Re: [RFC PATCH] mm: truncate: flush lru cache for evicted inode

[Hillf Danton <hdanton@sina.com>]

On Sat, 15 Jun 2024 21:44:54 +0100 Matthew Wilcox wrote:
> On Sat, Jun 15, 2024 at 07:59:53AM +0800, Hillf Danton wrote:
> > On Fri, 14 Jun 2024 14:42:20 +0100 Matthew Wilcox wrote:
> > > On Fri, Jun 14, 2024 at 09:18:56PM +0800, Hillf Danton wrote:
> > > > Flush lru cache to avoid folio->mapping uaf in case of inode teardown.
> > > 
> > > What?  inodes are supposed to have all their folios removed before
> > > being freed.  Part of removing a folio sets the folio->mapping to NULL.
> > > Where is the report?
> > >
> > Subject: Re: [syzbot] [nilfs?] [mm?] KASAN: slab-use-after-free Read in lru_add_fn
> > https://lore.kernel.org/lkml/000000000000cae276061aa12d5e@google.com/
> 
> Thanks.  This fix is wrong.  Of course syzbot says it fixes the problem,
> but you're just avoiding putting the folios into the situation where we
> have debug that would detect the problem.
> 
> I suspect this would trigger:
> 
Happy to test your idea.

> +++ b/fs/inode.c
> @@ -282,6 +282,7 @@ static struct inode *alloc_inode(struct super_block *sb)
>  void __destroy_inode(struct inode *inode)
>  {
>         BUG_ON(inode_has_buffers(inode));
> +       BUG_ON(inode->i_data.nrpages);
>         inode_detach_wb(inode);
>         security_inode_free(inode);
>         fsnotify_inode_delete(inode);
> 
> and what a real fix would look like would be calling clear_inode()
> before calling iput() in nilfs_put_root().  But I'm not an expert

Hm...given I_FREEING checked in clear_inode(), fix like this one could be
tried in midle 2026.

> in this layer of the VFS, so I might well be wrong.

#syz test https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git  83a7eefedc9b

--- x/mm/truncate.c
+++ y/mm/truncate.c
@@ -419,6 +419,9 @@ void truncate_inode_pages_range(struct a
 		truncate_folio_batch_exceptionals(mapping, &fbatch, indices);
 		folio_batch_release(&fbatch);
 	}
+
+	if (mapping_exiting(mapping))
+		lru_add_drain_all();
 }
 EXPORT_SYMBOL(truncate_inode_pages_range);
 
--- x/fs/inode.c
+++ y/fs/inode.c
@@ -282,6 +282,7 @@ static struct inode *alloc_inode(struct
 void __destroy_inode(struct inode *inode)
 {
 	BUG_ON(inode_has_buffers(inode));
+	BUG_ON(inode->i_data.nrpages);
 	inode_detach_wb(inode);
 	security_inode_free(inode);
 	fsnotify_inode_delete(inode);
--