| Message ID | 20181114180217.195917-1-ebiggers@kernel.org (mailing list archive) |
|---|---|
| State | Superseded, archived |
| Delegated to: | Jiri Kosina |
| Headers | show |
| Series | HID: uhid: prevent uhid_char_write() under KERNEL_DS | expand |
On Wed, Nov 14, 2018 at 10:03 AM Eric Biggers <ebiggers@kernel.org> wrote: > > From: Eric Biggers <ebiggers@google.com> > > When a UHID_CREATE command is written to the uhid char device, a > copy_from_user() is done from a user pointer embedded in the command. > When the address limit is KERNEL_DS, e.g. as is the case during > sendfile(), this can read from kernel memory. Therefore, UHID_CREATE > must not be allowed in this case. Hmm, instead of disallowing access, can we switch back to USER_DS before trying to use the user pointer? > > > For consistency and to make sure all current and future uhid commands > are covered, apply the restriction to uhid_char_write() as a whole > rather than to UHID_CREATE specifically. > > Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to > Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess > helpers fault on kernel addresses"), allowing this bug to be found. > > Reported-by: syzbot+72473edc9bf4eb1c6556@syzkaller.appspotmail.com > Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events") > Cc: <stable@vger.kernel.org> # v3.6+ > Cc: Jann Horn <jannh@google.com> > Signed-off-by: Eric Biggers <ebiggers@google.com> > --- > drivers/hid/uhid.c | 6 ++++++ > 1 file changed, 6 insertions(+) > > diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c > index 3c55073136064..e94c5e248b56e 100644 > --- a/drivers/hid/uhid.c > +++ b/drivers/hid/uhid.c > @@ -705,6 +705,12 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer, > int ret; > size_t len; > > + if (uaccess_kernel()) { /* payload may contain a __user pointer */ > + pr_err_once("%s: process %d (%s) called from kernel context, this is not allowed.\n", > + __func__, task_tgid_vnr(current), current->comm); > + return -EACCES; > + } > + > /* we need at least the "type" member of uhid_event */ > if (count < sizeof(__u32)) > return -EINVAL; > -- > 2.19.1.930.g4563a0d9d0-goog >
+cc Andy On Wed, Nov 14, 2018 at 7:03 PM Eric Biggers <ebiggers@kernel.org> wrote: > When a UHID_CREATE command is written to the uhid char device, a > copy_from_user() is done from a user pointer embedded in the command. > When the address limit is KERNEL_DS, e.g. as is the case during > sendfile(), this can read from kernel memory. Therefore, UHID_CREATE > must not be allowed in this case. > > For consistency and to make sure all current and future uhid commands > are covered, apply the restriction to uhid_char_write() as a whole > rather than to UHID_CREATE specifically. > > Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to > Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess > helpers fault on kernel addresses"), allowing this bug to be found. > > Reported-by: syzbot+72473edc9bf4eb1c6556@syzkaller.appspotmail.com Wheeeee, it found something! :) > Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events") > Cc: <stable@vger.kernel.org> # v3.6+ > Cc: Jann Horn <jannh@google.com> > Signed-off-by: Eric Biggers <ebiggers@google.com> > --- > drivers/hid/uhid.c | 6 ++++++ > 1 file changed, 6 insertions(+) > > diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c > index 3c55073136064..e94c5e248b56e 100644 > --- a/drivers/hid/uhid.c > +++ b/drivers/hid/uhid.c > @@ -705,6 +705,12 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer, > int ret; > size_t len; > > + if (uaccess_kernel()) { /* payload may contain a __user pointer */ > + pr_err_once("%s: process %d (%s) called from kernel context, this is not allowed.\n", > + __func__, task_tgid_vnr(current), current->comm); > + return -EACCES; > + } If this file can conceivably be opened by a process that doesn't have root privileges, this check should be something along the lines of ib_safe_file_access() or sg_check_file_access(). Checking for uaccess_kernel() prevents the symptom that syzkaller notices - a user being able to cause a kernel memory access -, but it doesn't deal with the case where a user opens a file descriptor to this device and tricks a more privileged process into writing into it (e.g. by passing it to a suid binary as stdout or stderr). Looking closer, I wonder whether this kind of behavior is limited to the UHID_CREATE request, which has a comment on it saying "/* Obsolete! Use UHID_CREATE2. */". If we could keep this kind of ugly kludge away from the code paths you're supposed to be using, that would be nice...
On Wed, Nov 14, 2018 at 07:18:39PM +0100, 'Jann Horn' via syzkaller-bugs wrote: > +cc Andy > > On Wed, Nov 14, 2018 at 7:03 PM Eric Biggers <ebiggers@kernel.org> wrote: > > When a UHID_CREATE command is written to the uhid char device, a > > copy_from_user() is done from a user pointer embedded in the command. > > When the address limit is KERNEL_DS, e.g. as is the case during > > sendfile(), this can read from kernel memory. Therefore, UHID_CREATE > > must not be allowed in this case. > > > > For consistency and to make sure all current and future uhid commands > > are covered, apply the restriction to uhid_char_write() as a whole > > rather than to UHID_CREATE specifically. > > > > Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to > > Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess > > helpers fault on kernel addresses"), allowing this bug to be found. > > > > Reported-by: syzbot+72473edc9bf4eb1c6556@syzkaller.appspotmail.com > > Wheeeee, it found something! :) > > > Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events") > > Cc: <stable@vger.kernel.org> # v3.6+ > > Cc: Jann Horn <jannh@google.com> > > Signed-off-by: Eric Biggers <ebiggers@google.com> > > --- > > drivers/hid/uhid.c | 6 ++++++ > > 1 file changed, 6 insertions(+) > > > > diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c > > index 3c55073136064..e94c5e248b56e 100644 > > --- a/drivers/hid/uhid.c > > +++ b/drivers/hid/uhid.c > > @@ -705,6 +705,12 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer, > > int ret; > > size_t len; > > > > + if (uaccess_kernel()) { /* payload may contain a __user pointer */ > > + pr_err_once("%s: process %d (%s) called from kernel context, this is not allowed.\n", > > + __func__, task_tgid_vnr(current), current->comm); > > + return -EACCES; > > + } > > If this file can conceivably be opened by a process that doesn't have > root privileges, this check should be something along the lines of > ib_safe_file_access() or sg_check_file_access(). > > Checking for uaccess_kernel() prevents the symptom that syzkaller > notices - a user being able to cause a kernel memory access -, but it > doesn't deal with the case where a user opens a file descriptor to > this device and tricks a more privileged process into writing into it > (e.g. by passing it to a suid binary as stdout or stderr). > Yep, I'll do that. > Looking closer, I wonder whether this kind of behavior is limited to > the UHID_CREATE request, which has a comment on it saying "/* > Obsolete! Use UHID_CREATE2. */". If we could keep this kind of ugly > kludge away from the code paths you're supposed to be using, that > would be nice... > I wanted to be careful, but yes AFAICS it can be limited to UHID_CREATE only, so I'll do that instead. - Eric
diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c index 3c55073136064..e94c5e248b56e 100644 --- a/drivers/hid/uhid.c +++ b/drivers/hid/uhid.c @@ -705,6 +705,12 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer, int ret; size_t len; + if (uaccess_kernel()) { /* payload may contain a __user pointer */ + pr_err_once("%s: process %d (%s) called from kernel context, this is not allowed.\n", + __func__, task_tgid_vnr(current), current->comm); + return -EACCES; + } + /* we need at least the "type" member of uhid_event */ if (count < sizeof(__u32)) return -EINVAL;