Re: [PATCH 2/2] erofs: avoid using multiple devices with different type

[Hongbo Li <lihongbo22@huawei.com>]

On 2025/5/13 19:34, Sheng Yong wrote:
> From: Sheng Yong <shengyong1@xiaomi.com>
> 
> For multiple devices, both primary and extra devices should be the
> same type. `erofs_init_device` has already guaranteed that if the
> primary is a file-backed device, extra devices should also be
> regular files.
> 
> However, if the primary is a block device while the extra device
> is a file-backed device, `erofs_init_device` will get an ENOTBLK,
> which is not treated as an error in `erofs_fc_get_tree`, and that
> leads to an UAF:
> 
>    erofs_fc_get_tree
>      get_tree_bdev_flags(erofs_fc_fill_super)
>        erofs_read_superblock
>          erofs_init_device  // sbi->dif0 is not inited yet,
>                             // return -ENOTBLK
>        deactivate_locked_super
>          free(sbi)
>      if (err is -ENOTBLK)
>        sbi->dif0.file = filp_open()  // sbi UAF
> 
> So if -ENOTBLK is hitted in `erofs_init_device`, it means the
> primary device must be a block device, and the extra device
> is not a block device. The error can be converted to -EINVAL.
> 
> Signed-off-by: Sheng Yong <shengyong1@xiaomi.com>
> ---
>   fs/erofs/super.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/erofs/super.c b/fs/erofs/super.c
> index 512877d7d855..16b5b1f66584 100644
> --- a/fs/erofs/super.c
> +++ b/fs/erofs/super.c
> @@ -165,8 +165,11 @@ static int erofs_init_device(struct erofs_buf *buf, struct super_block *sb,
>   				filp_open(dif->path, O_RDONLY | O_LARGEFILE, 0) :
>   				bdev_file_open_by_path(dif->path,
>   						BLK_OPEN_READ, sb->s_type, NULL);
> -		if (IS_ERR(file))
> +		if (IS_ERR(file)) {
> +			if (PTR_ERR(file) == -ENOTBLK)
> +				file = ERR_PTR(-EINVAL);
>   			return PTR_ERR(file);

Hi, Yong

Thank you, I think it is indeed a UAF problem. This fixes the problem 
introduced by fb176750266a ("erofs: add file-backed mount support"). How 
about considering adding the fixes tag?

In addition, I wonder may be we can only check the fc->s_fs_info (we can 
set it to NULL in .kill_sb) in erofs_fc_get_tree rather than change the 
error code. So this way we can reback the correct error message to user.

Thanks,
Hongbo

> +		}
>   
>   		if (!erofs_is_fileio_mode(sbi)) {
>   			dif->dax_dev = fs_dax_get_by_bdev(file_bdev(file),