CVE-2025-40054

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

f2fs: fix UAF issue in f2fs_merge_page_bio()

As JY reported in bugzilla [1],

Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000 pc : [0xffffffe51d249484] f2fs_is_cp_guaranteed+0x70/0x98 lr : [0xffffffe51d24adbc] f2fs_merge_page_bio+0x520/0x6d4 CPU: 3 UID: 0 PID: 6790 Comm: kworker/u16:3 Tainted: P B W OE 6.12.30-android16-5-maybe-dirty-4k #1 5f7701c9cbf727d1eebe77c89bbbeb3371e895e5 Tainted: [P]=PROPRIETARY_MODULE, [B]=BAD_PAGE, [W]=WARN, [O]=OOT_MODULE, [E]=UNSIGNED_MODULE Workqueue: writeback wb_workfn (flush-254:49) Call trace: f2fs_is_cp_guaranteed+0x70/0x98 f2fs_inplace_write_data+0x174/0x2f4 f2fs_do_write_data_page+0x214/0x81c f2fs_write_single_data_page+0x28c/0x764 f2fs_write_data_pages+0x78c/0xce4 do_writepages+0xe8/0x2fc __writeback_single_inode+0x4c/0x4b4 writeback_sb_inodes+0x314/0x540 __writeback_inodes_wb+0xa4/0xf4 wb_writeback+0x160/0x448 wb_workfn+0x2f0/0x5dc process_scheduled_works+0x1c8/0x458 worker_thread+0x334/0x3f0 kthread+0x118/0x1ac ret_from_fork+0x10/0x20

[1] https://bugzilla.kernel.org/show_bug.cgi?id=220575

The panic was caused by UAF issue w/ below race condition:

kworker

• writepages
• f2fs_write_cache_pages
• f2fs_write_single_data_page
• f2fs_do_write_data_page - f2fs_inplace_write_data
  • f2fs_merge_page_bio
  • add_inu_page : cache page #1 into bio & cache bio in io->bio_list
• f2fs_write_single_data_page
• f2fs_do_write_data_page - f2fs_inplace_write_data

  • f2fs_merge_page_bio

  • add_inu_page : cache page #2 into bio which is linked in io->bio_list write - f2fs_write_begin : write page #1 - f2fs_folio_wait_writeback - f2fs_submit_merged_ipu_write - f2fs_submit_write_bio : submit bio which inclues page #1 and #2

    			software IRQ
    			- f2fs_write_end_io
    			 - fscrypt_free_bounce_page
    			 : freed bounced page which belongs to page #2
    

  • inc_page_count( , WB_DATA_TYPE(data_folio), false) : data_folio points to fio->encrypted_page the bounced page can be freed before accessing it in f2fs_is_cp_guarantee()

It can reproduce w/ below testcase: Run below script in shell #1: for ((i=1;i>0;i++)) do xfs_io -f /mnt/f2fs/enc/file
-c "pwrite 0 32k" -c "fdatasync"

Run below script in shell #2: for ((i=1;i>0;i++)) do xfs_io -f /mnt/f2fs/enc/file
-c "pwrite 0 32k" -c "fdatasync"

So, in f2fs_merge_page_bio(), let's avoid using fio->encrypted_page after commit page into internal ipu cache.

Red Hat Enterprise Linux is not vulnerable to this flaw because the Flash-Friendly File System (F2FS) code is not included in supported enterprise kernels. The underlying use-after-free defect involves concurrent encrypted write operations handled by the F2FS driver, causing a race condition during writeback. Consequently, Red Hat environments do not encounter this issue in standard deployments.