Fix WaitEventSetWait() buffer overrun.

macdice · macdice · commit f1821b58fc8d · 2023-01-13T11:02:12.000+13:00
The WAIT_USE_EPOLL and WAIT_USE_KQUEUE implementations of WaitEventSetWaitBlock() confused the size of their internal buffer with the size of the caller's output buffer, and could ask the kernel for too many events. In fact the set of events retrieved from the kernel needs to be able to fit in both buffers, so take the smaller of the two. The WAIT_USE_POLL and WAIT_USE WIN32 implementations didn't have this confusion. This probably didn't come up before because we always used the same number in both places, but commit 7389aad calculates a dynamic size at construction time, while using MAXLISTEN for its output event buffer on the stack. That seems like a reasonable thing to want to do, so consider this to be a pre-existing bug worth fixing. As discovered by valgrind on skink. Back-patch to all supported releases for epoll, and to release 13 for the kqueue part, which copied the incorrect epoll code. Reviewed-by: Andres Freund <andres@anarazel.de> Discussion: https://postgr.es/m/901504.1673504836%40sss.pgh.pa.us
diff --git a/src/backend/storage/ipc/latch.c b/src/backend/storage/ipc/latch.c
@@ -1525,7 +1525,7 @@ WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
 
 	/* Sleep */
 	rc = epoll_wait(set->epoll_fd, set->epoll_ret_events,
-					nevents, cur_timeout);
+					Min(nevents, set->nevents_space), cur_timeout);
 
 	/* Check return code */
 	if (rc < 0)
@@ -1685,7 +1685,8 @@ WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
 
 	/* Sleep */
 	rc = kevent(set->kqueue_fd, NULL, 0,
-				set->kqueue_ret_events, nevents,
+				set->kqueue_ret_events,
+				Min(nevents, set->nevents_space),
 				timeout_p);
 
 	/* Check return code */
