linux-kernel - [PATCH RT 1/5] signal: Prevent double-free of user struct

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Date:   Fri, 21 Aug 2020 14:47:00 -0500
From:   zanussi@...nel.org
To:     LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Carsten Emde <C.Emde@...dl.org>,
        John Kacur <jkacur@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Daniel Wagner <wagi@...om.org>,
        Clark Williams <williams@...hat.com>,
        Pavel Machek <pavel@...x.de>, Tom Zanussi <zanussi@...nel.org>
Cc:     Matt Fleming <matt@...eblueprint.co.uk>
Subject: [PATCH RT 1/5] signal: Prevent double-free of user struct

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

v4.19.135-rt61-rc1 stable review patch.
If anyone has any objections, please let me know.

-----------

[ Upsteam commit 9567db2ebe566a93485e1a27d8759969d0002d7a ]

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@...eblueprint.co.uk>
Reported-by: Daniel Wagner <wagi@...om.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: Tom Zanussi <zanussi@...nel.org>
---
 kernel/signal.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 45748993f777..05c9b5a6b3ae 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -488,8 +488,8 @@ 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);
 }
-- 
2.17.1