seccomp: restore knotif->state when SECCOMP_ADDFD_FLAG_SEND is interrupted

In seccomp_notify_addfd(), when SECCOMP_ADDFD_FLAG_SEND is set in
addfd.flags, knotif->state is optimistically transitioned from
SECCOMP_NOTIFY_SENT to SECCOMP_NOTIFY_REPLIED before waking the tracee
and waiting on kaddfd.completion:

	if (addfd.flags & SECCOMP_ADDFD_FLAG_SEND) {
		knotif->state = SECCOMP_NOTIFY_REPLIED;
		...
	}

If wait_for_completion_interruptible(&kaddfd.completion) is interrupted
by a signal before the tracee dequeues the kaddfd request
(!list_empty(&kaddfd.list)), seccomp_notify_addfd() removes kaddfd from
knotif->addfd and returns -EINTR to the supervisor without having
installed the file descriptor.

However, knotif->state is left as SECCOMP_NOTIFY_REPLIED (with
knotif->error == 0 and knotif->val == 0). This causes two problems:

1. When the tracee runs in do_user_notif(), it checks
   if (knotif.state != SECCOMP_NOTIFY_REPLIED) after calling
   seccomp_handle_addfd(). Because knotif->state is already
   SECCOMP_NOTIFY_REPLIED, the tracee exits the notification wait loop
   prematurely and returns 0 from the trapped syscall without the file
   descriptor ever having been installed.

2. If the supervisor retries SECCOMP_IOCTL_NOTIF_ADDFD or
   SECCOMP_IOCTL_NOTIF_SEND before the tracee runs, it fails with
   -EINPROGRESS because knotif->state is no longer SECCOMP_NOTIFY_SENT.

Fix this by restoring knotif->state back to SECCOMP_NOTIFY_SENT if
SECCOMP_ADDFD_FLAG_SEND was set and kaddfd was not consumed before the
interrupted wait. Also add a seccomp_bpf selftest
(user_notification_addfd_send_interrupted) covering this race.

Tested in QEMU against Linux 7.3.0-rc3 using the
user_notification_addfd_send_interrupted selftest added in this patch
(./seccomp_bpf -t user_notification_addfd_send_interrupted): on the
unfixed kernel the test fails because the retry ioctl(listener,
SECCOMP_IOCTL_NOTIF_ADDFD, &addfd) returns -EINPROGRESS (-1) and the
tracee prematurely exits do_user_notif() with return value 0 instead of
FD 42, whereas with this patch applied the test passes.

Fixes: 0ae71c7720e3 ("seccomp: Support atomic "addfd + send reply"")
Cc: stable@vger.kernel.org
Reviewed-by: Bradley Morgan <brads@mainlining.org>
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@gmail.com>
Link: https://patch.msgid.link/20260922023204.2504832-1-benquike@gmail.com
Signed-off-by: Kees Cook <kees@kernel.org>

0 files changed, 0 insertions, 0 deletions