vfs: Fix possible escape from mount namespace (CVE-2015-2925)

debian/changelog

@@ -8,6 +8,11 @@ linux (4.2-1~exp2) UNRELEASED; urgency=medium
     directly
   * workqueue: Make flush_workqueue() available again to non GPL modules
     (Closes: #798311)
+  * vfs: Fix possible escape from mount namespace (CVE-2015-2925):
+    - namei: lift (open-coded) terminate_walk() in follow_dotdot_rcu() into
+      callers
+    - dcache: Handle escaped paths in prepend_path
+    - vfs: Test for and handle paths that are unreachable from their mnt_root
 
   [ Aurelien Jarno ]
   * [mips*el] Fix BPF assembly code for pre-R2 CPUs. (fixes FTBFS)

debian/patches/bugfix/all/dcache-handle-escaped-paths-in-prepend_path.patch

@@ -0,0 +1,58 @@
+From: "Eric W. Biederman" <ebiederm@xmission.com>
+Date: Sat, 15 Aug 2015 13:36:12 -0500
+Subject: dcache: Handle escaped paths in prepend_path
+Origin: https://git.kernel.org/linus/cde93be45a8a90d8c264c776fab63487b5038a65
+
+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@xmission.com>
+Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
+---
+ fs/dcache.c | 7 +++++++
+ 1 file changed, 7 insertions(+)
+
+--- a/fs/dcache.c
++++ b/fs/dcache.c
+@@ -2927,6 +2927,13 @@ restart:
+ 
+ 		if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) {
+ 			struct mount *parent = ACCESS_ONCE(mnt->mnt_parent);
++			/* Escaped? */
++			if (dentry != vfsmnt->mnt_root) {
++				bptr = *buffer;
++				blen = *buflen;
++				error = 3;
++				break;
++			}
+ 			/* Global root? */
+ 			if (mnt != parent) {
+ 				dentry = ACCESS_ONCE(mnt->mnt_mountpoint);

debian/patches/bugfix/all/dcache-reduce-the-scope-of-i_lock-in-d_splice_alias.patch

@@ -0,0 +1,65 @@
+From: "Eric W. Biederman" <ebiederm@xmission.com>
+Date: Sat, 15 Aug 2015 13:36:41 -0500
+Subject: dcache: Reduce the scope of i_lock in d_splice_alias
+Origin: https://git.kernel.org/linus/a03e283bf5c3d4851b4998122196ce9f849e6dfb
+
+i_lock is only needed until __d_find_any_alias calls dget on the alias
+dentry.  After that the reference to new ensures that dentry_kill and
+d_delete will not remove the inode from the dentry, and remove the
+dentry from the inode->d_entry list.
+
+The inode i_lock came to be held over the the __d_move calls in
+d_splice_alias through a series of introduction of locks with
+increasing smaller scope.  First it was the dcache_lock, then
+it was the dcache_inode_lock, and finally inode->i_lock.
+
+Furthermore inode->i_lock is not held over any other calls
+to d_move or __d_move so it can not provide any meaningful
+rename protection.
+
+Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
+Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
+---
+ fs/dcache.c | 7 +++----
+ 1 file changed, 3 insertions(+), 4 deletions(-)
+
+--- a/fs/dcache.c
++++ b/fs/dcache.c
+@@ -2719,7 +2719,7 @@ struct dentry *d_ancestor(struct dentry
+  * This helper attempts to cope with remotely renamed directories
+  *
+  * It assumes that the caller is already holding
+- * dentry->d_parent->d_inode->i_mutex, inode->i_lock and rename_lock
++ * dentry->d_parent->d_inode->i_mutex, and rename_lock
+  *
+  * Note: If ever the locking in lock_rename() changes, then please
+  * remember to update this too...
+@@ -2745,7 +2745,6 @@ out_unalias:
+ 	__d_move(alias, dentry, false);
+ 	ret = 0;
+ out_err:
+-	spin_unlock(&inode->i_lock);
+ 	if (m2)
+ 		mutex_unlock(m2);
+ 	if (m1)
+@@ -2791,10 +2790,11 @@ struct dentry *d_splice_alias(struct ino
+ 	if (S_ISDIR(inode->i_mode)) {
+ 		struct dentry *new = __d_find_any_alias(inode);
+ 		if (unlikely(new)) {
++			/* The reference to new ensures it remains an alias */
++			spin_unlock(&inode->i_lock);
+ 			write_seqlock(&rename_lock);
+ 			if (unlikely(d_ancestor(new, dentry))) {
+ 				write_sequnlock(&rename_lock);
+-				spin_unlock(&inode->i_lock);
+ 				dput(new);
+ 				new = ERR_PTR(-ELOOP);
+ 				pr_warn_ratelimited(
+@@ -2813,7 +2813,6 @@ struct dentry *d_splice_alias(struct ino
+ 			} else {
+ 				__d_move(new, dentry, false);
+ 				write_sequnlock(&rename_lock);
+-				spin_unlock(&inode->i_lock);
+ 				security_d_instantiate(new, inode);
+ 			}
+ 			iput(inode);

debian/patches/bugfix/all/vfs-test-for-and-handle-paths-that-are-unreachable-f.patch

@@ -0,0 +1,104 @@
+From: "Eric W. Biederman" <ebiederm@xmission.com>
+Date: Sat, 15 Aug 2015 20:27:13 -0500
+Subject: vfs: Test for and handle paths that are unreachable from their mnt_root
+Origin: https://git.kernel.org/linus/397d425dc26da728396e66d392d5dcb8dac30c37
+
+In rare cases a directory can be renamed out from under a bind mount.
+In those cases without special handling it becomes possible to walk up
+the directory tree to the root dentry of the filesystem and down
+from the root dentry to every other file or directory on the filesystem.
+
+Like division by zero .. from an unconnected path can not be given
+a useful semantic as there is no predicting at which path component
+the code will realize it is unconnected.  We certainly can not match
+the current behavior as the current behavior is a security hole.
+
+Therefore when encounting .. when following an unconnected path
+return -ENOENT.
+
+- Add a function path_connected to verify path->dentry is reachable
+  from path->mnt.mnt_root.  AKA to validate that rename did not do
+  something nasty to the bind mount.
+
+  To avoid races path_connected must be called after following a path
+  component to it's next path component.
+
+Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
+Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
+---
+ fs/namei.c | 27 +++++++++++++++++++++++++--
+ 1 file changed, 25 insertions(+), 2 deletions(-)
+
+diff --git a/fs/namei.c b/fs/namei.c
+index 1c2105e..29b9279 100644
+--- a/fs/namei.c
++++ b/fs/namei.c
+@@ -560,6 +560,24 @@ static int __nd_alloc_stack(struct nameidata *nd)
+ 	return 0;
+ }
+ 
++/**
++ * path_connected - Verify that a path->dentry is below path->mnt.mnt_root
++ * @path: nameidate to verify
++ *
++ * Rename can sometimes move a file or directory outside of a bind
++ * mount, path_connected allows those cases to be detected.
++ */
++static bool path_connected(const struct path *path)
++{
++	struct vfsmount *mnt = path->mnt;
++
++	/* Only bind mounts can have disconnected paths */
++	if (mnt->mnt_root == mnt->mnt_sb->s_root)
++		return true;
++
++	return is_subdir(path->dentry, mnt->mnt_root);
++}
++
+ static inline int nd_alloc_stack(struct nameidata *nd)
+ {
+ 	if (likely(nd->depth != EMBEDDED_LEVELS))
+@@ -1296,6 +1314,8 @@ static int follow_dotdot_rcu(struct nameidata *nd)
+ 				return -ECHILD;
+ 			nd->path.dentry = parent;
+ 			nd->seq = seq;
++			if (unlikely(!path_connected(&nd->path)))
++				return -ENOENT;
+ 			break;
+ 		} else {
+ 			struct mount *mnt = real_mount(nd->path.mnt);
+@@ -1396,7 +1416,7 @@ static void follow_mount(struct path *path)
+ 	}
+ }
+ 
+-static void follow_dotdot(struct nameidata *nd)
++static int follow_dotdot(struct nameidata *nd)
+ {
+ 	if (!nd->root.mnt)
+ 		set_root(nd);
+@@ -1412,6 +1432,8 @@ static void follow_dotdot(struct nameidata *nd)
+ 			/* rare case of legitimate dget_parent()... */
+ 			nd->path.dentry = dget_parent(nd->path.dentry);
+ 			dput(old);
++			if (unlikely(!path_connected(&nd->path)))
++				return -ENOENT;
+ 			break;
+ 		}
+ 		if (!follow_up(&nd->path))
+@@ -1419,6 +1441,7 @@ static void follow_dotdot(struct nameidata *nd)
+ 	}
+ 	follow_mount(&nd->path);
+ 	nd->inode = nd->path.dentry->d_inode;
++	return 0;
+ }
+ 
+ /*
+@@ -1634,7 +1657,7 @@ static inline int handle_dots(struct nameidata *nd, int type)
+ 		if (nd->flags & LOOKUP_RCU) {
+ 			return follow_dotdot_rcu(nd);
+ 		} else
+-			follow_dotdot(nd);
++			return follow_dotdot(nd);
+ 	}
+ 	return 0;
+ }

debian/patches/series

@@ -91,3 +91,6 @@ features/all/grsecurity/grkernsec_perf_harden.patch
 
 bugfix/all/gfs2-make-statistics-unsigned-suitable-for-use-with-.patch
 bugfix/all/workqueue-make-flush_workqueue-available-again-to-no.patch
+bugfix/all/dcache-handle-escaped-paths-in-prepend_path.patch
+bugfix/all/dcache-reduce-the-scope-of-i_lock-in-d_splice_alias.patch
+bugfix/all/vfs-test-for-and-handle-paths-that-are-unreachable-f.patch