[Devel] [PATCH RHEL7 COMMIT] ms/fs: Check for invalid i_uid in may_follow_link()

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-514.26.1.vz7.33.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-514.26.1.vz7.33.3
------>
commit fdbfb1c0a4a553def2a3103e9500e842d3cd8072
Author: Seth Forshee <seth.forshee at canonical.com>
Date:   Tue Jul 11 19:39:38 2017 +0400

    ms/fs: Check for invalid i_uid in may_follow_link()
    
    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.
    
    Signed-off-by: Seth Forshee <seth.forshee at canonical.com>
    Acked-by: Serge Hallyn <serge.hallyn at canonical.com>
    Signed-off-by: Eric W. Biederman <ebiederm at xmission.com>
    (cherry picked from commit 2d7f9e2ad35e4e7a3086231f19bfab33c6a8a64a)
    
    https://jira.sw.ru/browse/PSBM-40075
    
    Signed-off-by: Konstantin Khorenko <khorenko at virtuozzo.com>
---
 fs/namei.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/namei.c b/fs/namei.c
index ede7535..1ee459f 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -803,6 +803,7 @@ static inline int may_follow_link(struct path *link, struct nameidata *nd)
 {
 	const struct inode *inode;
 	const struct inode *parent;
+	kuid_t puid;
 
 	if (!sysctl_protected_symlinks)
 		return 0;
@@ -818,7 +819,8 @@ static inline int may_follow_link(struct path *link, struct nameidata *nd)
 		return 0;
 
 	/* Allowed if parent directory and link owner match. */
-	if (uid_eq(parent->i_uid, inode->i_uid))
+	puid = parent->i_uid;
+	if (uid_valid(puid) && uid_eq(puid, inode->i_uid))
 		return 0;
 
 	audit_log_link_denied("follow_link", link);