[PATCH] erofs: fix UAF issue in erofs_fileio_rq_submit()

[Gao Xiang]

Hi Chao,

On 2026/2/6 06:30, Chao Yu wrote:
> [    9.269940][ T3222] Call trace:
> [    9.269948][ T3222]  ext4_file_read_iter+0xac/0x108
> [    9.269979][ T3222]  vfs_iocb_iter_read+0xac/0x198
> [    9.269993][ T3222]  erofs_fileio_rq_submit+0x12c/0x180
> [    9.270008][ T3222]  erofs_fileio_submit_bio+0x14/0x24
> [    9.270030][ T3222]  z_erofs_runqueue+0x834/0x8ac
> [    9.270054][ T3222]  z_erofs_read_folio+0x120/0x220
> [    9.270083][ T3222]  filemap_read_folio+0x60/0x120
> [    9.270102][ T3222]  filemap_fault+0xcac/0x1060
> [    9.270119][ T3222]  do_pte_missing+0x2d8/0x1554
> [    9.270131][ T3222]  handle_mm_fault+0x5ec/0x70c
> [    9.270142][ T3222]  do_page_fault+0x178/0x88c
> [    9.270167][ T3222]  do_translation_fault+0x38/0x54
> [    9.270183][ T3222]  do_mem_abort+0x54/0xac
> [    9.270208][ T3222]  el0_da+0x44/0x7c
> [    9.270227][ T3222]  el0t_64_sync_handler+0x5c/0xf4
> [    9.270253][ T3222]  el0t_64_sync+0x1bc/0x1c0
> 
> erofs may encounter above panic when enabling file-backed mount w/ directio
> mount option, the root cause is it may suffer UAF in below race condition:
> 
> - z_erofs_read_folio                          wq s_dio_done_wq
>   - z_erofs_runqueue
>    - erofs_fileio_submit_bio
>     - erofs_fileio_rq_submit
>      - vfs_iocb_iter_read
>       - ext4_file_read_iter
>        - ext4_dio_read_iter
>         - iomap_dio_rw
>         : bio was submitted and return -EIOCBQUEUED
>                                                - dio_aio_complete_work
>                                                 - dio_complete
>                                                  - dio->iocb->ki_complete (erofs_fileio_ki_complete())
>                                                   - kfree(rq)
>                                                   : it frees iocb, iocb.ki_filp can be UAF in file_accessed().
>         - file_accessed
>         : access NULL file point
> 
> Introduce a reference count in struct erofs_fileio_rq, and initialize it
> as two, both erofs_fileio_ki_complete() and erofs_fileio_rq_submit() will
> decrease reference count, the last one decreasing the reference count
> to zero will free rq.
> 
> Cc: stable at kernel.org
> Fixes: fb176750266a ("erofs: add file-backed mount support")
> Signed-off-by: Chao Yu <chao at kernel.org>

Thanks for catching this:

I will update the subject as
"erofs: fix UAF issue for file-backed mounts w/ directio"

to make it more clear about the impacts.

Reviewed-by: Gao Xiang <hsiangkao at linux.alibaba.com>

Thanks,
Gao Xiang