commit 33adeaa3e2b9143c38884bc5aa65ded222ed274e
Author: Florian Weimer <fweimer@redhat.com>
Date:   Thu Sep 23 09:55:54 2021 +0200
 
    nptl: Avoid setxid deadlock with blocked signals in thread exit [BZ #28361]
    
    As part of the fix for bug 12889, signals are blocked during
    thread exit, so that application code cannot run on the thread that
    is about to exit.  This would cause problems if the application
    expected signals to be delivered after the signal handler revealed
    the thread to still exist, despite pthread_kill can no longer be used
    to send signals to it.  However, glibc internally uses the SIGSETXID
    signal in a way that is incompatible with signal blocking, due to the
    way the setxid handshake delays thread exit until the setxid operation
    has completed.  With a blocked SIGSETXID, the handshake can never
    complete, causing a deadlock.
    
    As a band-aid, restore the previous handshake protocol by not blocking
    SIGSETXID during thread exit.
    
    The new test sysdeps/pthread/tst-pthread-setuid-loop.c is based on
    a downstream test by Martin Osvald.
    
    Reviewed-by: Carlos O'Donell <carlos@redhat.com>
    Tested-by: Carlos O'Donell <carlos@redhat.com>
    (cherry picked from commit 2849e2f53311b66853cb5159b64cba2bddbfb854)
 
diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
index 33b426fc682300dc..bc213f0bc4e948bd 100644
--- a/nptl/pthread_create.c
+++ b/nptl/pthread_create.c
@@ -488,8 +488,16 @@ start_thread (void *arg)
 
   /* This prevents sending a signal from this thread to itself during
      its final stages.  This must come after the exit call above
-     because atexit handlers must not run with signals blocked.  */
-  __libc_signal_block_all (NULL);
+     because atexit handlers must not run with signals blocked.
+
+     Do not block SIGSETXID.  The setxid handshake below expects the
+     signal to be delivered.  (SIGSETXID cannot run application code,
+     nor does it use pthread_kill.)  Reuse the pd->sigmask space for
+     computing the signal mask, to save stack space.  */
+  __sigfillset (&pd->sigmask);
+  __sigdelset (&pd->sigmask, SIGSETXID);
+  INTERNAL_SYSCALL_CALL (rt_sigprocmask, SIG_BLOCK, &pd->sigmask, NULL,
+			 __NSIG_BYTES);
 
   /* Tell __pthread_kill_internal that this thread is about to exit.
      If there is a __pthread_kill_internal in progress, this delays
diff --git a/sysdeps/pthread/Makefile b/sysdeps/pthread/Makefile
index 48dba717a1cdc20a..d4bd2d4e3ee6a496 100644
--- a/sysdeps/pthread/Makefile
+++ b/sysdeps/pthread/Makefile
@@ -118,6 +118,7 @@ tests += tst-cnd-basic tst-mtx-trylock tst-cnd-broadcast \
 	 tst-unload \
 	 tst-unwind-thread \
 	 tst-pt-vfork1 tst-pt-vfork2 tst-vfork1x tst-vfork2x \
+	 tst-pthread-setuid-loop \
 	 tst-pthread_cancel-exited \
 	 tst-pthread_cancel-select-loop \
 	 tst-pthread_kill-exited \
diff --git a/sysdeps/pthread/tst-pthread-setuid-loop.c b/sysdeps/pthread/tst-pthread-setuid-loop.c
new file mode 100644
index 0000000000000000..fda2a49b7f0ccf81
--- /dev/null
+++ b/sysdeps/pthread/tst-pthread-setuid-loop.c
@@ -0,0 +1,61 @@
+/* Test that setuid, pthread_create, thread exit do not deadlock (bug 28361).
+   Copyright (C) 2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <support/check.h>
+#include <support/xthread.h>
+#include <unistd.h>
+
+/* How many threads to launch during each iteration.  */
+enum { threads = 4 };
+
+/* How many iterations to perform.  This value seems to reproduce
+   bug 28361 in a bout one in three runs.  */
+enum { iterations = 5000 };
+
+/* Cache of the real user ID used by setuid_thread.  */
+static uid_t uid;
+
+/* Start routine for the threads.  */
+static void *
+setuid_thread (void *closure)
+{
+  TEST_COMPARE (setuid (uid), 0);
+  return NULL;
+}
+
+static int
+do_test (void)
+{
+  /* The setxid machinery is still invoked even if the UID is
+     unchanged.  (The kernel might reset other credentials as part of
+     the system call.)  */
+  uid = getuid ();
+
+  for (int i = 0; i < iterations; ++i)
+    {
+      pthread_t thread_ids[threads];
+      for (int j = 0; j < threads; ++j)
+        thread_ids[j] = xpthread_create (NULL, setuid_thread, NULL);
+      for (int j = 0; j < threads; ++j)
+        xpthread_join (thread_ids[j]);
+    }
+
+  return 0;
+}
+
+#include <support/test-driver.c>