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Subject[PATCH RT 1/6] signal: Prevent double-free of user struct
5.4.54-rt33-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: Matt Fleming <matt@codeblueprint.co.uk>

The way user struct reference counting works changed significantly with,

fda31c50292a ("signal: avoid double atomic counter increments for user accounting")

Now user structs are only freed once the last pending signal is
dequeued. Make sigqueue_free_current() follow this new convention to
avoid freeing the user struct multiple times and triggering this
warning:

refcount_t: underflow; use-after-free.
WARNING: CPU: 0 PID: 6794 at lib/refcount.c:288 refcount_dec_not_one+0x45/0x50
Call Trace:
refcount_dec_and_lock_irqsave+0x16/0x60
free_uid+0x31/0xa0
__dequeue_signal+0x17c/0x190
dequeue_signal+0x5a/0x1b0
do_sigtimedwait+0x208/0x250
__x64_sys_rt_sigtimedwait+0x6f/0xd0
do_syscall_64+0x72/0x200
entry_SYSCALL_64_after_hwframe+0x49/0xbe

Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
Reported-by: Daniel Wagner <wagi@monom.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
kernel/signal.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index def2e8e37f1f..aa924f0141cf 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -494,8 +494,8 @@ static void sigqueue_free_current(struct sigqueue *q)

up = q->user;
if (rt_prio(current->normal_prio) && !put_task_cache(current, q)) {
- atomic_dec(&up->sigpending);
- free_uid(up);
+ if (atomic_dec_and_test(&up->sigpending))
+ free_uid(up);
} else
__sigqueue_free(q);
}
--
2.28.0

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