linux-kernel - [PATCH] VFS: add config options to enable link restrictions

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Date:	Fri, 26 Oct 2012 11:50:21 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org
Subject: [PATCH] VFS: add config options to enable link restrictions

There are situations where devices running without initrds may need
very early protection from link vulnerabilities, so make these sysctls
configurable at build time, since 561ec64ae67e ("VFS: don't do protected
{sym,hard}links by default") has disabled the protections by default.

Cc: Alexander Viro <viro@...iv.linux.org.uk>
Cc: stable@...r.kernel.org
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 fs/Kconfig |   39 +++++++++++++++++++++++++++++++++++++++
 fs/namei.c |    6 ++++--
 2 files changed, 43 insertions(+), 2 deletions(-)

diff --git a/fs/Kconfig b/fs/Kconfig
index f95ae3a..46ae2dc 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -276,4 +276,43 @@ endif # NETWORK_FILESYSTEMS
 source "fs/nls/Kconfig"
 source "fs/dlm/Kconfig"
 
+config PROTECTED_SYMLINKS
+	bool "Protect symlink following in sticky world-writable dirs"
+	default n
+	help
+	  Solve the class of ToCToU symlink race vulnerabilities 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_symlinks.
+
+	  See Documentation/sysctl/fs.txt for details.
+
+config PROTECTED_SYMLINKS_SYSCTL
+	int
+	default "1" if PROTECTED_SYMLINKS
+	default "0"
+
+config PROTECTED_HARDLINKS
+	bool "Protect hardlink creation to non-accessible files"
+	default n
+	help
+	  Solve the class of ToCToU hardlink race vulnerabilities by
+	  permitting hardlinks to be created only when to a regular file
+	  that is owned by the user, or is readable and writable by the
+	  user. Also blocks users from "pinning" vulnerable setuid/setgid
+	  programs from being upgraded by the administrator.
+
+	  When PROC_SYSCTL is enabled, this setting can also be controlled
+	  via /proc/sys/kernel/protected_hardlinks.
+
+	  See Documentation/sysctl/fs.txt for details.
+
+config PROTECTED_HARDLINKS_SYSCTL
+	int
+	default "1" if PROTECTED_HARDLINKS
+	default "0"
+
 endmenu
diff --git a/fs/namei.c b/fs/namei.c
index 937f9d5..21854df 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -705,8 +705,10 @@ static inline void put_link(struct nameidata *nd, struct path *link, void *cooki
 	path_put(link);
 }
 
-int sysctl_protected_symlinks __read_mostly = 0;
-int sysctl_protected_hardlinks __read_mostly = 0;
+int sysctl_protected_symlinks __read_mostly =
+	CONFIG_PROTECTED_SYMLINKS_SYSCTL;
+int sysctl_protected_hardlinks __read_mostly =
+	CONFIG_PROTECTED_HARDLINKS_SYSCTL;
 
 /**
  * may_follow_link - Check symlink following for unsafe situations
-- 
1.7.9.5


-- 
Kees Cook
Chrome OS Security
--
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