UBUNTU: SAUCE: fs: block hardlinks to non-accessible sources

Hardlinks can be abused in a similar fashion to symlinks above, but they
are not limited to world-writable directories. If /etc and /home are on
the same partition, a regular user can create a hardlink to /etc/shadow in
their home directory. While it retains the original owner and permissions,
it is possible for privileged programs that are otherwise symlink-safe
to mistakenly access the file through its hardlink. Additionally, a very
minor untraceable quota-bypassing local denial of service is possible by
an attacker exhausting disk space by filling a world-writable directory
with hardlinks.

The solution is to not allow the creation of hardlinks to files that a
given user would be unable to read or write originally, or are otherwise
sensitive.

Some links to the history of its discussion:

  1997 Dec, Yuri Kuzmenko http://lkml.org/lkml/1997/12/29/20
  2002 Apr, Chris Wright http://lkml.org/lkml/2002/4/13/99

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 atd, courier, and other unknown applications that use this
   feature.
   - These applications are easy to spot and can be tested and
     fixed. Applications that are vulnerable to hardlink attacks by not
     having the change aren't.
 - Applications should correctly drop privileges before attempting to
   access user files.
   - 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-nonaccess-hardlinks, as well as
a ratelimited deprecation warning.

Signed-off-by: Kees Cook <kees.cook@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Leann Ogasawara <leann.ogasawara@canonical.com>

5 files changed, 62 insertions, 7 deletions

diff --git a/include/linux/security.h b/include/linux/security.h
index a06d5685326..edc137ff41f 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -79,6 +79,10 @@ extern int cap_task_setioprio(struct task_struct *p, int ioprio);
 extern int cap_task_setnice(struct task_struct *p, int nice);
 extern int cap_syslog(int type, bool from_file);
 extern int cap_vm_enough_memory(struct mm_struct *mm, long pages);
+#ifdef CONFIG_SECURITY_PATH
+extern int cap_path_link(struct dentry *old_dentry, struct path *new_dir,
+			 struct dentry *new_dentry);
+#endif
 
 struct msghdr;
 struct sk_buff;
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 313bc9a8255..9be395856eb 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -89,6 +89,7 @@ extern int max_threads;
 extern int core_uses_pid;
 extern int suid_dumpable;
 extern int weak_sticky_symlinks;
+extern int weak_nonaccess_hardlinks;
 extern char core_pattern[];
 extern unsigned int core_pipe_limit;
 extern int pid_max;
@@ -1471,6 +1472,15 @@ static struct ctl_table fs_table[] = {
 		.mode		= 0644,
 		.proc_handler	= &proc_dointvec,
 	},
+#ifdef CONFIG_SECURITY_PATH
+	{
+		.procname	= "weak-nonaccess-hardlinks",
+		.data		= &weak_nonaccess_hardlinks,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= &proc_dointvec,
+	},
+#endif
 #if defined(CONFIG_BINFMT_MISC) || defined(CONFIG_BINFMT_MISC_MODULE)
 	{
 		.procname	= "binfmt_misc",
diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
index 814b086f072..82e222dce8b 100644
--- a/security/apparmor/lsm.c
+++ b/security/apparmor/lsm.c
@@ -317,7 +317,10 @@ static int apparmor_path_link(struct dentry *old_dentry, struct path *new_dir,
 			      struct dentry *new_dentry)
 {
 	struct aa_profile *profile;
-	int error = 0;
+	int error = 0, rc;
+
+	if ( (rc = cap_path_link(old_dentry, new_dir, new_dentry)) )
+		return rc;
 
 	if (!mediated_filesystem(old_dentry->d_inode))
 		return 0;
diff --git a/security/capability.c b/security/capability.c
index ff34291111a..76593e227c1 100644
--- a/security/capability.c
+++ b/security/capability.c
@@ -250,12 +250,6 @@ static int cap_path_symlink(struct path *dir, struct dentry *dentry,
 	return 0;
 }
 
-static int cap_path_link(struct dentry *old_dentry, struct path *new_dir,
-			 struct dentry *new_dentry)
-{
-	return 0;
-}
-
 static int cap_path_rename(struct path *old_path, struct dentry *old_dentry,
 			   struct path *new_path, struct dentry *new_dentry)
 {
diff --git a/security/commoncap.c b/security/commoncap.c
index 167f170e8e9..b906188f349 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -31,6 +31,8 @@
 
 /* sysctl for symlink permissions checking */
 int weak_sticky_symlinks;
+/* sysctl for hardlink permissions checking */
+int weak_nonaccess_hardlinks;
 
 /*
  * If a non-root user executes a setuid-root binary in
@@ -305,6 +307,48 @@ int cap_inode_follow_link(struct dentry *dentry,
 	return 0;
 }
 
+#ifdef CONFIG_SECURITY_PATH
+/*
+ * cap_path_link - verify that hardlinking is allowed
+ * @old_dentry: the source inode/dentry to hardlink from
+ * @new_dir: target directory
+ * @new_dentry: the target inode/dentry to hardlink to
+ *
+ * Block hardlink when all of:
+ *  - fsuid does not match inode
+ *  - not CAP_FOWNER
+ *  - and at least one of:
+ *    - inode is not a regular file
+ *    - inode is setuid
+ *    - inode is setgid and group-exec
+ *    - access failure for read or write
+ *
+ * Returns 0 if successful, -ve on error.
+ */
+int cap_path_link(struct dentry *old_dentry, struct path *new_dir,
+		  struct dentry *new_dentry)
+{
+	struct inode *inode = old_dentry->d_inode;
+	const int mode = inode->i_mode;
+	const struct cred *cred = current_cred();
+
+	if (weak_nonaccess_hardlinks) return 0;
+
+	if (cred->fsuid != inode->i_uid &&
+	    (!S_ISREG(mode) || (mode & S_ISUID) ||
+	     ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) ||
+	     (generic_permission(inode, MAY_READ | MAY_WRITE, NULL))) &&
+	    !capable(CAP_FOWNER)) {
+		printk_ratelimited(KERN_INFO "deprecated non-accessible"
+			" hardlink creation was attempted by: %s\n",
+			current->comm);
+		return -EPERM;
+	}
+
+	return 0;
+}
+#endif /* CONFIG_SECURITY_PATH */
+
 /*
  * Calculate the new process capability sets from the capability sets attached
  * to a file.