Commit 0a36bad017 ("release_task: kill the no longer needed
get/put_pid(thread_pid)") removed the reference around proc_flush_pid().
It assumed that free_pids(post.pids) at the end of release_task() would
keep thread_pid alive until then.
That assumption is wrong. __change_pid() only records a detached PID in
post.pids when pid_has_task() is false for every PIDTYPE. If another task
still uses the exiting task's PID as its process group or session ID,
__unhash_process() removes the exiting task's PIDTYPE_PID link but leaves
the PID out of post.pids. release_task() therefore holds no reference to
it after dropping tasklist_lock.
The other task can then remove the remaining PIDTYPE links. Its
free_pids() call schedules delayed_put_pid(), and the RCU callback can free
the PID before the first release_task() reaches proc_flush_pid().
An unprivileged reproducer races wait4(-1) against setsid() to trigger this
ordering. Three of three fresh v7.2 KASAN boots reported:
BUG: KASAN: slab-use-after-free in
proc_invalidate_siblings_dcache+0x3e2/0x3f0
Read of size 8 by task h7_pid_reaper/1921
Call Trace:
proc_invalidate_siblings_dcache
release_task
wait_consider_task
__do_wait
do_wait
kernel_wait4
Freed by task 0:
kmem_cache_free
put_pid
delayed_put_pid
rcu_core
Last potentially related work creation:
__call_rcu_common
free_pids
ksys_setsid
KASAN identified a 144-byte object from the pid cache and located the bad
read 80 bytes into the freed object, matching pid->inodes. With an
explicit reference, three of three fresh boots completed without a KASAN
report. The concurrent RCU callback dropped its reference while
proc_flush_pid() was protected, and the balancing put_pid() performed the
final free afterward.
Take a reference before __unhash_process() clears p->thread_pid and release
it after proc_flush_pid() completes.
A tested source reproducer is available privately on request. No
controlled read or write, information leak, or privilege escalation is
claimed. The mainline patch applies directly to v6.19.y and newer;
v6.16.y through v6.18.y need a context-adjusted backport.
Fixes: 0a36bad017 ("release_task: kill the no longer needed get/put_pid(thread_pid)")
Reported-by: syzbot+0aee5e8066eddbbe7397@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=0aee5e8066eddbbe7397
Reported-by: syzbot+e8b3520b53e78e90034e@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?extid=e8b3520b53e78e90034e
Cc: stable@vger.kernel.org # see patch description, needs adjustments for 6.16.y-6.18.y
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
Link: https://patch.msgid.link/20260831001221.3755948-1-4ncienth@gmail.com
Acked-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Bradley Morgan <brads@mainlining.org>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
cdd812d068