CVE-2023-52438

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

binder: fix use-after-free in shinker's callback

The mmap read lock is used during the shrinker's callback, which means that using alloc->vma pointer isn't safe as it can race with munmap(). As of commit dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in munmap") the mmap lock is downgraded after the vma has been isolated.

I was able to reproduce this issue by manually adding some delays and triggering page reclaiming through the shrinker's debug sysfs. The following KASAN report confirms the UAF:

================================================================== BUG: KASAN: slab-use-after-free in zappagerange_single+0x470/0x4b8 Read of size 8 at addr ffff356ed50e50f0 by task bash/478

CPU: 1 PID: 478 Comm: bash Not tainted 6.6.0-rc5-00055-g1c8b86a3799f-dirty #70 Hardware name: linux,dummy-virt (DT) Call trace: zappagerangesingle+0x470/0x4b8 binderallocfreepage+0x608/0xadc _listlruwalkone+0x130/0x3b0 listlruwalknode+0xc4/0x22c bindershrinkscan+0x108/0x1dc shrinkerdebugfsscanwrite+0x2b4/0x500 fullproxywrite+0xd4/0x140 vfswrite+0x1ac/0x758 ksyswrite+0xf0/0x1dc _arm64sys_write+0x6c/0x9c

Allocated by task 492: kmemcachealloc+0x130/0x368 vmareaalloc+0x2c/0x190 mmapregion+0x258/0x18bc dommap+0x694/0xa60 vmmmappgoff+0x170/0x29c ksysmmappgoff+0x290/0x3a0 _arm64sys_mmap+0xcc/0x144

Freed by task 491: kmemcachefree+0x17c/0x3c8 vmareafreercucb+0x74/0x98 rcucore+0xa38/0x26d4 rcucoresi+0x10/0x1c _do_softirq+0x2fc/0xd24

Last potentially related work creation: _callrcucommon.constprop.0+0x6c/0xba0 callrcu+0x10/0x1c vmareafree+0x18/0x24 removevma+0xe4/0x118 dovmialignmunmap.isra.0+0x718/0xb5c dovmimunmap+0xdc/0x1fc _vmmunmap+0x10c/0x278 _arm64sys_munmap+0x58/0x7c

Fix this issue by performing instead a vmalookup() which will fail to find the vma that was isolated before the mmap lock downgrade. Note that this option has better performance than upgrading to a mmap write lock which would increase contention. Plus, mmapwrite_trylock() has been recently removed anyway.