Vulnerability: fs/overlayfs/dir.c in the OverlayFS filesystem implementation in the Linux kernel before 4.6 does not properly verify the upper dentry before proceeding with unlink and rename system-call processing, which allows local users to cause a denial of service (system crash) via a rename system call that specifies a self-hardlink.
Vulnerability ID: CVE-2016-6197
CWE: CWE-20
Severity score: 6.9
Project: https://github.com/torvalds/linux
Published: 2016-08-06
File: fs/overlayfs/dir.c
Commit message: ovl: verify upper dentry before unlink and rename

Unlink and rename in overlayfs checked the upper dentry for staleness by
verifying upper->d_parent against upperdir.  However the dentry can go
stale also by being unhashed, for example.

Expand the verification to actually look up the name again (under parent
lock) and check if it matches the upper dentry.  This matches what the VFS
does before passing the dentry to filesytem's unlink/rename methods, which
excludes any inconsistency caused by overlayfs.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Commit author: Miklos Szeredi

Vulnerable version (before fix):
 static int ovl_remove_upper(struct dentry *dentry, bool is_dir) { 	struct dentry *upperdir = ovl_dentry_upper(dentry->d_parent); 	struct inode *dir = upperdir->d_inode; 	struct dentry *upper = ovl_dentry_upper(dentry); 	int err;  	inode_lock_nested(dir, I_MUTEX_PARENT); 	err = -ESTALE; 	if (upper->d_parent == upperdir) { 		/* Don't let d_delete() think it can reset d_inode */ 		dget(upper); 		if (is_dir) 			err = vfs_rmdir(dir, upper); 		else 			err = vfs_unlink(dir, upper, NULL); 		dput(upper); 		ovl_dentry_version_inc(dentry->d_parent); 	}  	/* 	 * Keeping this dentry hashed would mean having to release static int ovl_remove_upper(struct dentry *dentry, bool is_dir) 	 */ 	if (!err) 		d_drop(dentry); 	inode_unlock(dir);  	return err; static int ovl_rename2(struct inode *olddir, struct dentry *old,  	trap = lock_rename(new_upperdir, old_upperdir);  	olddentry = ovl_dentry_upper(old); 	newdentry = ovl_dentry_upper(new); 	if (newdentry) { 		if (opaquedir) { 			newdentry = opaquedir; 			opaquedir = NULL; 		} else { 			dget(newdentry); 		} 	} else { 		new_create = true; 		newdentry = lookup_one_len(new->d_name.name, new_upperdir, 					   new->d_name.len); 		err = PTR_ERR(newdentry); 		if (IS_ERR(newdentry)) 			goto out_unlock; 	}  	err = -ESTALE; 	if (olddentry->d_parent != old_upperdir) 		goto out_dput; 	if (newdentry->d_parent != new_upperdir) 		goto out_dput; 	if (olddentry == trap) 		goto out_dput; 	if (newdentry == trap) static int ovl_rename2(struct inode *olddir, struct dentry *old,  out_dput: 	dput(newdentry); out_unlock: 	unlock_rename(new_upperdir, old_upperdir); out_revert_creds:

Fixed version (after fix):
 static int ovl_remove_upper(struct dentry *dentry, bool is_dir) { 	struct dentry *upperdir = ovl_dentry_upper(dentry->d_parent); 	struct inode *dir = upperdir->d_inode; 	struct dentry *upper; 	int err;  	inode_lock_nested(dir, I_MUTEX_PARENT); 	upper = lookup_one_len(dentry->d_name.name, upperdir, 			       dentry->d_name.len); 	err = PTR_ERR(upper); 	if (IS_ERR(upper)) 		goto out_unlock;  	err = -ESTALE; 	if (upper == ovl_dentry_upper(dentry)) { 		if (is_dir) 			err = vfs_rmdir(dir, upper); 		else 			err = vfs_unlink(dir, upper, NULL); 		ovl_dentry_version_inc(dentry->d_parent); 	} 	dput(upper);  	/* 	 * Keeping this dentry hashed would mean having to release static int ovl_remove_upper(struct dentry *dentry, bool is_dir) 	 */ 	if (!err) 		d_drop(dentry); out_unlock: 	inode_unlock(dir);  	return err; static int ovl_rename2(struct inode *olddir, struct dentry *old,  	trap = lock_rename(new_upperdir, old_upperdir);   	olddentry = lookup_one_len(old->d_name.name, old_upperdir, 				   old->d_name.len); 	err = PTR_ERR(olddentry); 	if (IS_ERR(olddentry)) 		goto out_unlock;  	err = -ESTALE; 	if (olddentry != ovl_dentry_upper(old)) 		goto out_dput_old;  	newdentry = lookup_one_len(new->d_name.name, new_upperdir, 				   new->d_name.len); 	err = PTR_ERR(newdentry); 	if (IS_ERR(newdentry)) 		goto out_dput_old;  	err = -ESTALE; 	if (ovl_dentry_upper(new)) { 		if (opaquedir) { 			if (newdentry != opaquedir) 				goto out_dput; 		} else { 			if (newdentry != ovl_dentry_upper(new)) 				goto out_dput; 		} 	} else { 		new_create = true; 		if (!d_is_negative(newdentry) && 		    (!new_opaque || !ovl_is_whiteout(newdentry))) 			goto out_dput; 	}  	if (olddentry == trap) 		goto out_dput; 	if (newdentry == trap) static int ovl_rename2(struct inode *olddir, struct dentry *old,  out_dput: 	dput(newdentry); out_dput_old: 	dput(olddentry); out_unlock: 	unlock_rename(new_upperdir, old_upperdir); out_revert_creds:

Patch:
patch: @@ -596,21 +596,25 @@ static int ovl_remove_upper(struct dentry *dentry, bool is_dir)
 {
 	struct dentry *upperdir = ovl_dentry_upper(dentry->d_parent);
 	struct inode *dir = upperdir->d_inode;
-	struct dentry *upper = ovl_dentry_upper(dentry);
+	struct dentry *upper;
 	int err;
 
 	inode_lock_nested(dir, I_MUTEX_PARENT);
+	upper = lookup_one_len(dentry->d_name.name, upperdir,
+			       dentry->d_name.len);
+	err = PTR_ERR(upper);
+	if (IS_ERR(upper))
+		goto out_unlock;
+
 	err = -ESTALE;
-	if (upper->d_parent == upperdir) {
-		/* Don't let d_delete() think it can reset d_inode */
-		dget(upper);
+	if (upper == ovl_dentry_upper(dentry)) {
 		if (is_dir)
 			err = vfs_rmdir(dir, upper);
 		else
 			err = vfs_unlink(dir, upper, NULL);
-		dput(upper);
 		ovl_dentry_version_inc(dentry->d_parent);
 	}
+	dput(upper);
 
 	/*
 	 * Keeping this dentry hashed would mean having to release
@@ -620,6 +624,7 @@ static int ovl_remove_upper(struct dentry *dentry, bool is_dir)
 	 */
 	if (!err)
 		d_drop(dentry);
+out_unlock:
 	inode_unlock(dir);
 
 	return err;
@@ -840,29 +845,39 @@ static int ovl_rename2(struct inode *olddir, struct dentry *old,
 
 	trap = lock_rename(new_upperdir, old_upperdir);
 
-	olddentry = ovl_dentry_upper(old);
-	newdentry = ovl_dentry_upper(new);
-	if (newdentry) {
+
+	olddentry = lookup_one_len(old->d_name.name, old_upperdir,
+				   old->d_name.len);
+	err = PTR_ERR(olddentry);
+	if (IS_ERR(olddentry))
+		goto out_unlock;
+
+	err = -ESTALE;
+	if (olddentry != ovl_dentry_upper(old))
+		goto out_dput_old;
+
+	newdentry = lookup_one_len(new->d_name.name, new_upperdir,
+				   new->d_name.len);
+	err = PTR_ERR(newdentry);
+	if (IS_ERR(newdentry))
+		goto out_dput_old;
+
+	err = -ESTALE;
+	if (ovl_dentry_upper(new)) {
 		if (opaquedir) {
-			newdentry = opaquedir;
-			opaquedir = NULL;
+			if (newdentry != opaquedir)
+				goto out_dput;
 		} else {
-			dget(newdentry);
+			if (newdentry != ovl_dentry_upper(new))
+				goto out_dput;
 		}
 	} else {
 		new_create = true;
-		newdentry = lookup_one_len(new->d_name.name, new_upperdir,
-					   new->d_name.len);
-		err = PTR_ERR(newdentry);
-		if (IS_ERR(newdentry))
-			goto out_unlock;
+		if (!d_is_negative(newdentry) &&
+		    (!new_opaque || !ovl_is_whiteout(newdentry)))
+			goto out_dput;
 	}
 
-	err = -ESTALE;
-	if (olddentry->d_parent != old_upperdir)
-		goto out_dput;
-	if (newdentry->d_parent != new_upperdir)
-		goto out_dput;
 	if (olddentry == trap)
 		goto out_dput;
 	if (newdentry == trap)
@@ -925,6 +940,8 @@ static int ovl_rename2(struct inode *olddir, struct dentry *old,
 
 out_dput:
 	dput(newdentry);
+out_dput_old:
+	dput(olddentry);
 out_unlock:
 	unlock_rename(new_upperdir, old_upperdir);
 out_revert_creds:
path: fs/overlayfs/dir.c