Issue #8020: Avoid a crash where the small objects allocator would read
non-Python managed memory while it is being modified by another thread.
Patch by Matt Bandy.
diff --git a/Objects/obmalloc.c b/Objects/obmalloc.c
index bbdc946..3916262 100644
--- a/Objects/obmalloc.c
+++ b/Objects/obmalloc.c
@@ -682,11 +682,19 @@
obmalloc in a small constant time, independent of the number of arenas
obmalloc controls. Since this test is needed at every entry point, it's
extremely desirable that it be this fast.
+
+Since Py_ADDRESS_IN_RANGE may be reading from memory which was not allocated
+by Python, it is important that (POOL)->arenaindex is read only once, as
+another thread may be concurrently modifying the value without holding the
+GIL. To accomplish this, the arenaindex_temp variable is used to store
+(POOL)->arenaindex for the duration of the Py_ADDRESS_IN_RANGE macro's
+execution. The caller of the macro is responsible for declaring this
+variable.
*/
#define Py_ADDRESS_IN_RANGE(P, POOL) \
- ((POOL)->arenaindex < maxarenas && \
- (uptr)(P) - arenas[(POOL)->arenaindex].address < (uptr)ARENA_SIZE && \
- arenas[(POOL)->arenaindex].address != 0)
+ ((arenaindex_temp = (POOL)->arenaindex) < maxarenas && \
+ (uptr)(P) - arenas[arenaindex_temp].address < (uptr)ARENA_SIZE && \
+ arenas[arenaindex_temp].address != 0)
/* This is only useful when running memory debuggers such as
@@ -945,6 +953,9 @@
block *lastfree;
poolp next, prev;
uint size;
+#ifndef Py_USING_MEMORY_DEBUGGER
+ uint arenaindex_temp;
+#endif
if (p == NULL) /* free(NULL) has no effect */
return;
@@ -1167,6 +1178,9 @@
void *bp;
poolp pool;
size_t size;
+#ifndef Py_USING_MEMORY_DEBUGGER
+ uint arenaindex_temp;
+#endif
if (p == NULL)
return PyObject_Malloc(nbytes);
@@ -1867,8 +1881,10 @@
int
Py_ADDRESS_IN_RANGE(void *P, poolp pool)
{
- return pool->arenaindex < maxarenas &&
- (uptr)P - arenas[pool->arenaindex].address < (uptr)ARENA_SIZE &&
- arenas[pool->arenaindex].address != 0;
+ uint arenaindex_temp = pool->arenaindex;
+
+ return arenaindex_temp < maxarenas &&
+ (uptr)P - arenas[arenaindex_temp].address < (uptr)ARENA_SIZE &&
+ arenas[arenaindex_temp].address != 0;
}
#endif