linux-kernel - [PATCH 4.7 05/59] fs: Check for invalid i_uid in may_follow

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Message-Id: <20160912152128.980769300@linuxfoundation.org>
Date:   Mon, 12 Sep 2016 17:29:25 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Seth Forshee <seth.forshee@...onical.com>,
        Serge Hallyn <serge.hallyn@...onical.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: [PATCH 4.7 05/59] fs: Check for invalid i_uid in may_follow_link()

4.7-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Seth Forshee <seth.forshee@...onical.com>

commit 2d7f9e2ad35e4e7a3086231f19bfab33c6a8a64a upstream.

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@...onical.com>
Acked-by: Serge Hallyn <serge.hallyn@...onical.com>
Signed-off-by: Eric W. Biederman <ebiederm@...ssion.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 fs/namei.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/fs/namei.c
+++ b/fs/namei.c
@@ -901,6 +901,7 @@ static inline int may_follow_link(struct
 {
 	const struct inode *inode;
 	const struct inode *parent;
+	kuid_t puid;

 	if (!sysctl_protected_symlinks)
 		return 0;
@@ -916,7 +917,8 @@ static inline int may_follow_link(struct
 		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;

 	if (nd->flags & LOOKUP_RCU)