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>

1 files changed, 13 insertions, 2 deletions

diff --git a/signal/sigaction.c b/signal/sigaction.c
index 811062ae96..81ae550315 100644
--- a/signal/sigaction.c
+++ b/signal/sigaction.c
@@ -16,8 +16,9 @@
    <https://www.gnu.org/licenses/>.  */
 
 #include <errno.h>
-#include <signal.h>
 #include <internal-signals.h>
+#include <libc-lock.h>
+#include <signal.h>
 
 /* If ACT is not NULL, change the action for SIG to *ACT.
    If OACT is not NULL, put the old action for SIG in *OACT.  */
@@ -30,7 +31,17 @@ __sigaction (int sig, const struct sigaction *act, struct sigaction *oact)
       return -1;
     }
 
-  return __libc_sigaction (sig, act, oact);
+  internal_sigset_t set;
+
+  if (sig == SIGABRT)
+    __abort_lock_wrlock (&set);
+
+  int r = __libc_sigaction (sig, act, oact);
+
+  if (sig == SIGABRT)
+    __abort_lock_unlock (&set);
+
+  return r;
 }
 libc_hidden_def (__sigaction)
 weak_alias (__sigaction, sigaction)