commit 79474303223c5665bec75ffbdb2a86ee04a2514b
Author: Nikita Popov <npv1310@gmail.com>
Date:   Mon Aug 9 20:17:34 2021 +0530

    librt: fix NULL pointer dereference (bug 28213)
    
    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).
    
    Signed-off-by: Nikita Popov <npv1310@gmail.com>
    Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
    (cherry picked from commit b805aebd42364fe696e417808a700fdb9800c9e8)

diff --git a/sysdeps/unix/sysv/linux/mq_notify.c b/sysdeps/unix/sysv/linux/mq_notify.c
index 9799dcdaa479a1d5..eccae2e4c6cdfefa 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);