Commit e7980f68 authored by Alexander Richardson's avatar Alexander Richardson
Browse files

dix/privates.c: Avoid undefined behaviour after realloc()



Adding the offset between the realloc result and the old allocation to
update pointers into the new allocation is undefined behaviour: the
old pointers are no longer valid after realloc() according to the C
standard. While this works on almost all architectures and compilers,
it causes  problems on architectures that track pointer bounds (e.g.
CHERI or Arm's Morello): the DevPrivateKey pointers will still have the
bounds of the previous allocation and therefore any dereference will
result in a run-time trap.

I found this due to a crash (dereferencing an invalid capability) while
trying to run `XVnc` on a CHERI-RISC-V system. With this commit I can
successfully connect to the XVnc instance running inside a QEMU with a
VNC viewer on my host.

This also changes the check whether the allocation was moved to use
uintptr_t instead of a pointer since according to the C standard:
"The value of a pointer becomes indeterminate when the object it
points to (or just past) reaches the end of its lifetime." Casting to an
integer type avoids this undefined behaviour.
Signed-off-by: Alexander Richardson's avatarAlex Richardson <Alexander.Richardson@cl.cam.ac.uk>
parent 617f591f
Pipeline #398716 passed with stages
in 7 minutes and 26 seconds
......@@ -155,14 +155,13 @@ dixMovePrivates(PrivatePtr *privates, int new_offset, unsigned bytes)
static Bool
fixupOneScreen(ScreenPtr pScreen, FixupFunc fixup, unsigned bytes)
{
intptr_t dist;
char *old;
uintptr_t old;
char *new;
DevPrivateKey *keyp, key;
DevPrivateType type;
int size;
old = (char *) pScreen->devPrivates;
old = (uintptr_t) pScreen->devPrivates;
size = global_keys[PRIVATE_SCREEN].offset;
if (!fixup (&pScreen->devPrivates, size, bytes))
return FALSE;
......@@ -182,9 +181,7 @@ fixupOneScreen(ScreenPtr pScreen, FixupFunc fixup, unsigned bytes)
if (fixup == dixMovePrivates)
new += bytes;
dist = new - old;
if (dist) {
if ((uintptr_t) new != old) {
for (type = PRIVATE_XSELINUX; type < PRIVATE_LAST; type++)
/* Walk the privates list, being careful as the
......@@ -199,10 +196,11 @@ fixupOneScreen(ScreenPtr pScreen, FixupFunc fixup, unsigned bytes)
* is contained within the allocation. Privates
* stored elsewhere will be left alone
*/
if (old <= (char *) key && (char *) key < old + size)
if (old <= (uintptr_t) key && (uintptr_t) key < old + size)
{
/* Compute new location of key */
key = (DevPrivateKey) ((char *) key + dist);
/* Compute new location of key (deriving from the new
* allocation to avoid UB) */
key = (DevPrivateKey) (new + ((uintptr_t) key - old));
/* Patch the list */
*keyp = key;
......
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