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From: Matt Fleming <matt@codeblueprint.co.uk>
Date: Tue, 7 Apr 2020 10:54:13 +0100
Subject: [PATCH] signal: Prevent double-free of user struct
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/5.10/older/patches-5.10.8-rt24.tar.xz
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>
---
kernel/signal.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -495,8 +495,8 @@ static void sigqueue_free_current(struct
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);
}
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