[PATCH 11/19] fs: Ensure the mounter of a filesystem is privileged towards its inodes

[Serge E. Hallyn]

Quoting Seth Forshee (seth.forshee at canonical.com):
> The mounter of a filesystem should be privileged towards the
> inodes of that filesystem. Extend the checks in
> inode_owner_or_capable() and capable_wrt_inode_uidgid() to
> permit access by users priviliged in the user namespace of the
> inode's superblock.
> 
> Signed-off-by: Seth Forshee <seth.forshee at canonical.com>

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

> ---
>  fs/inode.c          |  3 +++
>  kernel/capability.c | 13 +++++++++----
>  2 files changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/inode.c b/fs/inode.c
> index 1be5f9003eb3..01c036fe1950 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -1962,6 +1962,9 @@ bool inode_owner_or_capable(const struct inode *inode)
>  	ns = current_user_ns();
>  	if (ns_capable(ns, CAP_FOWNER) && kuid_has_mapping(ns, inode->i_uid))
>  		return true;
> +
> +	if (ns_capable(inode->i_sb->s_user_ns, CAP_FOWNER))
> +		return true;
>  	return false;
>  }
>  EXPORT_SYMBOL(inode_owner_or_capable);
> diff --git a/kernel/capability.c b/kernel/capability.c
> index 45432b54d5c6..5137a38a5670 100644
> --- a/kernel/capability.c
> +++ b/kernel/capability.c
> @@ -437,13 +437,18 @@ EXPORT_SYMBOL(file_ns_capable);
>   *
>   * Return true if the current task has the given capability targeted at
>   * its own user namespace and that the given inode's uid and gid are
> - * mapped into the current user namespace.
> + * mapped into the current user namespace, or if the current task has
> + * the capability towards the user namespace of the inode's superblock.
>   */
>  bool capable_wrt_inode_uidgid(const struct inode *inode, int cap)
>  {
> -	struct user_namespace *ns = current_user_ns();
> +	struct user_namespace *ns;
>  
> -	return ns_capable(ns, cap) && kuid_has_mapping(ns, inode->i_uid) &&
> -		kgid_has_mapping(ns, inode->i_gid);
> +	ns = current_user_ns();
> +	if (ns_capable(ns, cap) && kuid_has_mapping(ns, inode->i_uid) &&
> +	    kgid_has_mapping(ns, inode->i_gid))
> +		return true;
> +
> +	return ns_capable(inode->i_sb->s_user_ns, cap);
>  }
>  EXPORT_SYMBOL(capable_wrt_inode_uidgid);
> -- 
> 1.9.1
> 
> --
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