[PATCH 1/2] cgroup: fix spurious SIGKILL of CLONE_INTO_CGROUP children

[Etienne Perot <eperot@google.com>]

Since commit b69bb476dee9 ("cgroup: fix race between fork and
cgroup.kill"), the fork path snapshots the kill_seq of the child's
future cgroup into kargs->kill_seq, and cgroup_post_fork() SIGKILLs
the child if that cgroup's kill_seq has changed in the meantime, to
catch forks racing with a cgroup.kill sweep.

For CLONE_INTO_CGROUP, however, the snapshot in cgroup_css_set_fork()
is taken before the target cgroup has been resolved: kargs->cgrp is
always NULL at this point (it is only set at the end of the function).
So the "if (kargs->cgrp)" branch is dead code and the snapshot always
records the kill_seq of the parent's cgroup. cgroup_post_fork() then
compares it with the kill_seq of the target cgroup, so the child gets
SIGKILLed whenever the two cgroups have been killed a different number
of times.

As a result, once cgroup.kill has been written to a cgroup, every
child subsequently cloned into it with clone3(CLONE_INTO_CGROUP) is
killed on the spot, for as long as the cgroup exists: kill_seq is not
exposed to userspace and never resets.

Re-snapshot kill_seq from the target cgroup once it has been resolved,
and drop the dead branch at the early snapshot site.

This does not reopen the race fixed by b69bb476dee9. For
CLONE_INTO_CGROUP, everything from the snapshot to the check in
cgroup_post_fork() runs with cgroup_mutex held, and kill_seq is
only ever incremented under cgroup_mutex.

Fixes: b69bb476dee9 ("cgroup: fix race between fork and cgroup.kill")
Cc: stable@vger.kernel.org
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Assisted-by: LLM
Signed-off-by: Etienne Perot <eperot@google.com>
---
 kernel/cgroup/cgroup.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index c3a12fee7528..2d532bf2c0c7 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -6873,10 +6873,7 @@ static int cgroup_css_set_fork(struct kernel_clone_args *kargs)
 	spin_lock_irq(&css_set_lock);
 	cset = task_css_set(current);
 	get_css_set(cset);
-	if (kargs->cgrp)
-		kargs->kill_seq = kargs->cgrp->kill_seq;
-	else
-		kargs->kill_seq = cset->dfl_cgrp->kill_seq;
+	kargs->kill_seq = cset->dfl_cgrp->kill_seq;
 	spin_unlock_irq(&css_set_lock);
 
 	if (!(kargs->flags & CLONE_INTO_CGROUP)) {
@@ -6940,6 +6937,7 @@ static int cgroup_css_set_fork(struct kernel_clone_args *kargs)
 
 	put_css_set(cset);
 	kargs->cgrp = dst_cgrp;
+	kargs->kill_seq = dst_cgrp->kill_seq;
 	return ret;
 
 err:
-- 
2.55.0.897.gb25b4bd76c-goog