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Date: Fri, 17 Feb 2012 15:24:32 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Kees Cook <keescook@...omium.org>
Cc: linux-kernel@...r.kernel.org, Alexander Viro <viro@...iv.linux.org.uk>,
Rik van Riel <riel@...hat.com>, Federica Teodori
<federica.teodori@...glemail.com>, Lucian Adrian Grijincu
<lucian.grijincu@...il.com>, Ingo Molnar <mingo@...e.hu>, Peter Zijlstra
<a.p.zijlstra@...llo.nl>, Eric Paris <eparis@...hat.com>, Randy Dunlap
<rdunlap@...otime.net>, Dan Rosenberg <drosenberg@...curity.com>,
linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org,
kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v2012.2] fs: symlink restrictions on sticky directories
On Sat, 7 Jan 2012 10:55:48 -0800
Kees Cook <keescook@...omium.org> wrote:
> 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
>
> The solution is to permit symlinks to only be followed when outside
> a sticky world-writable directory, or when the uid of the symlink and
> follower match, or when the directory owner matches the symlink's owner.
>
> Some pointers to the history of earlier discussion that I could find:
>
> 1996 Aug, Zygo Blaxell
> http://marc.info/?l=bugtraq&m=87602167419830&w=2
> 1996 Oct, Andrew Tridgell
> http://lkml.indiana.edu/hypermail/linux/kernel/9610.2/0086.html
> 1997 Dec, Albert D Cahalan
> http://lkml.org/lkml/1997/12/16/4
> 2005 Feb, Lorenzo Hern__ndez Garc__a-Hierro
> http://lkml.indiana.edu/hypermail/linux/kernel/0502.0/1896.html
> 2010 May, Kees Cook
> https://lkml.org/lkml/2010/5/30/144
>
> Past objections and rebuttals could be summarized as:
>
> - Violates POSIX.
> - POSIX didn't consider this situation and it's not useful to follow
> a broken specification at the cost of security.
> - Might break unknown applications that use this feature.
> - Applications that break because of the change are easy to spot and
> fix. Applications that are vulnerable to symlink ToCToU by not having
> the change aren't. Additionally, no applications have yet been found
> that rely on this behavior.
> - Applications should just use mkstemp() or O_CREATE|O_EXCL.
> - True, but applications are not perfect, and new software is written
> all the time that makes these mistakes; blocking this flaw at the
> kernel is a single solution to the entire class of vulnerability.
> - This should live in the core VFS.
> - This should live in an LSM. (https://lkml.org/lkml/2010/5/31/135)
> - This should live in an LSM.
> - This should live in the core VFS. (https://lkml.org/lkml/2010/8/2/188)
>
> This patch is based on the patch in Openwall and grsecurity, along with
> suggestions from Al Viro. I have added a sysctl to enable the protected
> behavior, documentation, and an audit notification.
Looks reasonable to me.
It's a viropatch. I shall merge it into 3.4-rc1 if nothing happens to
prevent that.
> ...
>
> +config PROTECTED_STICKY_SYMLINKS
> + bool "Evaluate vulnerable symlink conditions"
> + default y
> + help
> + 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 malicious symlink belonging to another user).
> +
> + Enabling this adds the logic to examine these dangerous symlink
> + conditions. Whether or not the dangerous symlink situations are
> + allowed is controlled by PROTECTED_STICKY_SYMLINKS_ENABLED.
> +
> +config PROTECTED_STICKY_SYMLINKS_ENABLED
> + depends on PROTECTED_STICKY_SYMLINKS
> + bool "Disallow symlink following in sticky world-writable dirs"
> + default y
> + help
> + Solve ToCToU symlink race vulnerablities by permitting symlinks
> + to be followed only when outside a sticky world-writable directory,
> + or when the uid of the symlink and follower match, or when the
> + directory and symlink owners match.
> +
> + When PROC_SYSCTL is enabled, this setting can also be controlled
> + via /proc/sys/kernel/protected_sticky_symlinks.
I think I disagree with this. If the person compiling the kernel
includes the feature in his kernel via the time-honoured process of
"wtf is that thing? Yeah, whatev", it gets turned on by default. This
could easily result in weird failures which would take a *long* time
for an unsuspecting person to debug.
Would it not be kinder to our users to start this out as
turned-off-at-runtime unless the kernel configurer has deliberately
gone in and enabled it?
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -109,6 +109,9 @@ extern int sysctl_nr_trim_pages;
> #ifdef CONFIG_BLOCK
> extern int blk_iopoll_enabled;
> #endif
> +#ifdef CONFIG_PROTECTED_STICKY_SYMLINKS
> +extern int sysctl_protected_sticky_symlinks;
> +#endif
>
Grumble. Yes, it's a site of much badness. Let's not worsen things.
From: Andrew Morton <akpm@...ux-foundation.org>
Subject: fs-symlink-restrictions-on-sticky-directories-fix
move sysctl_protected_sticky_symlinks declaration into .h
Cc: Kees Cook <keescook@...omium.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
--- a/kernel/sysctl.c~fs-symlink-restrictions-on-sticky-directories-fix
+++ a/kernel/sysctl.c
@@ -109,9 +109,6 @@ extern int sysctl_nr_trim_pages;
#ifdef CONFIG_BLOCK
extern int blk_iopoll_enabled;
#endif
-#ifdef CONFIG_PROTECTED_STICKY_SYMLINKS
-extern int sysctl_protected_sticky_symlinks;
-#endif
/* Constants used for minimum and maximum */
#ifdef CONFIG_LOCKUP_DETECTOR
--- a/include/linux/fs.h~fs-symlink-restrictions-on-sticky-directories-fix
+++ a/include/linux/fs.h
@@ -422,6 +422,7 @@ extern unsigned long get_max_files(void)
extern int sysctl_nr_open;
extern struct inodes_stat_t inodes_stat;
extern int leases_enable, lease_break_time;
+extern int sysctl_protected_sticky_symlinks;
struct buffer_head;
typedef int (get_block_t)(struct inode *inode, sector_t iblock,
_
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