From 7bddc2ba16a2a15773c2ea8947059afa27727764 Mon Sep 17 00:00:00 2001 From: Alan Coopersmith Date: Mon, 16 Sep 2013 21:47:16 -0700 Subject: [PATCH] Avoid use-after-free in dix/dixfonts.c: doImageText() [CVE-2013-4396] Save a pointer to the passed in closure structure before copying it and overwriting the *c pointer to point to our copy instead of the original. If we hit an error, once we free(c), reset c to point to the original structure before jumping to the cleanup code that references *c. Since one of the errors being checked for is whether the server was able to malloc(c->nChars * itemSize), the client can potentially pass a number of characters chosen to cause the malloc to fail and the error path to be taken, resulting in the read from freed memory. Since the memory is accessed almost immediately afterwards, and the X server is mostly single threaded, the odds of the free memory having invalid contents are low with most malloc implementations when not using memory debugging features, but some allocators will definitely overwrite the memory there, leading to a likely crash. Reported-by: Pedro Ribeiro Signed-off-by: Alan Coopersmith Reviewed-by: Julien Cristau --- dix/dixfonts.c | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/dix/dixfonts.c b/dix/dixfonts.c index feb765d..2e34d37 100644 --- a/dix/dixfonts.c +++ b/dix/dixfonts.c @@ -1425,6 +1425,7 @@ doImageText(ClientPtr client, ITclosurePtr c) GC *pGC; unsigned char *data; ITclosurePtr new_closure; + ITclosurePtr old_closure; /* We're putting the client to sleep. We need to save some state. Similar problem to that handled @@ -1436,12 +1437,14 @@ doImageText(ClientPtr client, ITclosurePtr c) err = BadAlloc; goto bail; } + old_closure = c; *new_closure = *c; c = new_closure; data = malloc(c->nChars * itemSize); if (!data) { free(c); + c = old_closure; err = BadAlloc; goto bail; } @@ -1452,6 +1455,7 @@ doImageText(ClientPtr client, ITclosurePtr c) if (!pGC) { free(c->data); free(c); + c = old_closure; err = BadAlloc; goto bail; } @@ -1464,6 +1468,7 @@ doImageText(ClientPtr client, ITclosurePtr c) FreeScratchGC(pGC); free(c->data); free(c); + c = old_closure; err = BadAlloc; goto bail; } -- 1.7.9.2