On Tue, Apr 13, 2021 at 06:01:51PM +0200, Rodrigo Campos wrote:
It is possible for the thread with the seccomp filter attached (target) to be waken up by an addfd message, but the list be empty. This happens when the addfd ioctl on the other side (seccomp agent) is interrupted by a signal such as SIGURG. In that case, the target erroneously and prematurely returns from the syscall to userspace even though the seccomp agent didn't ask for it.
This happens in the following scenario:
seccomp_notify_addfd() | seccomp_do_user_notification() | | err = wait_for_completion_interruptible(&n.ready); complete(&knotif->ready); | ret = wait_for_completion_interruptible(&kaddfd.completion); | // interrupted | | mutex_lock(&filter->notify_lock); | list_del(&kaddfd.list); | mutex_unlock(&filter->notify_lock); | | mutex_lock(&match->notify_lock); | // This is false, addfd is false | if (addfd && n.state != SECCOMP_NOTIFY_REPLIED) | | ret = n.val; | err = n.error; | flags = n.flags;
So, the process blocked in seccomp_do_user_notification() will see a response. As n is 0 initialized and wasn't set, it will see a 0 as return value from the syscall.
The seccomp agent, when retrying the interrupted syscall, will see an ENOENT error as the notification no longer exists (it was already answered by this bug).
This patch fixes the issue by splitting the if in two parts: if we were woken up and the state is not replied, we will always do a "goto wait". And if that happens and there is an addfd element on the list, we will add the fd before "goto wait".
This issue is present since 5.9, when addfd was added.
Fixes: 7cf97b1254550 Cc: stable@vger.kernel.org # 5.9+ Signed-off-by: Rodrigo Campos rodrigo@kinvolk.io
So the agent will see the return value from wait_for_completion_interruptible() and know that the addfd wasn't successful and the target will notice that no addfd request has actually been added and essentially try again. Seems like a decent fix and can be backported cleanly. I assume seccomp testsuite passes.
Acked-by: Christian Brauner christian.brauner@ubuntu.com