CVE-2024-49885

In the Linux kernel, the following vulnerability has been resolved:

mm, slub: avoid zeroing kmalloc redzone

Since commit 946fa0dbf2d8 ("mm/slub: extend redzone check to extra allocated kmalloc space than requested"), setting origsize treats the wasted space (objectsize - origsize) as a redzone. However with initon_free=1 we clear the full object->size, including the redzone.

Additionally we clear the object metadata, including the stored origsize, making it zero, which makes checkobject() treat the whole object as a redzone.

These issues lead to the following BUG report with "slubdebug=FUZ initon_free=1":

[ 0.000000] ============================================================================= [ 0.000000] BUG kmalloc-8 (Not tainted): kmalloc Redzone overwritten [ 0.000000] ----------------------------------------------------------------------------- [ 0.000000] [ 0.000000] 0xffff000010032858-0xffff00001003285f @offset=2136. First byte 0x0 instead of 0xcc [ 0.000000] FIX kmalloc-8: Restoring kmalloc Redzone 0xffff000010032858-0xffff00001003285f=0xcc [ 0.000000] Slab 0xfffffdffc0400c80 objects=36 used=23 fp=0xffff000010032a18 flags=0x3fffe0000000200(workingset|node=0|zone=0|lastcpupid=0x1ffff) [ 0.000000] Object 0xffff000010032858 @offset=2136 fp=0xffff0000100328c8 [ 0.000000] [ 0.000000] Redzone ffff000010032850: cc cc cc cc cc cc cc cc ........ [ 0.000000] Object ffff000010032858: cc cc cc cc cc cc cc cc ........ [ 0.000000] Redzone ffff000010032860: cc cc cc cc cc cc cc cc ........ [ 0.000000] Padding ffff0000100328b4: 00 00 00 00 00 00 00 00 00 00 00 00 ............ [ 0.000000] CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 6.11.0-rc3-next-20240814-00004-g61844c55c3f4 #144 [ 0.000000] Hardware name: NXP i.MX95 19X19 board (DT) [ 0.000000] Call trace: [ 0.000000] dumpbacktrace+0x90/0xe8 [ 0.000000] showstack+0x18/0x24 [ 0.000000] dumpstacklvl+0x74/0x8c [ 0.000000] dumpstack+0x18/0x24 [ 0.000000] printtrailer+0x150/0x218 [ 0.000000] checkobject+0xe4/0x454 [ 0.000000] freetopartiallist+0x2f8/0x5ec

To address the issue, use origsize to clear the used area. And restore the value of origsize after clear the remaining area.

When CONFIGSLUBDEBUG not defined, (getorigsize()' directly returns s->objectsize. So when using memset to init the area, the size can simply be origsize, as origsize returns objectsize when CONFIGSLUBDEBUG not enabled. And origsize can never be bigger than objectsize.