[PATCH v2] erofs: avoid using multiple devices with different type

[Gao Xiang]

On 2025/5/15 09:48, Sheng Yong wrote:
> From: Sheng Yong <shengyong1 at 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.
> 
> Fixes: fb176750266a ("erofs: add file-backed mount support")
> Signed-off-by: Sheng Yong <shengyong1 at xiaomi.com>

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

Thanks,
Gao Xiang