[PATCH v6] fs: allow protected cross-uid sticky symlinks

[Kees Cook <kees.cook@canonical.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 opening a file through a given
symlink (i.e. a root process opens 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 opened when outside a sticky
world-writable directory, or when the uid of the symlink and opener 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 Hernández García-Hierro
  http://lkml.indiana.edu/hypermail/linux/kernel/0502.0/1896.html

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.
 - 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 patch is based on the patch in Openwall and grsecurity, but with the
scope changed to be only "opening" a symlink.  I have added a sysctl to
enable the protected behavior, documentation, and a ratelimited warning.

Signed-off-by: Kees Cook <kees.cook@canonical.com>
---

v2:
 - dropped redundant S_ISLNK check.
 - moved sysctl extern into security.h.
 - asked to include CC to linux-fsdevel.

v3:
 - move into VFS core.
 - add CONFIG entry for build-time default.
 - rename sysctl, invert logic.
 - use get_task_comm for task name.
 - lock dentry when checking parent.

v4:
 - limit check to leaf symlink opening.

v5:
 - Kconfig whitespace regressed (thanks to Randy Dunlap for pointing it out)

v6:
 - move sysctl extern to fs.h.
---
 Documentation/sysctl/fs.txt |   15 ++++++++++
 fs/Kconfig                  |   15 ++++++++++
 fs/namei.c                  |   61 +++++++++++++++++++++++++++++++++++++++++++
 include/linux/fs.h          |    1 +
 kernel/sysctl.c             |    9 ++++++
 5 files changed, 101 insertions(+), 0 deletions(-)

diff --git a/Documentation/sysctl/fs.txt b/Documentation/sysctl/fs.txt
index 6268250..9986bce 100644
--- a/Documentation/sysctl/fs.txt
+++ b/Documentation/sysctl/fs.txt
@@ -32,6 +32,7 @@ Currently, these files are in /proc/sys/fs:
 - nr_open
 - overflowuid
 - overflowgid
+- protected-sticky-symlinks
 - suid_dumpable
 - super-max
 - super-nr
@@ -158,6 +159,20 @@ The default is 65534.
 
 ==============================================================
 
+protected-sticky-symlinks:
+
+Opening symlinks in sticky world-writable directories (like /tmp) can be
+dangerous due to time-of-check-time-of-use races that frequently result
+in security vulnerabilities.
+
+The default value is "0", leaving the behavior of symlink opening
+unchanged from POSIX.  A value of "1" will enable the protection, causing
+symlinks to be openable only if outside a sticky world-writable directory,
+or if the symlink and the opener's uid match, or if the symlink and its
+directory are owned by the same uid.
+
+==============================================================
+
 suid_dumpable:
 
 This value can be used to query and set the core dump mode for setuid
diff --git a/fs/Kconfig b/fs/Kconfig
index 5f85b59..b2cdff3 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -256,3 +256,18 @@ source "fs/nls/Kconfig"
 source "fs/dlm/Kconfig"
 
 endmenu
+
+config PROTECTED_STICKY_SYMLINKS
+	bool "Protect symlink opening 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 opening a given symlink (i.e. a root process opens a
+	  malicious symlink belonging to another user).
+
+	  Enabling this solves the problem by permitting symlinks to only
+	  be opened when outside a sticky world-writable directory, or
+	  when the uid of the symlink and opener match, or when the
+	  directory and symlink owners match.
diff --git a/fs/namei.c b/fs/namei.c
index 868d0cb..ee9d493 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -32,6 +32,7 @@
 #include <linux/fcntl.h>
 #include <linux/device_cgroup.h>
 #include <linux/fs_struct.h>
+#include <linux/ratelimit.h>
 #include <asm/uaccess.h>
 
 #include "internal.h"
@@ -530,6 +531,60 @@ static inline void path_to_nameidata(struct path *path, struct nameidata *nd)
 	nd->path.dentry = path->dentry;
 }
 
+int protected_sticky_symlinks = CONFIG_PROTECTED_STICKY_SYMLINKS;
+
+/**
+ * may_open_sticky_symlink - Check symlink opening 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 opened when outside a sticky
+ * world-writable directory, or when the uid of the symlink and opener
+ * match, or when the directory owner matches the symlink's owner.
+ *
+ * Returns 0 if opening the symlink is allowed, -ve on error.
+ */
+static __always_inline int
+may_open_sticky_symlink(struct dentry *dentry, struct nameidata *nameidata)
+{
+	int error = 0;
+	const struct inode *parent;
+	const struct inode *inode;
+	const struct cred *cred;
+
+	if (!protected_sticky_symlinks)
+		return 0;
+
+	/* owner and opener match? */
+	cred = current_cred();
+	inode = dentry->d_inode;
+	if (cred->fsuid == inode->i_uid)
+		return 0;
+
+	/* check parent directory mode and owner */
+	spin_lock(&dentry->d_lock);
+	parent = dentry->d_parent->d_inode;
+	if ((parent->i_mode & (S_ISVTX|S_IWOTH)) == (S_ISVTX|S_IWOTH) &&
+	    parent->i_uid != inode->i_uid) {
+		error = -EACCES;
+	}
+	spin_unlock(&dentry->d_lock);
+
+	if (error) {
+		char name[sizeof(current->comm)];
+		printk_ratelimited(KERN_NOTICE "non-matching-uid symlink "
+			"opening attempted in sticky world-writable "
+			"directory by %s (fsuid %d)\n",
+			get_task_comm(name, current), cred->fsuid);
+	}
+	return error;
+}
+
 static __always_inline int
 __do_follow_link(struct path *path, struct nameidata *nd, void **p)
 {
@@ -1844,6 +1899,12 @@ reval:
 			goto exit_dput;
 		if (count++ == 32)
 			goto exit_dput;
+
+		/* check if this symlink is in a sticky world-write dir */
+		error = may_open_sticky_symlink(path.dentry, &nd);
+		if (error)
+			goto exit_dput;
+
 		/*
 		 * This is subtle. Instead of calling do_follow_link() we do
 		 * the thing by hands. The reason is that this way we have zero
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 3428393..0daecd6 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -405,6 +405,7 @@ extern void __init files_init(unsigned long);
 extern struct files_stat_struct files_stat;
 extern int get_max_files(void);
 extern int sysctl_nr_open;
+extern int protected_sticky_symlinks;
 extern struct inodes_stat_t inodes_stat;
 extern int leases_enable, lease_break_time;
 #ifdef CONFIG_DNOTIFY
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 997080f..431f013 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1455,6 +1455,15 @@ static struct ctl_table fs_table[] = {
 #endif
 #endif
 	{
+		.procname	= "protected-sticky-symlinks",
+		.data		= &protected_sticky_symlinks,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= &zero,
+		.extra2		= &one,
+	},
+	{
 		.procname	= "suid_dumpable",
 		.data		= &suid_dumpable,
 		.maxlen		= sizeof(int),
-- 
1.7.0.4


-- 
Kees Cook
Ubuntu Security Team