Avoid use-after-free in dix/dixfonts.c: doImageText() [CVE-2013-4396] from…

Avoid use-after-free in dix/dixfonts.c: doImageText() [CVE-2013-4396] from xorg/Xserver http://lists.x.org/archives/xorg-announce/2013-October/002332.html 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.

Avoid use-after-free in dix/dixfonts.c: doImageText() [CVE-2013-4396] from…
72790a55 · Mike DePaulo · Mike Gabriel · df4a3b72 · 72790a55
Commit 72790a55 authored Feb 08, 2015 by Mike DePaulo Committed by Mike Gabriel Feb 14, 2015
Hide whitespace changes
Inline Side-by-side

Showing with 5 additions and 0 deletions

dixfonts.c nx-X11/programs/Xserver/dix/dixfonts.c +5 -0

No files found.
--- a/nx-X11/programs/Xserver/dix/dixfonts.c
+++ b/nx-X11/programs/Xserver/dix/dixfonts.c
@@ -1559,6 +1559,7 @@ doImageText(ClientPtr client, register 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
@@ -1571,6 +1572,7 @@ doImageText(ClientPtr client, register ITclosurePtr c)
 		err = BadAlloc;
 		goto bail;
 	    }
+	    old_closure = c;
 	    *new_closure = *c;
 	    c = new_closure;

@@ -1578,6 +1580,7 @@ doImageText(ClientPtr client, register ITclosurePtr c)
 	    if (!data)
 	    {
 		xfree(c);
+		c = old_closure;
 		err = BadAlloc;
 		goto bail;
 	    }
@@ -1589,6 +1592,7 @@ doImageText(ClientPtr client, register ITclosurePtr c)
 	    {
 		xfree(c->data);
 		xfree(c);
+		c = old_closure;
 		err = BadAlloc;
 		goto bail;
 	    }
@@ -1602,6 +1606,7 @@ doImageText(ClientPtr client, register ITclosurePtr c)
 		FreeScratchGC(pGC);
 		xfree(c->data);
 		xfree(c);
+		c = old_closure;
 		err = BadAlloc;
 		goto bail;
 	    }