aa7f98d53e (skmsg: Avoid lock_sock() in sk_psock_backlog()) > Reported-by: Jiri Olsa <jolsa@kernel.org> > Signed-off-by: John Fastabend <john.fastabend@gmail.com> > --- > net/core/skmsg.c | 12 ++++++++---- > 1 file changed, 8 insertions(+), 4 deletions(-) > > diff --git a/net/core/skmsg.c b/net/core/skmsg.c > index a0659fc29bcc..6c31eefbd777 100644 > --- a/net/core/skmsg.c > +++ b/net/core/skmsg.c > @@ -612,12 +612,18 @@ static int sk_psock_skb_ingress_self(struct sk_psock *psock, struct sk_buff *skb > static int sk_psock_handle_skb(struct sk_psock *psock, struct sk_buff *skb, > u32 off, u32 len, bool ingress) > { > + int err = 0; > + > if (!ingress) { > if (!sock_writeable(psock->sk)) > return -EAGAIN; > return skb_send_sock(psock->sk, skb, off, len); > } > - return sk_psock_skb_ingress(psock, skb, off, len); > + skb_get(skb); > + err = sk_psock_skb_ingress(psock, skb, off, len); > + if (err < 0) > + kfree_skb(skb); > + return err; > } > > static void sk_psock_skb_state(struct sk_psock *psock, > @@ -685,9 +691,7 @@ static void sk_psock_backlog(struct work_struct *work) > } while (len); > > skb = skb_dequeue(&psock->ingress_skb); > - if (!ingress) { > - kfree_skb(skb); > - } > + kfree_skb(skb); > } > end: > mutex_unlock(&psock->work_mutex); Tested-by: Xu Kuohai <xukuohai@huawei.com>
On Fri, Sep 01, 2023 at 01:21:37PM -0700, John Fastabend wrote: > There is a race where skb's from the sk_psock_backlog can be referenced > after userspace side has already skb_consumed() the sk_buff and its > refcnt dropped to zer0 causing use after free. > > The flow is the following, > > while ((skb = skb_peek(&psock->ingress_skb)) > sk_psock_handle_Skb(psock, skb, ..., ingress) > if (!ingress) ... > sk_psock_skb_ingress > sk_psock_skb_ingress_enqueue(skb) > msg->skb = skb > sk_psock_queue_msg(psock, msg) > skb_dequeue(&psock->ingress_skb) > > The sk_psock_queue_msg() puts the msg on the ingress_msg queue. This is > what the application reads when recvmsg() is called. An application can > read this anytime after the msg is placed on the queue. The recvmsg > hook will also read msg->skb and then after user space reads the msg > will call consume_skb(skb) on it effectively free'ing it. > > But, the race is in above where backlog queue still has a reference to > the skb and calls skb_dequeue(). If the skb_dequeue happens after the > user reads and free's the skb we have a use after free. > > The !ingress case does not suffer from this problem because it uses > sendmsg_*(sk, msg) which does not pass the sk_buff further down the > stack. > > The following splat was observed with 'test_progs -t sockmap_listen': > > [ 1022.710250][ T2556] general protection fault, ... > ... > [ 1022.712830][ T2556] Workqueue: events sk_psock_backlog > [ 1022.713262][ T2556] RIP: 0010:skb_dequeue+0x4c/0x80 > [ 1022.713653][ T2556] Code: ... > ... > [ 1022.720699][ T2556] Call Trace: > [ 1022.720984][ T2556] <TASK> > [ 1022.721254][ T2556] ? die_addr+0x32/0x80^M > [ 1022.721589][ T2556] ? exc_general_protection+0x25a/0x4b0 > [ 1022.722026][ T2556] ? asm_exc_general_protection+0x22/0x30 > [ 1022.722489][ T2556] ? skb_dequeue+0x4c/0x80 > [ 1022.722854][ T2556] sk_psock_backlog+0x27a/0x300 > [ 1022.723243][ T2556] process_one_work+0x2a7/0x5b0 > [ 1022.723633][ T2556] worker_thread+0x4f/0x3a0 > [ 1022.723998][ T2556] ? __pfx_worker_thread+0x10/0x10 > [ 1022.724386][ T2556] kthread+0xfd/0x130 > [ 1022.724709][ T2556] ? __pfx_kthread+0x10/0x10 > [ 1022.725066][ T2556] ret_from_fork+0x2d/0x50 > [ 1022.725409][ T2556] ? __pfx_kthread+0x10/0x10 > [ 1022.725799][ T2556] ret_from_fork_asm+0x1b/0x30 > [ 1022.726201][ T2556] </TASK> > > To fix we add an skb_get() before passing the skb to be enqueued in > the engress queue. This bumps the skb->users refcnt so that consume_skb > and kfree_skb will not immediately free the sk_buff. With this we can > be sure the skb is still around when we do the dequeue. Then we just > need to decrement the refcnt or free the skb in the backlog case which > we do by calling kfree_skb() on the ingress case as well as the sendmsg > case. > > Before locking change from fixes tag we had the sock locked so we > couldn't race with user and there was no issue here. > > Fixes: 799aa7f98d53e (skmsg: Avoid lock_sock() in sk_psock_backlog()) Hi John, A minor nit from my side. I think the usual format for a fixes tag is follows. Fixes: 799aa7f98d53e ("skmsg: Avoid lock_sock() in sk_psock_backlog()") > Reported-by: Jiri Olsa <jolsa@kernel.org> > Signed-off-by: John Fastabend <john.fastabend@gmail.com> ...
On Sat, Sep 02, 2023 at 12:24:01AM +0300, Eduard Zingerman wrote: SNIP > > > static void sk_psock_skb_state(struct sk_psock *psock, > > > @@ -685,9 +691,7 @@ static void sk_psock_backlog(struct work_struct *work) > > > } while (len); > > > > > > skb = skb_dequeue(&psock->ingress_skb); > > > - if (!ingress) { > > > - kfree_skb(skb); > > > - } > > > + kfree_skb(skb); > > > } > > > end: > > > mutex_unlock(&psock->work_mutex); > > > -- > > > 2.33.0 > > > > > > > there's no crash wit with fix, but I noticed I occasionally get FAIL > > > > Please note this patch: > https://lore.kernel.org/bpf/20230901031037.3314007-1-xukuohai@huaweicloud.com/ > Which should fix the test in question. ah right it does, thanks Tested-by: Jiri Olsa <jolsa@kernel.org> jirka > > > #212/78 sockmap_listen/sockmap Unix test_unix_redir:OK > > ./test_progs:vsock_unix_redir_connectible:1501: ingress: write: Transport endpoint is not connected > > vsock_unix_redir_connectible:FAIL:1501 > > ./test_progs:vsock_unix_redir_connectible:1501: egress: write: Transport endpoint is not connected > > vsock_unix_redir_connectible:FAIL:1501 > > #212/79 sockmap_listen/sockmap VSOCK test_vsock_redir:FAIL > > #212/80 sockmap_listen/sockhash IPv4 TCP test_insert_invalid:OK > > > > no idea if it's related > > > > jirka >
Hello: This patch was applied to bpf/bpf.git (master) by Daniel Borkmann <daniel@iogearbox.net>: On Fri, 1 Sep 2023 13:21:37 -0700 you wrote: > There is a race where skb's from the sk_psock_backlog can be referenced > after userspace side has already skb_consumed() the sk_buff and its > refcnt dropped to zer0 causing use after free. > > The flow is the following, > > while ((skb = skb_peek(&psock->ingress_skb)) > sk_psock_handle_Skb(psock, skb, ..., ingress) > if (!ingress) ... > sk_psock_skb_ingress > sk_psock_skb_ingress_enqueue(skb) > msg->skb = skb > sk_psock_queue_msg(psock, msg) > skb_dequeue(&psock->ingress_skb) > > [...] Here is the summary with links: - [bpf] bpf: sockmap, fix skb refcnt race after locking changes https://git.kernel.org/bpf/bpf/c/a454d84ee20b You are awesome, thank you!
diff --git a/net/core/skmsg.c b/net/core/skmsg.c index a0659fc29bcc..6c31eefbd777 100644 --- a/net/core/skmsg.c +++ b/net/core/skmsg.c @@ -612,12 +612,18 @@ static int sk_psock_skb_ingress_self(struct sk_psock *psock, struct sk_buff *skb static int sk_psock_handle_skb(struct sk_psock *psock, struct sk_buff *skb, u32 off, u32 len, bool ingress) { + int err = 0; + if (!ingress) { if (!sock_writeable(psock->sk)) return -EAGAIN; return skb_send_sock(psock->sk, skb, off, len); } - return sk_psock_skb_ingress(psock, skb, off, len); + skb_get(skb); + err = sk_psock_skb_ingress(psock, skb, off, len); + if (err < 0) + kfree_skb(skb); + return err; } static void sk_psock_skb_state(struct sk_psock *psock, @@ -685,9 +691,7 @@ static void sk_psock_backlog(struct work_struct *work) } while (len); skb = skb_dequeue(&psock->ingress_skb); - if (!ingress) { - kfree_skb(skb); - } + kfree_skb(skb); } end: mutex_unlock(&psock->work_mutex);
There is a race where skb's from the sk_psock_backlog can be referenced after userspace side has already skb_consumed() the sk_buff and its refcnt dropped to zer0 causing use after free. The flow is the following, while ((skb = skb_peek(&psock->ingress_skb)) sk_psock_handle_Skb(psock, skb, ..., ingress) if (!ingress) ... sk_psock_skb_ingress sk_psock_skb_ingress_enqueue(skb) msg->skb = skb sk_psock_queue_msg(psock, msg) skb_dequeue(&psock->ingress_skb) The sk_psock_queue_msg() puts the msg on the ingress_msg queue. This is what the application reads when recvmsg() is called. An application can read this anytime after the msg is placed on the queue. The recvmsg hook will also read msg->skb and then after user space reads the msg will call consume_skb(skb) on it effectively free'ing it. But, the race is in above where backlog queue still has a reference to the skb and calls skb_dequeue(). If the skb_dequeue happens after the user reads and free's the skb we have a use after free. The !ingress case does not suffer from this problem because it uses sendmsg_*(sk, msg) which does not pass the sk_buff further down the stack. The following splat was observed with 'test_progs -t sockmap_listen': [ 1022.710250][ T2556] general protection fault, ... ... [ 1022.712830][ T2556] Workqueue: events sk_psock_backlog [ 1022.713262][ T2556] RIP: 0010:skb_dequeue+0x4c/0x80 [ 1022.713653][ T2556] Code: ... ... [ 1022.720699][ T2556] Call Trace: [ 1022.720984][ T2556] <TASK> [ 1022.721254][ T2556] ? die_addr+0x32/0x80^M [ 1022.721589][ T2556] ? exc_general_protection+0x25a/0x4b0 [ 1022.722026][ T2556] ? asm_exc_general_protection+0x22/0x30 [ 1022.722489][ T2556] ? skb_dequeue+0x4c/0x80 [ 1022.722854][ T2556] sk_psock_backlog+0x27a/0x300 [ 1022.723243][ T2556] process_one_work+0x2a7/0x5b0 [ 1022.723633][ T2556] worker_thread+0x4f/0x3a0 [ 1022.723998][ T2556] ? __pfx_worker_thread+0x10/0x10 [ 1022.724386][ T2556] kthread+0xfd/0x130 [ 1022.724709][ T2556] ? __pfx_kthread+0x10/0x10 [ 1022.725066][ T2556] ret_from_fork+0x2d/0x50 [ 1022.725409][ T2556] ? __pfx_kthread+0x10/0x10 [ 1022.725799][ T2556] ret_from_fork_asm+0x1b/0x30 [ 1022.726201][ T2556] </TASK> To fix we add an skb_get() before passing the skb to be enqueued in the engress queue. This bumps the skb->users refcnt so that consume_skb and kfree_skb will not immediately free the sk_buff. With this we can be sure the skb is still around when we do the dequeue. Then we just need to decrement the refcnt or free the skb in the backlog case which we do by calling kfree_skb() on the ingress case as well as the sendmsg case. Before locking change from fixes tag we had the sock locked so we couldn't race with user and there was no issue here. Fixes: 799aa7f98d53e (skmsg: Avoid lock_sock() in sk_psock_backlog()) Reported-by: Jiri Olsa <jolsa@kernel.org> Signed-off-by: John Fastabend <john.fastabend@gmail.com> --- net/core/skmsg.c | 12 ++++++++---- 1 file changed, 8 insertions(+), 4 deletions(-)