Re: [PATCH 07/19] fs: Check for invalid i_uid in may_follow_link()

["Serge E. Hallyn" <serge.hallyn@ubuntu.com>]

On Wed, Dec 02, 2015 at 09:40:07AM -0600, Seth Forshee wrote:
> Filesystem uids which don't map into a user namespace may result
> in inode->i_uid being INVALID_UID. A symlink and its parent
> could have different owners in the filesystem can both get
> mapped to INVALID_UID, which may result in following a symlink
> when this would not have otherwise been permitted when protected
> symlinks are enabled.
> 
> Add a new helper function, uid_valid_eq(), and use this to
> validate that the ids in may_follow_link() are both equal and
> valid. Also add an equivalent helper for gids, which is
> currently unused.
> 
> Signed-off-by: Seth Forshee <seth.forshee@canonical.com>

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

> ---
>  fs/namei.c             |  2 +-
>  include/linux/uidgid.h | 10 ++++++++++
>  2 files changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 288e8a74bf88..4ccafd391697 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -902,7 +902,7 @@ static inline int may_follow_link(struct nameidata *nd)
>  		return 0;
>  
>  	/* Allowed if parent directory and link owner match. */
> -	if (uid_eq(parent->i_uid, inode->i_uid))
> +	if (uid_valid_eq(parent->i_uid, inode->i_uid))
>  		return 0;
>  
>  	if (nd->flags & LOOKUP_RCU)
> diff --git a/include/linux/uidgid.h b/include/linux/uidgid.h
> index 03835522dfcb..e09529fe2668 100644
> --- a/include/linux/uidgid.h
> +++ b/include/linux/uidgid.h
> @@ -117,6 +117,16 @@ static inline bool gid_valid(kgid_t gid)
>  	return __kgid_val(gid) != (gid_t) -1;
>  }
>  
> +static inline bool uid_valid_eq(kuid_t left, kuid_t right)
> +{
> +	return uid_eq(left, right) && uid_valid(left);
> +}
> +
> +static inline bool gid_valid_eq(kgid_t left, kgid_t right)
> +{
> +	return gid_eq(left, right) && gid_valid(left);
> +}
> +
>  #ifdef CONFIG_USER_NS
>  
>  extern kuid_t make_kuid(struct user_namespace *from, uid_t uid);
> -- 
> 1.9.1
> 
> --
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