fix extremely rare but dangerous race condition in robust mutexes
if new shared mappings of files/devices/shared memory can be made
between the time a robust mutex is unlocked and its subsequent removal
from the pending slot in the robustlist header, the kernel can
inadvertently corrupt data in the newly-mapped pages when the process
terminates. i am fixing the bug by using the same global vm lock
mechanism that was used to fix the race condition with unmapping
barriers after pthread_barrier_wait returns.
diff --git a/src/thread/pthread_barrier_wait.c b/src/thread/pthread_barrier_wait.c
index 6052925..5e60338 100644
--- a/src/thread/pthread_barrier_wait.c
+++ b/src/thread/pthread_barrier_wait.c
@@ -1,22 +1,7 @@
#include "pthread_impl.h"
-static int vmlock[2];
-
-void __vm_lock(int inc)
-{
- for (;;) {
- int v = vmlock[0];
- if (inc*v < 0) __wait(vmlock, vmlock+1, v, 1);
- else if (a_cas(vmlock, v, v+inc)==v) break;
- }
-}
-
-void __vm_unlock(void)
-{
- int inc = vmlock[0]>0 ? -1 : 1;
- if (a_fetch_add(vmlock, inc)==-inc && vmlock[1])
- __wake(vmlock, -1, 1);
-}
+void __vm_lock_impl(int);
+void __vm_unlock_impl(void);
static int pshared_barrier_wait(pthread_barrier_t *b)
{
@@ -41,7 +26,7 @@
__wait(&b->_b_count, &b->_b_waiters2, v, 0);
}
- __vm_lock(+1);
+ __vm_lock_impl(+1);
/* Ensure all threads have a vm lock before proceeding */
if (a_fetch_add(&b->_b_count, -1)==1-limit) {
@@ -62,7 +47,7 @@
if (v==INT_MIN+1 || (v==1 && w))
__wake(&b->_b_lock, 1, 0);
- __vm_unlock();
+ __vm_unlock_impl();
return ret;
}
diff --git a/src/thread/pthread_mutex_unlock.c b/src/thread/pthread_mutex_unlock.c
index fdf9fc1..5fc0f4e 100644
--- a/src/thread/pthread_mutex_unlock.c
+++ b/src/thread/pthread_mutex_unlock.c
@@ -1,5 +1,8 @@
#include "pthread_impl.h"
+void __vm_lock_impl(int);
+void __vm_unlock_impl(void);
+
int pthread_mutex_unlock(pthread_mutex_t *m)
{
pthread_t self;
@@ -20,11 +23,14 @@
self->robust_list.pending = &m->_m_next;
*(void **)m->_m_prev = m->_m_next;
if (m->_m_next) ((void **)m->_m_next)[-1] = m->_m_prev;
+ __vm_lock_impl(+1);
}
}
cont = a_swap(&m->_m_lock, 0);
- if (robust)
+ if (robust) {
self->robust_list.pending = 0;
+ __vm_unlock_impl();
+ }
if (waiters || cont<0)
__wake(&m->_m_lock, 1, 0);
return 0;
diff --git a/src/thread/vmlock.c b/src/thread/vmlock.c
new file mode 100644
index 0000000..aba9e31
--- /dev/null
+++ b/src/thread/vmlock.c
@@ -0,0 +1,22 @@
+#include "pthread_impl.h"
+
+static int vmlock[2];
+
+void __vm_lock(int inc)
+{
+ for (;;) {
+ int v = vmlock[0];
+ if (inc*v < 0) __wait(vmlock, vmlock+1, v, 1);
+ else if (a_cas(vmlock, v, v+inc)==v) break;
+ }
+}
+
+void __vm_unlock(void)
+{
+ int inc = vmlock[0]>0 ? -1 : 1;
+ if (a_fetch_add(vmlock, inc)==-inc && vmlock[1])
+ __wake(vmlock, -1, 1);
+}
+
+weak_alias(__vm_lock, __vm_lock_impl);
+weak_alias(__vm_unlock, __vm_unlock_impl);