[PATCH v3] fs/kernfs: null-ptr deref in simple_xattrs_free()

[Will Rosenberg]

There exists a null pointer dereference in simple_xattrs_free() as
part of the __kernfs_new_node() routine. Within __kernfs_new_node(),
err_out4 calls simple_xattr_free(), but kn->iattr may be NULL if
__kernfs_setattr() was never called. As a result, the first argument to
simple_xattrs_free() may be NULL + 0x38, and no NULL check is done
internally, causing an incorrect pointer dereference.

Add a check to ensure kn->iattr is not NULL, meaning __kernfs_setattr()
has been called and kn->iattr is allocated. Note that struct kernfs_node
kn is allocated with kmem_cache_zalloc, so we can assume kn->iattr will
be NULL if not allocated.

An alternative fix could be to not call simple_xattr_free() at all. As
was previously discussed during the initial patch, simple_xattr_free()
is not strictly needed and is included to be consistent with
kernfs_free_rcu(), which also helps the function maintain correctness if
changes are made in __kernfs_new_node().

Reported-by: syzbot+6aaf7f48ae034ab0ea97@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=6aaf7f48ae034ab0ea97
Fixes: 382b1e8f30f7 ("kernfs: fix memory leak of kernfs_iattrs in __kernfs_new_node")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Will Rosenberg <whrosenb@xxxxxxx>
---
 fs/kernfs/dir.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 5c0efd6b239f..29baeeb97871 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -681,8 +681,10 @@ static struct kernfs_node *__kernfs_new_node(struct kernfs_root *root,
 	return kn;
 
  err_out4:
-	simple_xattrs_free(&kn->iattr->xattrs, NULL);
-	kmem_cache_free(kernfs_iattrs_cache, kn->iattr);
+	if (kn->iattr) {
+		simple_xattrs_free(&kn->iattr->xattrs, NULL);
+		kmem_cache_free(kernfs_iattrs_cache, kn->iattr);
+	}
  err_out3:
 	spin_lock(&root->kernfs_idr_lock);
 	idr_remove(&root->ino_idr, (u32)kernfs_ino(kn));

base-commit: d358e5254674b70f34c847715ca509e46eb81e6f
-- 
2.34.1