Hi,

On Fri, Sep 24, 2021 at 7:35 PM Pavel Machek pavel@denx.de wrote:

Hi!

...

From: Zhen Lei thunder.leizhen@huawei.com Subject: nilfs2: use refcount_dec_and_lock() to fix potential UAF

When the refcount is decreased to 0, the resource reclamation branch is entered. Before CPU0 reaches the race point (1), CPU1 may obtain the spinlock and traverse the rbtree to find 'root', see nilfs_lookup_root(). Although CPU1 will call refcount_inc() to increase the refcount, it is obviously too late. CPU0 will release 'root' directly, CPU1 then accesses 'root' and triggers UAF.

Use refcount_dec_and_lock() to ensure that both the operations of decrease refcount to 0 and link deletion are lock protected eliminates this risk.

 CPU0                      CPU1

nilfs_put_root(): <-------- (1) spin_lock(&nilfs->ns_cptree_lock); rb_erase(&root->rb_node, &nilfs->ns_cptree); spin_unlock(&nilfs->ns_cptree_lock);

kfree(root); <-------- use-after-free

...

There is no reproduction program, and the above is only theoretical analysis.

Ok, so we have a theoretical bug, and fix already on its way to stable. But ... is it correct?

...

+++ a/fs/nilfs2/the_nilfs.c @@ -792,14 +792,13 @@ nilfs_find_or_create_root(struct the_nil

void nilfs_put_root(struct nilfs_root *root) {

• if (refcount_dec_and_test(&root->count)) {
  

•         struct the_nilfs *nilfs = root->nilfs;
  

• struct the_nilfs *nilfs = root->nilfs;
  
  

•         nilfs_sysfs_delete_snapshot_group(root);
  

•         spin_lock(&nilfs->ns_cptree_lock);
  

• if (refcount_dec_and_lock(&root->count, &nilfs->ns_cptree_lock)) {
          rb_erase(&root->rb_node, &nilfs->ns_cptree);
          spin_unlock(&nilfs->ns_cptree_lock);
  

•         nilfs_sysfs_delete_snapshot_group(root);
          iput(root->ifile);
  
          kfree(root);
  

spin_lock() is deleted, but spin_unlock() is not affected. This means unbalanced locking, right?

It's okay. spin_lock() is integrated into refcount_dec_and_lock(), which was originally refcount_dec_and_test().

Thanks, Ryusuke Konishi

Best regards, Pavel -- DENX Software Engineering GmbH, Managing Director: Wolfgang Denk HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany