[PATCH v3 1/4] namei: Fix use after free in kern_path_locked

[Stephen Brennan]

In 0ee50b47532a ("namei: change filename_parentat() calling
conventions"), filename_parentat() was made to always call putname() on
the  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.

Switch to using __filename_parentat() to avoid freeing the filename, and
wrap the function with a getname and putname instead. Remove
filename_parentat() as it is now unused, and it is inherently broken.

Fixes: 0ee50b47532a ("namei: change filename_parentat() calling conventions")
Link: https://lore.kernel.org/linux-fsdevel/YTfVE7IbbTV71Own@xxxxxxxxxxxxxxxxxxxxx/
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Reported-by: syzbot+fb0d60a179096e8c2731@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Stephen Brennan <stephen.s.brennan@xxxxxxxxxx>
---
 fs/namei.c | 25 ++++++++++++-------------
 1 file changed, 12 insertions(+), 13 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index d049d3972695..d6340fabaab4 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2538,25 +2538,15 @@ static int __filename_parentat(int dfd, struct filename *name,
 	return retval;
 }
 
-static int filename_parentat(int dfd, struct filename *name,
-				unsigned int flags, struct path *parent,
-				struct qstr *last, int *type)
-{
-	int retval = __filename_parentat(dfd, name, flags, parent, last, type);
-
-	putname(name);
-	return retval;
-}
-
 /* does lookup, returns the object with parent locked */
-struct dentry *kern_path_locked(const char *name, struct path *path)
+static struct dentry *__kern_path_locked(struct filename *name,
+					 struct path *path)
 {
 	struct dentry *d;
 	struct qstr last;
 	int type, error;
 
-	error = filename_parentat(AT_FDCWD, getname_kernel(name), 0, path,
-				    &last, &type);
+	error = __filename_parentat(AT_FDCWD, name, 0, path, &last, &type);
 	if (error)
 		return ERR_PTR(error);
 	if (unlikely(type != LAST_NORM)) {
@@ -2572,6 +2562,15 @@ struct dentry *kern_path_locked(const char *name, struct path *path)
 	return d;
 }
 
+struct dentry *kern_path_locked(const char *name, struct path *path)
+{
+	struct filename *filename = getname_kernel(name);
+	struct dentry *res = __kern_path_locked(filename, path);
+
+	putname(filename);
+	return res;
+}
+
 int kern_path(const char *name, unsigned int flags, struct path *path)
 {
 	return filename_lookup(AT_FDCWD, getname_kernel(name),
-- 
2.30.2