fix apparmor dereferencing potentially freed dentry, sanitize __d_path() API

__d_path() API is asking for trouble and in case of apparmor d_namespace_path()
getting just that.  The root cause is that when __d_path() misses the root
it had been told to look for, it stores the location of the most remote ancestor
in *root.  Without grabbing references.  Sure, at the moment of call it had
been pinned down by what we have in *path.  And if we raced with umount -l, we
could have very well stopped at vfsmount/dentry that got freed as soon as
prepend_path() dropped vfsmount_lock.

It is safe to compare these pointers with pre-existing (and known to be still
alive) vfsmount and dentry, as long as all we are asking is "is it the same
address?".  Dereferencing is not safe and apparmor ended up stepping into
that.  d_namespace_path() really wants to examine the place where we stopped,
even if it's not connected to our namespace.  As the result, it looked
at ->d_sb->s_magic of a dentry that might've been already freed by that point.
All other callers had been careful enough to avoid that, but it's really
a bad interface - it invites that kind of trouble.

The fix is fairly straightforward, even though it's bigger than I'd like:
	* prepend_path() root argument becomes const.
	* __d_path() is never called with NULL/NULL root.  It was a kludge
to start with.  Instead, we have an explicit function - d_absolute_root().
Same as __d_path(), except that it doesn't get root passed and stops where
it stops.  apparmor and tomoyo are using it.
	* __d_path() returns NULL on path outside of root.  The main
caller is show_mountinfo() and that's precisely what we pass root for - to
skip those outside chroot jail.  Those who don't want that can (and do)
use d_path().
	* __d_path() root argument becomes const.  Everyone agrees, I hope.
	* apparmor does *NOT* try to use __d_path() or any of its variants
when it sees that path->mnt is an internal vfsmount.  In that case it's
definitely not mounted anywhere and dentry_path() is exactly what we want
there.  Handling of sysctl()-triggered weirdness is moved to that place.
	* if apparmor is asked to do pathname relative to chroot jail
and __d_path() tells it we it's not in that jail, the sucker just calls
d_absolute_path() instead.  That's the other remaining caller of __d_path(),
BTW.
        * seq_path_root() does _NOT_ return -ENAMETOOLONG (it's stupid anyway -
the normal seq_file logics will take care of growing the buffer and redoing
the call of ->show() just fine).  However, if it gets path not reachable
from root, it returns SEQ_SKIP.  The only caller adjusted (i.e. stopped
ignoring the return value as it used to do).

Reviewed-by: John Johansen <john.johansen@canonical.com>
ACKed-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: stable@vger.kernel.org

1 files changed, 38 insertions, 27 deletions

diff --git a/security/apparmor/path.c b/security/apparmor/path.c
index 36cc0cc39e78..b566eba4a65c 100644
--- a/security/apparmor/path.c
+++ b/security/apparmor/path.c
@@ -57,23 +57,44 @@ static int prepend(char **buffer, int buflen, const char *str, int namelen)
 static int d_namespace_path(struct path *path, char *buf, int buflen,
 			    char **name, int flags)
 {
-	struct path root, tmp;
 	char *res;
-	int connected, error = 0;
+	int error = 0;
+	int connected = 1;
+
+	if (path->mnt->mnt_flags & MNT_INTERNAL) {
+		/* it's not mounted anywhere */
+		res = dentry_path(path->dentry, buf, buflen);
+		*name = res;
+		if (IS_ERR(res)) {
+			*name = buf;
+			return PTR_ERR(res);
+		}
+		if (path->dentry->d_sb->s_magic == PROC_SUPER_MAGIC &&
+		    strncmp(*name, "/sys/", 5) == 0) {
+			/* TODO: convert over to using a per namespace
+			 * control instead of hard coded /proc
+			 */
+			return prepend(name, *name - buf, "/proc", 5);
+		}
+		return 0;
+	}
 
-	/* Get the root we want to resolve too, released below */
+	/* resolve paths relative to chroot?*/
 	if (flags & PATH_CHROOT_REL) {
-		/* resolve paths relative to chroot */
+		struct path root;
 		get_fs_root(current->fs, &root);
-	} else {
-		/* resolve paths relative to namespace */
-		root.mnt = current->nsproxy->mnt_ns->root;
-		root.dentry = root.mnt->mnt_root;
-		path_get(&root);
+		res = __d_path(path, &root, buf, buflen);
+		if (res && !IS_ERR(res)) {
+			/* everything's fine */
+			*name = res;
+			path_put(&root);
+			goto ok;
+		}
+		path_put(&root);
+		connected = 0;
 	}
 
-	tmp = root;
-	res = __d_path(path, &tmp, buf, buflen);
+	res = d_absolute_path(path, buf, buflen);
 
 	*name = res;
 	/* handle error conditions - and still allow a partial path to
@@ -84,7 +105,10 @@ static int d_namespace_path(struct path *path, char *buf, int buflen,
 		*name = buf;
 		goto out;
 	}
+	if (!our_mnt(path->mnt))
+		connected = 0;
 
+ok:
 	/* Handle two cases:
 	 * 1. A deleted dentry && profile is not allowing mediation of deleted
 	 * 2. On some filesystems, newly allocated dentries appear to the
@@ -97,10 +121,7 @@ static int d_namespace_path(struct path *path, char *buf, int buflen,
 			goto out;
 	}
 
-	/* Determine if the path is connected to the expected root */
-	connected = tmp.dentry == root.dentry && tmp.mnt == root.mnt;
-
-	/* If the path is not connected,
+	/* If the path is not connected to the expected root,
 	 * check if it is a sysctl and handle specially else remove any
 	 * leading / that __d_path may have returned.
 	 * Unless
@@ -112,17 +133,9 @@ static int d_namespace_path(struct path *path, char *buf, int buflen,
 	 *     namespace root.
 	 */
 	if (!connected) {
-		/* is the disconnect path a sysctl? */
-		if (tmp.dentry->d_sb->s_magic == PROC_SUPER_MAGIC &&
-		    strncmp(*name, "/sys/", 5) == 0) {
-			/* TODO: convert over to using a per namespace
-			 * control instead of hard coded /proc
-			 */
-			error = prepend(name, *name - buf, "/proc", 5);
-		} else if (!(flags & PATH_CONNECT_PATH) &&
+		if (!(flags & PATH_CONNECT_PATH) &&
 			   !(((flags & CHROOT_NSCONNECT) == CHROOT_NSCONNECT) &&
-			     (tmp.mnt == current->nsproxy->mnt_ns->root &&
-			      tmp.dentry == tmp.mnt->mnt_root))) {
+			     our_mnt(path->mnt))) {
 			/* disconnected path, don't return pathname starting
 			 * with '/'
 			 */
@@ -133,8 +146,6 @@ static int d_namespace_path(struct path *path, char *buf, int buflen,
 	}
 
 out:
-	path_put(&root);
-
 	return error;
 }