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 by exercising kernel/seccomp.c
and verifying the fix with KASAN enabled.
Fixes: 0ae71c7720e3 ("seccomp: Support atomic "addfd + send reply"")
Cc: [email protected]
Reviewed-by: Bradley Morgan <[email protected]>
Assisted-by: LLM
Signed-off-by: Hui Peng <[email protected]>
---
Changes in v4:
- Restore Reviewed-by: Bradley Morgan <[email protected]> and Cc:
[email protected] tags from v1 (and restore Bradley Morgan to Cc),
and add Assisted-by: LLM tag.
- Fix Fixes: tag SHA and subject (0ae71c7720e3 ("seccomp: Support atomic
"addfd + send reply"")).
- Fix truncated word "interrupted" in the Subject line.
Changes in v3:
- Actually include the tools/testing/selftests/seccomp/seccomp_bpf.c
regression test in the patch diff (v2 accidentally omitted the selftest
hunk), with detailed comments explaining the race and test setup.
Changes in v2:
- Add user_notification_addfd_send_interrupted regression test to
tools/testing/selftests/seccomp/seccomp_bpf.c as requested by Kees Cook.
kernel/seccomp.c | 2 +
tools/testing/selftests/seccomp/seccomp_bpf.c | 161 ++++++++++++++++++
2 files changed, 163 insertions(+)
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 86cf4460d69e..94c7ba80a8f7 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -1808,10 +1808,13 @@ static long seccomp_notify_addfd(struct seccomp_filter
*filter,
* We need to check again if the addfd request has been handled,
* and if not, we will remove it from the queue.
*/
- if (list_empty(&kaddfd.list))
+ if (list_empty(&kaddfd.list)) {
ret = kaddfd.ret;
- else
+ } else {
list_del(&kaddfd.list);
+ if (addfd.flags & SECCOMP_ADDFD_FLAG_SEND)
+ knotif->state = SECCOMP_NOTIFY_SENT;
+ }
out_unlock:
mutex_unlock(&filter->notify_lock);
diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c
b/tools/testing/selftests/seccomp/seccomp_bpf.c
index 0622bc2acad4..e565728f9eb2 100644
--- a/tools/testing/selftests/seccomp/seccomp_bpf.c
+++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
@@ -4368,6 +4368,173 @@ TEST(user_notification_addfd_rlimit)
close(memfd);
}
+static void sigusr1_handler(int signo)
+{
+}
+
+/*
+ * Verify that when SECCOMP_IOCTL_NOTIF_ADDFD with SECCOMP_ADDFD_FLAG_SEND is
+ * interrupted by a signal before the tracee dequeues the addfd request,
+ * knotif->state is restored from SECCOMP_NOTIFY_REPLIED back to
+ * SECCOMP_NOTIFY_SENT so that:
+ * 1. The woken tracee sees knotif->state == SECCOMP_NOTIFY_SENT in
+ * do_user_notif() and goes back to sleep instead of prematurely
+ * returning 0 from the trapped syscall without the FD installed.
+ * 2. The supervisor can retry SECCOMP_IOCTL_NOTIF_ADDFD (or
+ * SECCOMP_IOCTL_NOTIF_SEND) instead of failing with -EINPROGRESS.
+ *
+ * To deterministically hit the race window where the supervisor sleeps in
+ * wait_for_completion_interruptible(&kaddfd.completion) after waking the
+ * tracee (complete(&knotif->ready)) but before the tracee runs
+ * seccomp_handle_addfd(), pin all processes to a single CPU and enforce a
+ * strict 3-tier scheduling priority hierarchy on that CPU:
+ * - Supervisor: SCHED_FIFO priority 99 (highest)
+ * - Signal helper (sig_pid): SCHED_FIFO priority 50 (middle)
+ * - Tracee (pid): SCHED_IDLE (lowest)
+ */
+TEST(user_notification_addfd_send_interrupted)
+{
+ /*
+ * Save parent_pid before user_notif_syscall(__NR_getppid, ...) installs
+ * the seccomp filter on the calling process; children inherit that
+ * filter, so sig_pid must not call getppid().
+ */
+ pid_t pid, sig_pid, parent_pid = getpid();
+ long ret;
+ int status, listener, memfd;
+ struct seccomp_notif_addfd addfd = {};
+ struct seccomp_notif req = {};
+ struct sigaction sa = {};
+ struct sched_param sp_tracee_idle = { .sched_priority = 0 };
+ struct sched_param sp_supervisor_fifo = { .sched_priority = 99 };
+ struct sched_param sp_sig_helper_fifo = { .sched_priority = 50 };
+ struct timespec delay = { .tv_nsec = 15000000 };
+ cpu_set_t cpuset;
+ int cpu;
+
+ /* Pin the supervisor (and its future child processes) to one CPU. */
+ cpu = sched_getcpu();
+ if (cpu >= 0) {
+ CPU_ZERO(&cpuset);
+ CPU_SET(cpu, &cpuset);
+ sched_setaffinity(0, sizeof(cpuset), &cpuset);
+ }
+
+ sa.sa_handler = sigusr1_handler;
+ ASSERT_EQ(sigaction(SIGUSR1, &sa, NULL), 0);
+
+ memfd = memfd_create("test", 0);
+ ASSERT_GE(memfd, 0);
+
+ ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
+ ASSERT_EQ(0, ret) {
+ TH_LOG("Kernel does not support PR_SET_NO_NEW_PRIVS!");
+ }
+
+ /*
+ * Follow the convention of other user_notification_* tests in this
+ * file by trapping __NR_getppid: because the filter is installed on
+ * the supervisor before fork(), the trapped syscall must be a
+ * side-effect-free syscall that the supervisor itself never invokes.
+ * Even though getppid() does not normally return an FD,
+ * SECCOMP_ADDFD_FLAG_SEND replaces the trapped syscall's return value
+ * with the newly installed FD number (42).
+ */
+ listener = user_notif_syscall(__NR_getppid,
+ SECCOMP_FILTER_FLAG_NEW_LISTENER);
+ ASSERT_GE(listener, 0);
+
+ pid = fork();
+ ASSERT_GE(pid, 0);
+
+ if (pid == 0) {
+ /*
+ * Tracee: invoke __NR_getppid as a dummy trigger syscall to
+ * trap into do_user_notif(). Verify that the syscall returns
+ * the injected FD number (42) and that FD 42 is open.
+ */
+ ret = syscall(__NR_getppid);
+ exit(ret != 42 || fcntl(42, F_GETFD) < 0);
+ }
+
+ /* Wait for the tracee to trap in do_user_notif(). */
+ ASSERT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
+
+ /*
+ * Demote the tracee to SCHED_IDLE and promote the supervisor to
+ * SCHED_FIFO(99) on the same CPU.
+ */
+ sched_setscheduler(pid, SCHED_IDLE, &sp_tracee_idle);
+ if (sched_setscheduler(0, SCHED_FIFO, &sp_supervisor_fifo) != 0) {
+ close(listener);
+ close(memfd);
+ kill(pid, SIGKILL);
+ waitpid(pid, NULL, 0);
+ SKIP(return, "SCHED_FIFO requires CAP_SYS_NICE");
+ }
+
+ addfd.srcfd = memfd;
+ addfd.newfd_flags = O_CLOEXEC;
+ addfd.newfd = 42;
+ addfd.id = req.id;
+ addfd.flags = SECCOMP_ADDFD_FLAG_SETFD | SECCOMP_ADDFD_FLAG_SEND;
+
+ /*
+ * Fork a signal helper on the same CPU and set it to SCHED_FIFO(50).
+ * Because the supervisor is currently running at SCHED_FIFO(99) on
+ * this CPU, sig_pid is queued on the runqueue but cannot run until the
+ * supervisor blocks inside the kernel.
+ *
+ * When the supervisor invokes ioctl(SECCOMP_IOCTL_NOTIF_ADDFD) below:
+ * 1. seccomp_notify_addfd() sets knotif->state =
SECCOMP_NOTIFY_REPLIED,
+ * wakes the tracee (SCHED_IDLE), and blocks in
+ * wait_for_completion_interruptible(&kaddfd.completion).
+ * 2. The CPU scheduler immediately runs sig_pid (SCHED_FIFO 50)
+ * ahead of the woken tracee (SCHED_IDLE).
+ * 3. sig_pid sends SIGUSR1 to parent_pid, waking the supervisor
+ * (SCHED_FIFO 99), which immediately preempts sig_pid, aborts the
+ * wait with -ERESTARTSYS (-EINTR), removes kaddfd from
+ * knotif->addfd, and restores knotif->state = SECCOMP_NOTIFY_SENT
+ * before the tracee has executed a single instruction.
+ */
+ sig_pid = fork();
+ ASSERT_GE(sig_pid, 0);
+ if (sig_pid == 0) {
+ sched_setscheduler(0, SCHED_FIFO, &sp_sig_helper_fifo);
+ kill(parent_pid, SIGUSR1);
+ _exit(0);
+ }
+
+ EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_ADDFD, &addfd), -1);
+ EXPECT_EQ(errno, EINTR);
+ EXPECT_EQ(waitpid(sig_pid, &status, 0), sig_pid);
+
+ /*
+ * Restore normal scheduling and sleep briefly so the woken tracee
+ * runs in do_user_notif(). With knotif->state restored to
+ * SECCOMP_NOTIFY_SENT, the tracee must loop back to sleep waiting for
+ * the notification reply rather than returning 0 from __NR_getppid.
+ */
+ sched_setscheduler(0, SCHED_OTHER, &sp_tracee_idle);
+ sched_setscheduler(pid, SCHED_OTHER, &sp_tracee_idle);
+ nanosleep(&delay, NULL);
+
+ /*
+ * Retry SECCOMP_IOCTL_NOTIF_ADDFD. Because knotif->state is
+ * SECCOMP_NOTIFY_SENT, the retry succeeds (returns 42) instead of
+ * failing with -EINPROGRESS, installs FD 42 into the tracee, and wakes
+ * the tracee to complete the syscall with return value 42.
+ */
+ EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_ADDFD, &addfd), 42);
+
+ EXPECT_EQ(waitpid(pid, &status, 0), pid);
+ EXPECT_EQ(true, WIFEXITED(status));
+ EXPECT_EQ(0, WEXITSTATUS(status));
+
+ close(listener);
+ close(memfd);
+}
+
#ifndef SECCOMP_USER_NOTIF_FD_SYNC_WAKE_UP
#define SECCOMP_USER_NOTIF_FD_SYNC_WAKE_UP (1UL << 0)
#define SECCOMP_IOCTL_NOTIF_SET_FLAGS SECCOMP_IOW(4, __u64)
--
2.49.0
