[PATCH v2] Yama: verify inode is symlink to avoid bind mounts

[PATCH v2] Yama: verify inode is symlink to avoid bind mounts

[Kees Cook]

The inode_follow_link LSM hook is called in bind mount situations as
well as for symlink situations, so we must explicitly check for the
inode being a symlink to not reject bind mounts in 1777 directories,
which seems to be a common NFSv4 configuration.

Signed-off-by: Kees Cook <kees.cook@canonical.com>
---
v2:
 - actually set inode in time to use it.  *face palm*
---
 security/yama/yama_lsm.c |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)

diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
index 3b76386..51c6a3a 100644
--- a/security/yama/yama_lsm.c
+++ b/security/yama/yama_lsm.c
@@ -116,9 +116,13 @@ static int yama_inode_follow_link(struct dentry *dentry,
 	if (!protected_sticky_symlinks)
 		return 0;
 
+	/* if inode isn't a symlink, don't try to evaluate blocking it */
+	inode = dentry->d_inode;
+	if (!S_ISLNK(inode->i_mode))
+		return 0;
+
 	/* owner and follower match? */
 	cred = current_cred();
-	inode = dentry->d_inode;
 	if (cred->fsuid == inode->i_uid)
 		return 0;
 
-- 
1.7.1


-- 
Kees Cook
Ubuntu Security Team

Re: [PATCH v2] Yama: verify inode is symlink to avoid bind mounts

[James Morris]

On Tue, 13 Jul 2010, Kees Cook wrote:

> The inode_follow_link LSM hook is called in bind mount situations as
> well as for symlink situations, so we must explicitly check for the
> inode being a symlink to not reject bind mounts in 1777 directories,
> which seems to be a common NFSv4 configuration.
> 
> Signed-off-by: Kees Cook <kees.cook@canonical.com>
> ---
> v2:
>  - actually set inode in time to use it.  *face palm*

Applied to
git://git.kernel.org/pub/scm/linux/kernel/git/jmorris/security-testing-2.6#next


-- 
James Morris
<jmorris@namei.org>