stdlib: Make abort AS-safe (BZ 26275) azanella/bz26275-abort-as

The recursive lock used on abort does not synchronize with 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 raise might create
programs with a non-expected signal mask.

To fix the AS-safe, the raise is issues without changing the process
signal mask, and an AS-safe lock is used if a SIGABRT is installed or
the process is blocked or ignored.  The the signal mask change removal,
there is no need to use a recursive lock.  The lock is also on
both _Fork and posix_spawn, to avoid the spawn process to see the
abort handler as SIG_DFL.

The posix_spawn possible issue is 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.

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

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

3 files changed, 14 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 e8ed2babb9..5b53924dff 100644
--- a/sysdeps/unix/sysv/linux/spawni.c
+++ b/sysdeps/unix/sysv/linux/spawni.c
@@ -383,7 +383,9 @@ __spawnix (int *pid, const char *file,
   args.pidfd = 0;
   args.xflags = xflags;
 
-  internal_signal_block_all (&args.oldmask);
+  /* Avoid the abort to change the SIGABRT disposition to SIG_DFL for the
+     case POSIX_SPAWN_SETSIGDEF is not set and SIG_IGN is current handle.  */
+  __abort_lock_lock (&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
@@ -465,7 +467,7 @@ __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);