stdlib: Make abort/_Exit AS-safe (BZ 26275)

The recursive lock used on abort does not synchronize with a new process
creation (either by fork-like interfaces or posix_spawn ones), nor it
is reinitialized after fork().

Also, the SIGABRT unblock before raise() shows another race condition,
where a fork or posix_spawn() call by another thread, just after the
recursive lock release and before the SIGABRT signal, might create
programs with a non-expected signal mask.  With the default option
(without POSIX_SPAWN_SETSIGDEF), the process can see SIG_DFL for
SIGABRT, where it should be SIG_IGN.

To fix the AS-safe, raise() does not change the process signal mask,
and an AS-safe lock is used if a SIGABRT is installed or the process
is blocked or ignored.  With the signal mask change removal,
there is no need to use a recursive loc.  The lock is also taken on
both _Fork() and posix_spawn(), to avoid the spawn process to see the
abort handler as SIG_DFL.

A read-write lock is used to avoid serialize _Fork and posix_spawn
execution.  Both sigaction (SIGABRT) and abort() requires to lock
as writer (since both change the disposition).

The fallback is also simplified: there is no need to use a loop of
ABORT_INSTRUCTION after _exit() (if the syscall does not terminate the
process, the system is broken).

The proposed fix changes how setjmp works on a SIGABRT handler, where
glibc does not save the signal mask.  So usage like the below will now
always abort.

  static volatile int chk_fail_ok;
  static jmp_buf chk_fail_buf;

  static void
  handler (int sig)
  {
    if (chk_fail_ok)
      {
        chk_fail_ok = 0;
        longjmp (chk_fail_buf, 1);
      }
    else
      _exit (127);
  }
  [...]
  signal (SIGABRT, handler);
  [....]
  chk_fail_ok = 1;
  if (! setjmp (chk_fail_buf))
    {
      // Something that can calls abort, like a failed fortify function.
      chk_fail_ok = 0;
      printf ("FAIL\n");
    }

Such cases will need to use sigsetjmp instead.

The _dl_start_profile calls sigaction through _profil, and to avoid
pulling abort() on loader the call is replaced with __libc_sigaction.

Checked on x86_64-linux-gnu and aarch64-linux-gnu.

Reviewed-by: DJ Delorie <dj@redhat.com>

3 files changed, 16 insertions, 3 deletions

diff --git a/sysdeps/unix/sysv/linux/internal-signals.h b/sysdeps/unix/sysv/linux/internal-signals.h
index a6fae59aaa..6e3a3d7692 100644
--- a/sysdeps/unix/sysv/linux/internal-signals.h
+++ b/sysdeps/unix/sysv/linux/internal-signals.h
@@ -90,6 +90,15 @@ internal_signal_restore_set (const internal_sigset_t *set)
 			 __NSIG_BYTES);
 }
 
+static inline void
+internal_signal_unblock_signal (int sig)
+{
+  internal_sigset_t set;
+  internal_sigemptyset (&set);
+  internal_sigaddset (&set, sig);
+  INTERNAL_SYSCALL_CALL (rt_sigprocmask, SIG_UNBLOCK, &set, NULL,
+			 __NSIG_BYTES);
+}
 
 /* It is used on timer_create code directly on sigwaitinfo call, so it can not
    use the internal_sigset_t definitions.  */
diff --git a/sysdeps/unix/sysv/linux/internal-sigset.h b/sysdeps/unix/sysv/linux/internal-sigset.h
index 5d7020b42d..4b19affd75 100644
--- a/sysdeps/unix/sysv/linux/internal-sigset.h
+++ b/sysdeps/unix/sysv/linux/internal-sigset.h
@@ -21,7 +21,7 @@
 
 #include <sigsetops.h>
 
-typedef struct
+typedef struct _internal_sigset_t
 {
   unsigned long int __val[__NSIG_WORDS];
 } internal_sigset_t;
diff --git a/sysdeps/unix/sysv/linux/spawni.c b/sysdeps/unix/sysv/linux/spawni.c
index f57e92815e..1edf62c6d7 100644
--- a/sysdeps/unix/sysv/linux/spawni.c
+++ b/sysdeps/unix/sysv/linux/spawni.c
@@ -383,7 +383,11 @@ __spawnix (int *pid, const char *file,
   args.pidfd = 0;
   args.xflags = xflags;
 
-  internal_signal_block_all (&args.oldmask);
+  /* Avoid the potential issues if caller sets a SIG_IGN for SIGABRT, calls
+     abort, and another thread issues posix_spawn just after the sigaction
+     returns.  With default options (not setting POSIX_SPAWN_SETSIGDEF), the
+     process can still see SIG_DFL for SIGABRT, where it should be SIG_IGN.  */
+  __abort_lock_rdlock (&args.oldmask);
 
   /* The clone flags used will create a new child that will run in the same
      memory space (CLONE_VM) and the execution of calling thread will be
@@ -474,7 +478,7 @@ __spawnix (int *pid, const char *file,
   if ((ec == 0) && (pid != NULL))
     *pid = use_pidfd ? args.pidfd : new_pid;
 
-  internal_signal_restore_set (&args.oldmask);
+  __abort_lock_unlock (&args.oldmask);
 
   __pthread_setcancelstate (state, NULL);