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

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Date:	Tue, 13 Jul 2010 23:12:16 -0700
From:	Kees Cook <kees.cook@...onical.com>
To:	linux-security-module@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Yama: verify inode is symlink to avoid bind mounts

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@...onical.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
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