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.

1 files changed, 5 insertions, 0 deletions

diff --git a/nx-X11/programs/Xserver/dix/dixfonts.c b/nx-X11/programs/Xserver/dix/dixfonts.c
index 193f5558c..42fd647a4 100644
--- 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;
 	    }