linux-kernel - Re: [PATCH] fs: block cross-uid sticky symlinks

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Date:	Thu, 27 May 2010 23:40:26 -0500
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	Kees Cook <kees.cook@...onical.com>
Cc:	linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org, linux-doc@...r.kernel.org,
	Randy Dunlap <rdunlap@...otime.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>,
	Dave Young <hidave.darkstar@...il.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	James Morris <jmorris@...ei.org>,
	Eric Paris <eparis@...hat.com>,
	David Howells <dhowells@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Tim Gardner <tim.gardner@...onical.com>
Subject: Re: [PATCH] fs: block cross-uid sticky symlinks

Quoting Kees Cook (kees.cook@...onical.com):
> A long-standing class of security issues is the symlink-based
> time-of-check-time-of-use race, most commonly seen in world-writable
> directories like /tmp. The common method of exploitation of this flaw
> is to cross privilege boundaries when following a given symlink (i.e. a
> root process follows a symlink belonging to another user).  For a likely
> incomplete list of hundreds of examples across the years, please see:
> http://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp

That is quite a list :)

> +int cap_inode_follow_link(struct dentry *dentry,
> +			  struct nameidata *nameidata)
> +{
> +	const struct inode *parent = dentry->d_parent->d_inode;
> +	const struct inode *inode = dentry->d_inode;
> +	const struct cred *cred = current_cred();
> +
> +	if (weak_sticky_symlinks)
> +		return 0;
> +
> +	if (S_ISLNK(inode->i_mode) &&

Q: is the S_ISLNK() check actually needed?

In either case:

Acked-by: Serge E. Hallyn <serue@...ibm.com>

thanks,
-serge
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