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 zap_page_range_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:

zap_page_range_single+0x470/0x4b8

binder_alloc_free_page+0x608/0xadc

__list_lru_walk_one+0x130/0x3b0

list_lru_walk_node+0xc4/0x22c

binder_shrink_scan+0x108/0x1dc

shrinker_debugfs_scan_write+0x2b4/0x500

full_proxy_write+0xd4/0x140

vfs_write+0x1ac/0x758

ksys_write+0xf0/0x1dc

__arm64_sys_write+0x6c/0x9c

Allocated by task 492:

kmem_cache_alloc+0x130/0x368

vm_area_alloc+0x2c/0x190

mmap_region+0x258/0x18bc

do_mmap+0x694/0xa60

vm_mmap_pgoff+0x170/0x29c

ksys_mmap_pgoff+0x290/0x3a0

__arm64_sys_mmap+0xcc/0x144

Freed by task 491:

kmem_cache_free+0x17c/0x3c8

vm_area_free_rcu_cb+0x74/0x98

rcu_core+0xa38/0x26d4

rcu_core_si+0x10/0x1c

__do_softirq+0x2fc/0xd24

Last potentially related work creation:

__call_rcu_common.constprop.0+0x6c/0xba0

call_rcu+0x10/0x1c

vm_area_free+0x18/0x24

remove_vma+0xe4/0x118

do_vmi_align_munmap.isra.0+0x718/0xb5c

do_vmi_munmap+0xdc/0x1fc

__vm_munmap+0x10c/0x278

__arm64_sys_munmap+0x58/0x7c

Fix this issue by performing instead a vma_lookup() 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, mmap_write_trylock() has been

recently removed anyway.