Re: [PATCH] hfs/hfsplus: serialize B-tree close against folio release

[Bruno Produit]

On Fri, Sep 18, 2026 at 9:57 PM Viacheslav Dubeyko <slava@xxxxxxxxxxx> wrote:
>
> On Fri, 2026-09-18 at 15:08 +0200, Bruno Produit wrote:
> > From: Kyle Zeng <kylebot@xxxxxxxxxx>
> >
> > From: Kyle Zeng <kylebot@xxxxxxxxxx>
>
> Why do we have the same email two times here? :)
>

Sorry for the mix-up, I was unsure about git send-email.

> >
> > B-tree nodes with a zero reference count remain in the node hash
> > until
> > folio reclaim or tree teardown frees them.  The folio release
> > callbacks
> > remove nodes while holding hash_lock, but hfs_btree_close() walks and
> > frees the same hash without that lock.  Reclaim can therefore unhash
> > and
> > free a node after close has loaded its pointer, causing a use-after-
> > free
> > or double-free.
> >
> > Detach each node with hfs_bnode_unhash() while holding hash_lock
> > before
> > inspecting and freeing it.  Drop the lock before hfs_bnode_free() so
> > a
> > large tree is not freed while holding a spinlock.  Apply the same fix
> > to
> > the matching HFS+ implementation.
> >
> > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
>
> I think we need to have Cc: stable@xxxxxxxxxxxxxxx here too.
>

Added to CC

> > Reported-by: syzbot+d729df28d933979e017a@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://syzkaller.appspot.com/bug?extid=d729df28d933979e017a
> > Reported-by: syzbot+2eac7d175baf21e6a5d5@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://syzkaller.appspot.com/bug?extid=2eac7d175baf21e6a5d5
> > Reported-by: syzbot+7155b2fe09e033c91381@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://syzkaller.appspot.com/bug?extid=7155b2fe09e033c91381
> > Reported-by: syzbot+adeb387cede15eb11607@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://syzkaller.appspot.com/bug?extid=adeb387cede15eb11607
> > Reported-by: syzbot+ae7f2423f3648100506d@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://syzkaller.appspot.com/bug?extid=ae7f2423f3648100506d
>
> Does this fix really eliminates these reported issues? Have you tested
> all of these cases? It will be great to have this clarification in the
> commit message.
>

These crashes do not have test cases to reproduce as far as i
understand (nothing in the repro columns). They do all seem to be the
same UAF in the {hfs,hfsplus}_btree_close() (or
{hfs,hfsplus}_bnode_unhash()) and follow the same pattern, which is
why I included them. Let me know if I should remove this :)

> > Assisted-by: Codex:gpt-5.6-sol
> > Signed-off-by: Kyle Zeng <kylebot@xxxxxxxxxx>
> > Signed-off-by: Bruno Produit <bruno.produit@xxxxxxxxxxxxxxx>
> > ---
> >  fs/hfs/btree.c     | 11 ++++++++---
> >  fs/hfsplus/btree.c | 11 ++++++++---
> >  2 files changed, 16 insertions(+), 6 deletions(-)
> >
> > diff --git a/fs/hfs/btree.c b/fs/hfs/btree.c
> > index 41b4e8fc9..d85a1df9f 100644
> > --- a/fs/hfs/btree.c
> > +++ b/fs/hfs/btree.c
> > @@ -310,14 +310,19 @@ void hfs_btree_close(struct hfs_btree *tree)
> >               return;
> >
> >       for (i = 0; i < NODE_HASH_SIZE; i++) {
> > -             while ((node = tree->node_hash[i])) {
> > -                     tree->node_hash[i] = node->next_hash;
> > +             for (;;) {
>
> I really dislike this infinite loop pattern. Could we have a loop is
> limited by some value?
>

I'll send a new patch removing this pattern and keeping the while loop.


> > +                     spin_lock(&tree->hash_lock);
> > +                     node = tree->node_hash[i];
> > +                     if (node)
> > +                             hfs_bnode_unhash(node);
> > +                     spin_unlock(&tree->hash_lock);
> > +                     if (!node)
> > +                             break;
> >                       if (atomic_read(&node->refcnt))
> >                               pr_err("node %d:%d still has %d
> > user(s)!\n",
> >                                      node->tree->cnid, node->this,
> >                                      atomic_read(&node->refcnt));
> >                       hfs_bnode_free(node);
> > -                     tree->node_hash_cnt--;
> >               }
> >       }
> >       iput(tree->inode);
> > diff --git a/fs/hfsplus/btree.c b/fs/hfsplus/btree.c
> > index 2ea8cd565..a90c9f416 100644
> > --- a/fs/hfsplus/btree.c
> > +++ b/fs/hfsplus/btree.c
> > @@ -417,15 +417,20 @@ void hfs_btree_close(struct hfs_btree *tree)
> >               return;
> >
> >       for (i = 0; i < NODE_HASH_SIZE; i++) {
> > -             while ((node = tree->node_hash[i])) {
> > -                     tree->node_hash[i] = node->next_hash;
> > +             for (;;) {
>
> Ditto.
>
> Thanks,
> Slava.
>
> > +                     spin_lock(&tree->hash_lock);
> > +                     node = tree->node_hash[i];
> > +                     if (node)
> > +                             hfs_bnode_unhash(node);
> > +                     spin_unlock(&tree->hash_lock);
> > +                     if (!node)
> > +                             break;
> >                       if (atomic_read(&node->refcnt))
> >                               pr_crit("node %d:%d "
> >                                               "still has %d
> > user(s)!\n",
> >                                       node->tree->cnid, node-
> > >this,
> >                                       atomic_read(&node->refcnt));
> >                       hfs_bnode_free(node);
> > -                     tree->node_hash_cnt--;
> >               }
> >       }
> >       iput(tree->inode);
>