[PATCH 1/1][B] UBUNTU: SAUCE: overlayfs: ensure mounter privileges when reading directories

[Tyler Hicks]

From: Andy Whitcroft <apw at canonical.com>

BugLink: https://launchpad.net/bugs/1793458

When reading directory contents ensure the mounter has permissions for
the operation over the constituent parts (lower and upper). Where we are
in a namespace this ensures that the mounter (root in that namespace)
has permissions over the files and directories, preventing exposure of
protected files and directory contents.

CVE-2018-6559

Signed-off-by: Andy Whitcroft <apw at canonical.com>
[tyhicks: make use of new upstream check in ovl_permission() for copy-ups]
[tyhicks: make use of creator (mounter) creds hanging off the super block]
Signed-off-by: Tyler Hicks <tyhicks at canonical.com>
---
 fs/overlayfs/inode.c     |  5 +----
 fs/overlayfs/overlayfs.h |  2 ++
 fs/overlayfs/readdir.c   | 12 ++++++++++++
 fs/overlayfs/util.c      | 13 +++++++++++++
 4 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index 0dbbfedef54c..21905e30f50d 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -171,7 +171,6 @@ int ovl_permission(struct inode *inode, int mask)
 {
 	struct inode *upperinode = ovl_inode_upper(inode);
 	struct inode *realinode = upperinode ?: ovl_inode_lower(inode);
-	const struct cred *old_cred;
 	int err;
 
 	/* Careful in RCU walk mode */
@@ -188,15 +187,13 @@ int ovl_permission(struct inode *inode, int mask)
 	if (err)
 		return err;
 
-	old_cred = ovl_override_creds(inode->i_sb);
 	if (!upperinode &&
 	    !special_file(realinode->i_mode) && mask & MAY_WRITE) {
 		mask &= ~(MAY_WRITE | MAY_APPEND);
 		/* Make sure mounter can read file for copy up later */
 		mask |= MAY_READ;
 	}
-	err = inode_permission(realinode, mask);
-	revert_creds(old_cred);
+	err = ovl_creator_permission(inode->i_sb, realinode, mask);
 
 	return err;
 }
diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index b1917c414e93..7b06983a8447 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -204,6 +204,8 @@ void ovl_drop_write(struct dentry *dentry);
 struct dentry *ovl_workdir(struct dentry *dentry);
 const struct cred *ovl_override_creds(struct super_block *sb);
 struct super_block *ovl_same_sb(struct super_block *sb);
+int ovl_creator_permission(struct super_block *sb, struct inode *inode,
+			   int mode);
 bool ovl_can_decode_fh(struct super_block *sb);
 struct dentry *ovl_indexdir(struct super_block *sb);
 struct ovl_entry *ovl_alloc_entry(unsigned int numlower);
diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index e258c234f357..fa9469325334 100644
--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -369,6 +369,12 @@ static int ovl_dir_read_merged(struct dentry *dentry, struct list_head *list,
 		next = ovl_path_next(idx, dentry, &realpath);
 		rdd.is_upper = ovl_dentry_upper(dentry) == realpath.dentry;
 
+		err = ovl_creator_permission(dentry->d_sb,
+					     d_inode(realpath.dentry),
+					     MAY_READ);
+		if (err)
+			break;
+
 		if (next != -1) {
 			err = ovl_dir_read(&realpath, &rdd);
 			if (err)
@@ -688,6 +694,12 @@ static int ovl_iterate(struct file *file, struct dir_context *ctx)
 		ovl_dir_reset(file);
 
 	if (od->is_real) {
+		err = ovl_creator_permission(dentry->d_sb,
+					     file_inode(od->realfile),
+					     MAY_READ);
+		if (err)
+			return err;
+
 		/*
 		 * If parent is merge, then need to adjust d_ino for '..', if
 		 * dir is impure then need to adjust d_ino for copied up
diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
index 06119f34a69d..ea2c28fd7be5 100644
--- a/fs/overlayfs/util.c
+++ b/fs/overlayfs/util.c
@@ -50,6 +50,19 @@ struct super_block *ovl_same_sb(struct super_block *sb)
 	return ofs->same_sb;
 }
 
+int ovl_creator_permission(struct super_block *sb, struct inode *inode,
+			   int mode)
+{
+	const struct cred *old_cred;
+	int err = 0;
+
+	old_cred = ovl_override_creds(sb);
+	err = inode_permission(inode, mode);
+	revert_creds(old_cred);
+
+	return err;
+}
+
 bool ovl_can_decode_fh(struct super_block *sb)
 {
 	return (sb->s_export_op && sb->s_export_op->fh_to_dentry &&
-- 
2.7.4