[PATCH v2 1/3] namei: Fix use after free in kern_path_locked

[Stephen Brennan <stephen.s.brennan@xxxxxxxxxx>]

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.

In this case, filename_parentat() is fundamentally broken due to this
use after free. So, remove it, and rename __filename_parentat to
filename_parentat, migrating all callers. Adjust kern_path_locked to put
the filename once all users are done with it.

Fixes: 0ee50b47532a ("namei: change filename_parentat() calling conventions")
Link: https://lore.kernel.org/linux-fsdevel/YS9D4AlEsaCxLFV0@xxxxxxxxxxxxx/
Link: https://lore.kernel.org/linux-fsdevel/YS+csMTV2tTXKg3s@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>
Co-authored-by: Dmitry Kadashev <dkadashev@xxxxxxxxx>
---
 fs/namei.c | 47 ++++++++++++++++++++++-------------------------
 1 file changed, 22 insertions(+), 25 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index d049d3972695..f2af301cc79f 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2514,9 +2514,10 @@ static int path_parentat(struct nameidata *nd, unsigned flags,
 	return err;
 }
 
-static int __filename_parentat(int dfd, struct filename *name,
-				unsigned int flags, struct path *parent,
-				struct qstr *last, int *type)
+/* Note: this does not consume "name" */
+static int filename_parentat(int dfd, struct filename *name,
+			     unsigned int flags, struct path *parent,
+			     struct qstr *last, int *type)
 {
 	int retval;
 	struct nameidata nd;
@@ -2538,30 +2539,24 @@ 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)
 {
+	struct filename *filename;
 	struct dentry *d;
 	struct qstr last;
 	int type, error;
 
-	error = filename_parentat(AT_FDCWD, getname_kernel(name), 0, path,
-				    &last, &type);
-	if (error)
-		return ERR_PTR(error);
+	filename = getname_kernel(name);
+	error = filename_parentat(AT_FDCWD, filename, 0, path, &last, &type);
+	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 +2564,8 @@ 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;
 }
 
@@ -3634,7 +3631,7 @@ static struct dentry *__filename_create(int dfd, struct filename *name,
 	 */
 	lookup_flags &= LOOKUP_REVAL;
 
-	error = __filename_parentat(dfd, name, lookup_flags, path, &last, &type);
+	error = filename_parentat(dfd, name, lookup_flags, path, &last, &type);
 	if (error)
 		return ERR_PTR(error);
 
@@ -3996,7 +3993,7 @@ int do_rmdir(int dfd, struct filename *name)
 	int type;
 	unsigned int lookup_flags = 0;
 retry:
-	error = __filename_parentat(dfd, name, lookup_flags, &path, &last, &type);
+	error = filename_parentat(dfd, name, lookup_flags, &path, &last, &type);
 	if (error)
 		goto exit1;
 
@@ -4135,7 +4132,7 @@ int do_unlinkat(int dfd, struct filename *name)
 	struct inode *delegated_inode = NULL;
 	unsigned int lookup_flags = 0;
 retry:
-	error = __filename_parentat(dfd, name, lookup_flags, &path, &last, &type);
+	error = filename_parentat(dfd, name, lookup_flags, &path, &last, &type);
 	if (error)
 		goto exit1;
 
@@ -4683,13 +4680,13 @@ int do_renameat2(int olddfd, struct filename *from, int newdfd,
 		target_flags = 0;
 
 retry:
-	error = __filename_parentat(olddfd, from, lookup_flags, &old_path,
-					&old_last, &old_type);
+	error = filename_parentat(olddfd, from, lookup_flags, &old_path,
+				  &old_last, &old_type);
 	if (error)
 		goto put_names;
 
-	error = __filename_parentat(newdfd, to, lookup_flags, &new_path, &new_last,
-				&new_type);
+	error = filename_parentat(newdfd, to, lookup_flags, &new_path, &new_last,
+				  &new_type);
 	if (error)
 		goto exit1;
 
-- 
2.30.2