Re: [PATCH v3 1/2] vfs: allow dedupe of user owned read-only files

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Jun 07, 2018 at 10:38:53AM -0700, Mark Fasheh wrote:
> The permission check in vfs_dedupe_file_range() is too coarse - We
> only allow dedupe of the destination file if the user is root, or
> they have the file open for write.
> 
> This effectively limits a non-root user from deduping their own read-only
> files. In addition, the write file descriptor that the user is forced to
> hold open can prevent execution of files. As file data during a dedupe
> does not change, the behavior is unexpected and this has caused a number of
> issue reports. For an example, see:
> 
> https://github.com/markfasheh/duperemove/issues/129
> 
> So change the check so we allow dedupe on the target if:
> 
> - the root or admin is asking for it
> - the process has write access
> - the owner of the file is asking for the dedupe
> - the process could get write access
> 
> That way users can open read-only and still get dedupe.
> 
> Signed-off-by: Mark Fasheh <mfasheh@suse.de>

Looks ok, but could you please update the manpage for
ioctl_fideduperange to elaborate on when userspace can expect EPERM?

Acked-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> ---
>  fs/read_write.c | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/read_write.c b/fs/read_write.c
> index e83bd9744b5d..71e9077f8bc1 100644
> --- a/fs/read_write.c
> +++ b/fs/read_write.c
> @@ -1964,6 +1964,20 @@ int vfs_dedupe_file_range_compare(struct inode *src, loff_t srcoff,
>  }
>  EXPORT_SYMBOL(vfs_dedupe_file_range_compare);
>  
> +/* Check whether we are allowed to dedupe the destination file */
> +static bool allow_file_dedupe(struct file *file)
> +{
> +	if (capable(CAP_SYS_ADMIN))
> +		return true;
> +	if (file->f_mode & FMODE_WRITE)
> +		return true;
> +	if (uid_eq(current_fsuid(), file_inode(file)->i_uid))
> +		return true;
> +	if (!inode_permission(file_inode(file), MAY_WRITE))
> +		return true;
> +	return false;
> +}
> +
>  int vfs_dedupe_file_range(struct file *file, struct file_dedupe_range *same)
>  {
>  	struct file_dedupe_range_info *info;
> @@ -1972,7 +1986,6 @@ int vfs_dedupe_file_range(struct file *file, struct file_dedupe_range *same)
>  	u64 len;
>  	int i;
>  	int ret;
> -	bool is_admin = capable(CAP_SYS_ADMIN);
>  	u16 count = same->dest_count;
>  	struct file *dst_file;
>  	loff_t dst_off;
> @@ -2036,7 +2049,7 @@ int vfs_dedupe_file_range(struct file *file, struct file_dedupe_range *same)
>  
>  		if (info->reserved) {
>  			info->status = -EINVAL;
> -		} else if (!(is_admin || (dst_file->f_mode & FMODE_WRITE))) {
> +		} else if (!allow_file_dedupe(dst_file)) {
>  			info->status = -EINVAL;
>  		} else if (file->f_path.mnt != dst_file->f_path.mnt) {
>  			info->status = -EXDEV;
> -- 
> 2.15.1
> 
> --
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