CVE-2021-47256

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

mm/memory-failure: make sure wait for page writeback in memory_failure

Our syzkaller trigger the BUG_ON(!list_empty(&inode->i_wb_list)) in clear_inode:

kernel BUG at fs/inode.c:519! Internal error: Oops - BUG: 0 [#1] SMP Modules linked in: Process syz-executor.0 (pid: 249, stack limit = 0x00000000a12409d7) CPU: 1 PID: 249 Comm: syz-executor.0 Not tainted 4.19.95 Hardware name: linux,dummy-virt (DT) pstate: 80000005 (Nzcv daif -PAN -UAO) pc : clear_inode+0x280/0x2a8 lr : clear_inode+0x280/0x2a8 Call trace: clear_inode+0x280/0x2a8 ext4_clear_inode+0x38/0xe8 ext4_free_inode+0x130/0xc68 ext4_evict_inode+0xb20/0xcb8 evict+0x1a8/0x3c0 iput+0x344/0x460 do_unlinkat+0x260/0x410 __arm64_sys_unlinkat+0x6c/0xc0 el0_svc_common+0xdc/0x3b0 el0_svc_handler+0xf8/0x160 el0_svc+0x10/0x218 Kernel panic - not syncing: Fatal exception

A crash dump of this problem show that someone called __munlock_pagevec to clear page LRU without lock_page: do_mmap -> mmap_region -> do_munmap -> munlock_vma_pages_range -> __munlock_pagevec.

As a result memory_failure will call identify_page_state without wait_on_page_writeback. And after truncate_error_page clear the mapping of this page. end_page_writeback wont call sb_clear_inode_writeback to clear inode->i_wb_list. That will trigger BUG_ON in clear_inode!

Fix it by checking PageWriteback too to help determine should we skip wait_on_page_writeback.

Affected Software

References

• https://git.kernel.org/stable/c/28788dc5c70597395b6b451dae4549bbaa8e2c56
• https://git.kernel.org/stable/c/566345aaabac853aa866f53a219c4b02a6beb527
• https://git.kernel.org/stable/c/6d210d547adc2218ef8b5bcf23518c5f2f1fd872
• https://git.kernel.org/stable/c/9e379da727a7a031be9b877cde7b9c34a0fb8306
• https://git.kernel.org/stable/c/d05267fd27a5c4f54e06daefa3035995d765ca0c
• https://git.kernel.org/stable/c/e8675d291ac007e1c636870db880f837a9ea112a