Re: [musl] Protect pthreads' mutexes against use-after-destroy

[Rich Felker <dalias@libc.org>]

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On Tue, Jan 09, 2024 at 02:07:26PM -0500, Rich Felker wrote:
> On Tue, Jan 09, 2024 at 03:37:17PM +0100, jvoisin wrote:
> > Ohai,
> > 
> > as discussed on irc, Android's bionic has a check to prevent
> > use-after-destroy on phtread mutexes
> > (https://github.com/LineageOS/android_bionic/blob/e0aac7df6f58138dae903b5d456c947a3f8092ea/libc/bionic/pthread_mutex.cpp#L803),
> > and musl doesn't.
> > 
> > While odds are that this is a super-duper common bug, it would still be
> > nice to have this kind of protection, since it's cheap, and would
> > prevent/make it easy to diagnose weird states.
> > 
> > Is this something that should/could be implemented?
> > 
> > o/
> 
> I think you meant that the odds are it's not common. There's already
> enough complexity in the code paths for supporting all the different
> mutex types that my leaning would be, if we do any hardening for
> use-after-destroy, that it should probably just take the form of
> putting the object in a state that will naturally deadlock or error
> rather than adding extra checks to every path where it's used.
> 
> If OTOH we do want it to actually trap in all cases where it's used
> after destroy, the simplest way to achieve that is probably to set it
> up as a non-robust non-PI recursive or errorchecking mutex with
> invalid prev/next pointers and owner of 0x3fffffff. Then the only
> place that would actually have to have an explicit trap is trylock in
> the code path:
> 
>         if (own == 0x3fffffff) return ENOTRECOVERABLE;
> 
> where it could trap if type isn't robust. The unlock code path would
> trap on accessing invalid prev/next pointers.

Draft attached in case anyone wants to play with it. This could
probably be something we could consider to adopt.

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diff --git a/src/thread/pthread_mutex_destroy.c b/src/thread/pthread_mutex_destroy.c
index 8d1bf77b..c56e9fbd 100644
--- a/src/thread/pthread_mutex_destroy.c
+++ b/src/thread/pthread_mutex_destroy.c
@@ -6,5 +6,13 @@ int pthread_mutex_destroy(pthread_mutex_t *mutex)
 	 * type (tracking ownership), it might be in the pending slot of a
 	 * robust_list; wait for quiescence. */
 	if (mutex->_m_type > 128) __vm_wait();
+
+	/* Setup a non-robust errorchecking mutex in ownerdead state so
+	 * use after destruction can be trapped. */
+	mutex->_m_type = 3;
+	mutex->_m_prev = mutex->_m_next = 0;
+	mutex->_m_lock = 0x7fffffff;
+	mutex->_m_count = 0;
+
 	return 0;
 }
diff --git a/src/thread/pthread_mutex_trylock.c b/src/thread/pthread_mutex_trylock.c
index a24e7c58..8d6102cb 100644
--- a/src/thread/pthread_mutex_trylock.c
+++ b/src/thread/pthread_mutex_trylock.c
@@ -21,7 +21,11 @@ int __pthread_mutex_trylock_owner(pthread_mutex_t *m)
 			return 0;
 		}
 	}
-	if (own == 0x3fffffff) return ENOTRECOVERABLE;
+	if (own == 0x3fffffff) {
+		/* Catch use-after-destroy */
+		if (!(type & 8)) a_crash();
+		return ENOTRECOVERABLE;
+	}
 	if (own || (old && !(type & 4))) return EBUSY;
 
 	if (type & 128) {
diff --git a/src/thread/pthread_mutex_unlock.c b/src/thread/pthread_mutex_unlock.c
index b66423e6..fe159804 100644
--- a/src/thread/pthread_mutex_unlock.c
+++ b/src/thread/pthread_mutex_unlock.c
@@ -14,8 +14,11 @@ int __pthread_mutex_unlock(pthread_mutex_t *m)
 		self = __pthread_self();
 		old = m->_m_lock;
 		int own = old & 0x3fffffff;
-		if (own != self->tid)
+		if (own != self->tid) {
+			/* Catch use-after-destroy */
+			if (own == 0x3fffffff && !(type & 8)) a_crash();
 			return EPERM;
+		}
 		if ((type&3) == PTHREAD_MUTEX_RECURSIVE && m->_m_count)
 			return m->_m_count--, 0;
 		if ((type&4) && (old&0x40000000))