commit 3f489c2067c5824528212b0fc18b28d51332d906 upstream.

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.

Fixes: dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in munmap") Cc: stable@vger.kernel.org Cc: Liam Howlett liam.howlett@oracle.com Cc: Minchan Kim minchan@kernel.org Reviewed-by: Alice Ryhl aliceryhl@google.com Signed-off-by: Carlos Llamas cmllamas@google.com Link: https://lore.kernel.org/r/20231201172212.1813387-3-cmllamas@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org [cmllamas: use find_vma() instead of vma_lookup() as commit ce6d42f2e4a2 is missing in v5.10. This only works because we check the vma against our cached alloc->vma pointer.] Signed-off-by: Carlos Llamas cmllamas@google.com (cherry picked from commit aaa8cde67eba56b3ebe8c2c155e896f9ccaa8bb7) --- drivers/android/binder_alloc.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c index b6bf9caaf1d1..61406bfa454b 100644 --- a/drivers/android/binder_alloc.c +++ b/drivers/android/binder_alloc.c @@ -1002,7 +1002,9 @@ enum lru_status binder_alloc_free_page(struct list_head *item, goto err_mmget; if (!mmap_read_trylock(mm)) goto err_mmap_read_lock_failed; - vma = binder_alloc_get_vma(alloc); + vma = find_vma(mm, page_addr); + if (vma && vma != binder_alloc_get_vma(alloc)) + goto err_invalid_vma;

list_lru_isolate(lru, item); spin_unlock(lock); @@ -1028,6 +1030,8 @@ enum lru_status binder_alloc_free_page(struct list_head *item, mutex_unlock(&alloc->mutex); return LRU_REMOVED_RETRY;

+err_invalid_vma: + mmap_read_unlock(mm); err_mmap_read_lock_failed: mmput_async(mm); err_mmget: