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

[PATCH] 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>
---
 security/yama/yama_lsm.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
index 3b76386..c70eb10 100644
--- a/security/yama/yama_lsm.c
+++ b/security/yama/yama_lsm.c
@@ -116,6 +116,10 @@ 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 */
+	if (!S_ISLNK(inode->i_mode))
+		return 0;
+
 	/* owner and follower match? */
 	cred = current_cred();
 	inode = dentry->d_inode;
-- 
1.7.1


-- 
Kees Cook
Ubuntu Security Team

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

[Serge E. Hallyn]

Quoting Kees Cook (kees.cook@canonical.com):
> 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,

Are you sure about that??

If that's true, you might also expand the comment in
include/linux/security.h.

> which seems to be a common NFSv4 configuration.
> 
> Signed-off-by: Kees Cook <kees.cook@canonical.com>
> ---
>  security/yama/yama_lsm.c |    4 ++++
>  1 files changed, 4 insertions(+), 0 deletions(-)
> 
> diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
> index 3b76386..c70eb10 100644
> --- a/security/yama/yama_lsm.c
> +++ b/security/yama/yama_lsm.c
> @@ -116,6 +116,10 @@ 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 */
> +	if (!S_ISLNK(inode->i_mode))
> +		return 0;
> +
>  	/* owner and follower match? */
>  	cred = current_cred();
>  	inode = dentry->d_inode;
> -- 
> 1.7.1
> 
> 
> -- 
> Kees Cook
> Ubuntu Security Team
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Re: [PATCH] Yama: verify inode is symlink to avoid bind mounts

[Kees Cook]

On Tue, Jul 13, 2010 at 09:30:48PM -0500, Serge E. Hallyn wrote:
> Quoting Kees Cook (kees.cook@canonical.com):
> > 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,
> 
> Are you sure about that??
> 
> If that's true, you might also expand the comment in
> include/linux/security.h.
> 
> > which seems to be a common NFSv4 configuration.

Well, the issue is how the NFSv4 client deals with it. It seems to treat
the mounts from the NFSv4 root as a symlink when the server is using bind
mounts.  It's kind of weird:
https://launchpad.net/bugs/604407

-- 
Kees Cook
Ubuntu Security Team

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

[Kees Cook]

On Tue, Jul 13, 2010 at 03:30:21PM -0700, 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>
> ---
>  security/yama/yama_lsm.c |    4 ++++
>  1 files changed, 4 insertions(+), 0 deletions(-)
> 
> diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
> index 3b76386..c70eb10 100644
> --- a/security/yama/yama_lsm.c
> +++ b/security/yama/yama_lsm.c
> @@ -116,6 +116,10 @@ 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 */
> +	if (!S_ISLNK(inode->i_mode))
> +		return 0;
> +
>  	/* owner and follower match? */
>  	cred = current_cred();
>  	inode = dentry->d_inode;

Erg, please ignore this -- I tested a slightly different version of this
patch.  This version doesn't have inode set yet.  I will follow up with the
correct one in a moment...

-Kees

-- 
Kees Cook
Ubuntu Security Team