fs: block cross-uid sticky symlinks

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

security/commoncap.c int cap_inode_killpriv(struct dentry *dentry) · line 297

Quoted 1 line

security/commoncap.c +1 -0

+	if (S_ISLNK(inode->i_mode) &&

Q: is the S_ISLNK() check actually needed?

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 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.  I have added a sysctl to toggle the behavior back to the old handling via /proc/sys/fs/weak-sticky-symlinks, documentation, and a ratelimited warning.

Signed-off-by Kees Cook <[email protected]>

---
 Documentation/sysctl/kernel.txt |   16 ++++++++++++++++ include/linux/security.h        |    1 + kernel/sysctl.c                 |   10 ++++++++++ security/capability.c           |    6 ------ security/commoncap.c            |   25 +++++++++++++++++++++++++

 5 files changed, 52 insertions(+), 6 deletions(-)

Documentation/sysctl/kernel.txt +0 -0

index 3894eaa..6b059f6 100644
@@ -66,6 +66,7 @@ show up in /proc/sys/kernel:

 - threads-max
 - unknown_nmi_panic
 - version
+- weak-sticky-symlinks

 ==============================================================

Documentation/sysctl/kernel.txt +0 -0

@@ -526,3 +527,18 @@ A small number of systems do generate NMI's for bizarre random reasons such as

 power management so the default is off. That sysctl works like the existing panic controls already in that directory.

+==============================================================

Documentation/sysctl/kernel.txt +14 -0

+
+weak-sticky-symlinks:
+
+Following symlinks in world-writable sticky directories (like /tmp) can
+be dangerous due to time-of-check-time-of-use races that frequently result
+in security vulnerabilities.  By default, symlinks can only be followed in
+sticky world-writable directories if the symlink and the follower's uid
+match (or if the symlink is owned by the owner of the world-writable directory
+itself).
+
+The default value is "0".  To disable this protection, setting a value of "1"
+will allow symlinks in sticky world-writable directories to be followed by
+anyone.
+

include/linux/security.h +1 -0

index 0c88191..a06d568 100644
@@ -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);

kernel/sysctl.c +0 -0

index 997080f..bf2d68b 100644

kernel/sysctl.c +9 -0

  extern char core_pattern[];
  extern unsigned int core_pipe_limit;
  extern int pid_max;

@@ -1463,6 +1464,15 @@ 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_minmax,
+               .extra1         = &zero,
+               .extra2         = &one,
+       },
  #if defined(CONFIG_BINFMT_MISC) || defined(CONFIG_BINFMT_MISC_MODULE)
        {
                .procname       = "binfmt_misc",

security/capability.c +0 -6

index 8168e3e..ff34291 100644
@@ -169,12 +169,6 @@ 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;

security/commoncap.c +25 -1

index 4e01599..e7eb397 100644
@@ -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 +284,28 @@ 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|S_IWOTH)) == (S_ISVTX|S_IWOTH) &&
+           parent->i_uid != inode->i_uid &&
+           cred->fsuid != inode->i_uid) {
+               printk_ratelimited(KERN_NOTICE "non-matching-uid symlink "
+                       "following attempted in sticky-directory by "
+                       "%s (fsuid %d)\n", current->comm, cred->fsuid);
+               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

kernel/sysctl.c extern int sysctl_oom_dump_tasks; · line 91

Quoted 7 lines

kernel/sysctl.c +1 -0

@@ -88,6 +88,7 @@ extern int sysctl_oom_dump_tasks;
  extern int max_threads;
  extern int core_uses_pid;
  extern int suid_dumpable;
+extern int weak_sticky_symlinks;

It will be better to put in security.h