Re: [PATCH] ovl: fix null pointer when filesystem doesn't support direct IO

[Chengguang Xu]

在 2021/9/18 20:13, Huang Jianan 写道:
> From: Huang Jianan <huangjianan@xxxxxxxx>
>
> At present, overlayfs provides overlayfs inode to users. Overlayfs
> inode provides ovl_aops with noop_direct_IO to avoid open failure
> with O_DIRECT. But some compressed filesystems, such as erofs and
> squashfs, don't support direct_IO.
>
> Users who use f_mapping->a_ops->direct_IO to check O_DIRECT support,
> will read file through this way. This will cause overlayfs to access
> a non-existent direct_IO function and cause panic due to null pointer:
>
> Kernel panic - not syncing: CFI failure (target: 0x0)
> CPU: 6 PID: 247 Comm: loop0
> Call Trace:
>   panic+0x188/0x45c
>   __cfi_slowpath+0x0/0x254
>   __cfi_slowpath+0x200/0x254
>   generic_file_read_iter+0x14c/0x150
>   vfs_iocb_iter_read+0xac/0x164
>   ovl_read_iter+0x13c/0x2fc
>   lo_rw_aio+0x2bc/0x458
>   loop_queue_work+0x4a4/0xbc0
>   kthread_worker_fn+0xf8/0x1d0
>   loop_kthread_worker_fn+0x24/0x38
>   kthread+0x29c/0x310
>   ret_from_fork+0x10/0x30
>
> The filesystem may only support direct_IO for some file types. For
> example, erofs supports direct_IO for uncompressed files. So fall
> back to buffered io only when the file doesn't support direct_IO to
> fix this problem.


IMO, return error to user seems better option than fall back to

buffered io directly.


Thanks,

Chengguang


> Fixes: 5b910bd615ba ("ovl: fix GPF in swapfile_activate of file from overlayfs over xfs")
> Signed-off-by: Huang Jianan <huangjianan@xxxxxxxx>
> ---
>   fs/overlayfs/file.c | 4 ++++
>   1 file changed, 4 insertions(+)
>
> diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
> index d081faa55e83..998c60770b81 100644
> --- a/fs/overlayfs/file.c
> +++ b/fs/overlayfs/file.c
> @@ -296,6 +296,10 @@ static ssize_t ovl_read_iter(struct kiocb *iocb, struct iov_iter *iter)
>   	if (ret)
>   		return ret;
>   
> +	if ((iocb->ki_flags & IOCB_DIRECT) && (!real.file->f_mapping->a_ops ||
> +		!real.file->f_mapping->a_ops->direct_IO))
> +		iocb->ki_flags &= ~IOCB_DIRECT;
> +
>   	old_cred = ovl_override_creds(file_inode(file)->i_sb);
>   	if (is_sync_kiocb(iocb)) {
>   		ret = vfs_iter_read(real.file, iter, &iocb->ki_pos,