[PATCH] UBUNTU: SAUCE: fs: block cross-uid sticky symlinks

[Kees Cook]

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).

The solution is to not permit symlinks to be followed when users do not
match, but only in a world-writable sticky directory (with an additional
improvement that the directory owner's symlinks can always be followed,
regardless who is following them).

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. Also, please reference
    where POSIX says this.
- 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 grsecurity, which is similar to the
patch in Openwall.  I have added a sysctl to toggle the behavior back
to the old handling via /proc/sys/fs/weak-sticky-symlinks, as well as
a ratelimited deprecation warning.

Signed-off-by: Kees Cook <kees.cook at canonical.com>
---
 include/linux/security.h |    1 +
 kernel/sysctl.c          |    8 ++++++++
 security/capability.c    |    6 ------
 security/commoncap.c     |   23 +++++++++++++++++++++++
 4 files changed, 32 insertions(+), 6 deletions(-)

diff --git a/include/linux/security.h b/include/linux/security.h
index 3158dd9..92eca95 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -67,6 +67,7 @@ extern int cap_inode_setxattr(struct dentry *dentry, const char *name,
 extern int cap_inode_removexattr(struct dentry *dentry, const char *name);
 extern int cap_inode_need_killpriv(struct dentry *dentry);
 extern int cap_inode_killpriv(struct dentry *dentry);
+extern int cap_inode_follow_link(struct dentry *dentry, struct nameidata *nd);
 extern int cap_file_mmap(struct file *file, unsigned long reqprot,
 			 unsigned long prot, unsigned long flags,
 			 unsigned long addr, unsigned long addr_only);
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 8686b0f..36a104c 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -86,6 +86,7 @@ extern int sysctl_oom_dump_tasks;
 extern int max_threads;
 extern int core_uses_pid;
 extern int suid_dumpable;
+extern int weak_sticky_symlinks;
 extern char core_pattern[];
 extern unsigned int core_pipe_limit;
 extern int pid_max;
@@ -1416,6 +1417,13 @@ static struct ctl_table fs_table[] = {
 		.extra1		= &zero,
 		.extra2		= &two,
 	},
+	{
+		.procname	= "weak-sticky-symlinks",
+		.data		= &weak_sticky_symlinks,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= &proc_dointvec,
+	},
 #if defined(CONFIG_BINFMT_MISC) || defined(CONFIG_BINFMT_MISC_MODULE)
 	{
 		.procname	= "binfmt_misc",
diff --git a/security/capability.c b/security/capability.c
index 4875142..d4633f3 100644
--- a/security/capability.c
+++ b/security/capability.c
@@ -200,12 +200,6 @@ static int cap_inode_readlink(struct dentry *dentry)
 	return 0;
 }
 
-static int cap_inode_follow_link(struct dentry *dentry,
-				 struct nameidata *nameidata)
-{
-	return 0;
-}
-
 static int cap_inode_permission(struct inode *inode, int mask)
 {
 	return 0;
diff --git a/security/commoncap.c b/security/commoncap.c
index 6166973..83d5a18 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -29,6 +29,9 @@
 #include <linux/securebits.h>
 #include <linux/syslog.h>
 
+/* sysctl for symlink permissions checking */
+int weak_sticky_symlinks;
+
 /*
  * If a non-root user executes a setuid-root binary in
  * !secure(SECURE_NOROOT) mode, then we raise capabilities.
@@ -281,6 +284,27 @@ int cap_inode_killpriv(struct dentry *dentry)
 	return inode->i_op->removexattr(dentry, XATTR_NAME_CAPS);
 }
 
+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) && (parent->i_mode & S_ISVTX) &&
+	    (parent->i_mode & S_IWOTH) && (parent->i_uid != inode->i_uid) &&
+	    (cred->fsuid != inode->i_uid)) {
+		printk_ratelimited(KERN_INFO "deprecated sticky-directory"
+			" non-matching uid symlink following was attempted"
+			" by: %s\n", current->comm);
+		return -EACCES;
+	}
+	return 0;
+}
+
 /*
  * Calculate the new process capability sets from the capability sets attached
  * to a file.
-- 
1.7.0.4


-- 
Kees Cook
Ubuntu Security Team