[PATCH 09/19] fs: Refuse uid/gid changes which don't map into s_user_ns

[Serge E. Hallyn]

On Wed, Dec 02, 2015 at 09:40:09AM -0600, Seth Forshee wrote:
> Add checks to inode_change_ok to verify that uid and gid changes
> will map into the superblock's user namespace. If they do not
> fail with -EOVERFLOW. This cannot be overriden with ATTR_FORCE.
> 
> Signed-off-by: Seth Forshee <seth.forshee at canonical.com>

Acked-by: Serge Hallyn <serge.hallyn at canonical.com>

...  although i could see root on the host being upset that it can't
assign a uid not valid in the mounter's ns.  But it does seem safer.

> ---
>  fs/attr.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/fs/attr.c b/fs/attr.c
> index 6530ced19697..55b46e3aa888 100644
> --- a/fs/attr.c
> +++ b/fs/attr.c
> @@ -42,6 +42,17 @@ int inode_change_ok(const struct inode *inode, struct iattr *attr)
>  			return error;
>  	}
>  
> +	/*
> +	 * Verify that uid/gid changes are valid in the target namespace
> +	 * of the superblock. This cannot be overriden using ATTR_FORCE.
> +	 */
> +	if (ia_valid & ATTR_UID &&
> +	    from_kuid(inode->i_sb->s_user_ns, attr->ia_uid) == (uid_t)-1)
> +		return -EOVERFLOW;
> +	if (ia_valid & ATTR_GID &&
> +	    from_kgid(inode->i_sb->s_user_ns, attr->ia_gid) == (gid_t)-1)
> +		return -EOVERFLOW;
> +
>  	/* If force is set do it anyway. */
>  	if (ia_valid & ATTR_FORCE)
>  		return 0;
> -- 
> 1.9.1
> 
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