[PATCH] librt: fix NULL pointer dereference (bug 28213)

["Никита Попов via Libc-alpha" <libc-alpha@sourceware.org>]

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From c69f990e356dd8e756b0025e026d59db5af6e059 Mon Sep 17 00:00:00 2001
From: Nikita Popov <npv1310@gmail.com>
Date: Mon, 9 Aug 2021 17:15:52 +0500
Subject: [PATCH] librt: fix NULL pointer dereference (bug 28213)
To: libc-alpha@sourceware.org

Helper thread frees copied attribute on NOTIFY_REMOVED message received from the OS kernel. Unfortunately, it fails to check whether copied attribute actually exists (data.attr != NULL). This worked earlier because free() checks passed pointer before actually attempting to release corresponding memory. But __pthread_attr_destroy assumes pointer is not NULL. So passing NULL pointer to __pthread_attr_destroy will result in segmentation fault. This scenario is possible if notification->sigev_notify_attributes == NULL (which means default thread attributes should be used).
---
 sysdeps/unix/sysv/linux/mq_notify.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sysdeps/unix/sysv/linux/mq_notify.c b/sysdeps/unix/sysv/linux/mq_notify.c
index 9799dcdaa4..eccae2e4c6 100644
--- a/sysdeps/unix/sysv/linux/mq_notify.c
+++ b/sysdeps/unix/sysv/linux/mq_notify.c
@@ -131,7 +131,7 @@ helper_thread (void *arg)
 	       to wait until it is done with it.  */
 	    (void) __pthread_barrier_wait (&notify_barrier);
 	}
-      else if (data.raw[NOTIFY_COOKIE_LEN - 1] == NOTIFY_REMOVED)
+      else if (data.raw[NOTIFY_COOKIE_LEN - 1] == NOTIFY_REMOVED && data.attr != NULL)
 	{
 	  /* The only state we keep is the copy of the thread attributes.  */
 	  __pthread_attr_destroy (data.attr);
-- 
2.17.1