[PATCH] namei: Fix use after free in kern_path_locked

[Stephen Brennan <stephen.s.brennan@oracle.com>]

In 0ee50b47532a ("namei: change filename_parentat() calling
conventions"), filename_parentat() was made to always put the struct
filename before returning, and kern_path_locked() was migrated to this
calling convention. However, kern_path_locked() uses the "last"
parameter to lookup and potentially create a new dentry. The last
parameter contains the last component of the path and points within the
filename, which was recently freed at the end of filename_parentat().
Thus, when kern_path_locked() calls __lookup_hash(), it is using the
filename after it has already been freed.

To avoid this, switch back to __filename_parentat() and place a putname
at the end of the function, once all uses are completed.

Fixes: 0ee50b47532a ("namei: change filename_parentat() calling conventions")
Reported-by: syzbot+fb0d60a179096e8c2731@syzkaller.appspotmail.com
Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
---
 fs/namei.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index d049d3972695..a0122f0016a3 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2551,17 +2551,22 @@ static int filename_parentat(int dfd, struct filename *name,
 /* does lookup, returns the object with parent locked */
 struct dentry *kern_path_locked(const char *name, struct path *path)
 {
+	struct filename *filename;
 	struct dentry *d;
 	struct qstr last;
 	int type, error;
 
-	error = filename_parentat(AT_FDCWD, getname_kernel(name), 0, path,
+	filename = getname_kernel(name);
+	error = __filename_parentat(AT_FDCWD, filename, 0, path,
 				    &last, &type);
-	if (error)
-		return ERR_PTR(error);
+	if (error) {
+		d = ERR_PTR(error);
+		goto out;
+	}
 	if (unlikely(type != LAST_NORM)) {
 		path_put(path);
-		return ERR_PTR(-EINVAL);
+		d = ERR_PTR(-EINVAL);
+		goto out;
 	}
 	inode_lock_nested(path->dentry->d_inode, I_MUTEX_PARENT);
 	d = __lookup_hash(&last, path->dentry, 0);
@@ -2569,6 +2574,8 @@ struct dentry *kern_path_locked(const char *name, struct path *path)
 		inode_unlock(path->dentry->d_inode);
 		path_put(path);
 	}
+out:
+	putname(filename);
 	return d;
 }
 
-- 
2.30.2