CVE-2023-52438

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

binder: fix use-after-free in shinkers callback

The mmap read lock is used during the shrinkers callback, which means that using alloc->vma pointer isnt 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 shrinkers 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.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/3f489c2067c5824528212b0fc18b28d51332d906
• https://git.kernel.org/stable/c/8ad4d580e8aff8de2a4d57c5930fcc29f1ffd4a6
• https://git.kernel.org/stable/c/9fa04c93f24138747807fe75b5591bb680098f56
• https://git.kernel.org/stable/c/a49087ab93508b60d9b8add91707a22dda832869
• https://git.kernel.org/stable/c/a53e15e592b4dcc91c3a3b8514e484a0bdbc53a3
• https://git.kernel.org/stable/c/c8c1158ffb007197f31f9d9170cf13e4f34cbb5c
• https://git.kernel.org/stable/c/e074686e993ff1be5f21b085a3b1b4275ccd5727
• https://git.kernel.org/stable/c/3f489c2067c5824528212b0fc18b28d51332d906
• https://git.kernel.org/stable/c/8ad4d580e8aff8de2a4d57c5930fcc29f1ffd4a6
• https://git.kernel.org/stable/c/9fa04c93f24138747807fe75b5591bb680098f56
• https://git.kernel.org/stable/c/a49087ab93508b60d9b8add91707a22dda832869
• https://git.kernel.org/stable/c/a53e15e592b4dcc91c3a3b8514e484a0bdbc53a3
• https://git.kernel.org/stable/c/c8c1158ffb007197f31f9d9170cf13e4f34cbb5c
• https://git.kernel.org/stable/c/e074686e993ff1be5f21b085a3b1b4275ccd5727
• https://lists.debian.org/debian-lts-announce/2024/06/msg00016.html