[CVE-2015-2925][Precise][PATCH 1/2] dcache: Handle escaped paths in prepend_path

[Luis Henriques]

From: "Eric W. Biederman" <ebiederm at xmission.com>

commit cde93be45a8a90d8c264c776fab63487b5038a65 upstream.

A rename can result in a dentry that by walking up d_parent
will never reach it's mnt_root.  For lack of a better term
I call this an escaped path.

prepend_path is called by four different functions __d_path,
d_absolute_path, d_path, and getcwd.

__d_path only wants to see paths are connected to the root it passes
in.  So __d_path needs prepend_path to return an error.

d_absolute_path similarly wants to see paths that are connected to
some root.  Escaped paths are not connected to any mnt_root so
d_absolute_path needs prepend_path to return an error greater
than 1.  So escaped paths will be treated like paths on lazily
unmounted mounts.

getcwd needs to prepend "(unreachable)" so getcwd also needs
prepend_path to return an error.

d_path is the interesting hold out.  d_path just wants to print
something, and does not care about the weird cases.  Which raises
the question what should be printed?

Given that <escaped_path>/<anything> should result in -ENOENT I
believe it is desirable for escaped paths to be printed as empty
paths.  As there are not really any meaninful path components when
considered from the perspective of a mount tree.

So tweak prepend_path to return an empty path with an new error
code of 3 when it encounters an escaped path.

Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>
Signed-off-by: Al Viro <viro at zeniv.linux.org.uk>
CVE-2015-2925
BugLink: https://bugs.launchpad.net/bugs/1441108
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 fs/dcache.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/dcache.c b/fs/dcache.c
index a419b148c220..2d4bdd87d9e6 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2453,6 +2453,8 @@ static int prepend_path(const struct path *path,
 {
 	struct dentry *dentry = path->dentry;
 	struct vfsmount *vfsmnt = path->mnt;
+	char *orig_buffer = *buffer;
+	int orig_len = *buflen;
 	bool slash = false;
 	int error = 0;
 
@@ -2460,6 +2462,14 @@ static int prepend_path(const struct path *path,
 		struct dentry * parent;
 
 		if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) {
+			/* Escaped? */
+			if (dentry != vfsmnt->mnt_root) {
+				*buffer = orig_buffer;
+				*buflen = orig_len;
+				slash = false;
+				error = 3;
+				goto global_root;
+			}
 			/* Global root? */
 			if (!mnt_has_parent(vfsmnt))
 				goto global_root;