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Date: Wed, 07 Dec 2011 10:22:07 -0800
From: Randy Dunlap <rdunlap@...otime.net>
To: Kees Cook <keescook@...omium.org>
CC: Andrew Morton <akpm@...ux-foundation.org>,
Al Viro <viro@...iv.linux.org.uk>,
linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-doc@...r.kernel.org, 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>,
Dan Rosenberg <drosenberg@...curity.com>,
kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v2011.1] fs: symlink restrictions on sticky directories
On 12/06/2011 03:58 PM, Kees Cook wrote:
> Documentation/sysctl/fs.txt | 21 ++++++++++++
> fs/Kconfig | 15 ++++++++
> fs/namei.c | 77 +++++++++++++++++++++++++++++++++++++++---
> kernel/sysctl.c | 10 +++++
> 4 files changed, 117 insertions(+), 6 deletions(-)
> diff --git a/fs/Kconfig b/fs/Kconfig
> index 5f4c45d..74b9e49 100644
> --- a/fs/Kconfig
> +++ b/fs/Kconfig
> @@ -278,3 +278,18 @@ source "fs/nls/Kconfig"
> source "fs/dlm/Kconfig"
>
> endmenu
> +
> +config PROTECTED_STICKY_SYMLINKS
> + bool "Protect symlink following in sticky world-writable directories"
> + 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 solves the problem by permitting symlinks to only
better:
Enabling this solves the problem by permitting symlinks to be followed
only when the uid ...
> + be followed 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.
> diff --git a/fs/namei.c b/fs/namei.c
> index 5008f01..c4d0bfc 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -624,10 +625,69 @@ static inline void put_link(struct nameidata *nd, struct path *link, void *cooki
> +
> +/**
> + * may_follow_link - Check symlink following for unsafe situations
> + * @dentry: The inode/dentry of the symlink
> + * @nameidata: The path data of the symlink
> + *
> + * In the case of the protected_sticky_symlinks sysctl being enabled,
> + * CAP_DAC_OVERRIDE needs to be specifically ignored if the symlink is
> + * in a sticky world-writable directory. This is to protect privileged
> + * processes from failing races against path names that may change out
> + * from under them by way of other users creating malicious symlinks.
> + * It will permit symlinks to only be followed when outside a sticky
similar:
It will permit 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 owner matches the symlink's owner.
> + *
> + * Returns 0 if following the symlink is allowed, -ve on error.
> + */
> +static inline int
> +may_follow_link(struct dentry *dentry, struct nameidata *nameidata)
--
~Randy
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