Re: [REGRESSION][BISECTED] Crash with Bad page state for FUSE/Flatpak related applications since v6.13

[Matthew Wilcox]

On Mon, Feb 10, 2025 at 02:12:35PM -0500, Josef Bacik wrote:
> From: Josef Bacik <josef@xxxxxxxxxxxxxx>
> Date: Mon, 10 Feb 2025 14:06:40 -0500
> Subject: [PATCH] fuse: drop extra put of folio when using pipe splice
> 
> In 3eab9d7bc2f4 ("fuse: convert readahead to use folios"), I converted
> us to using the new folio readahead code, which drops the reference on
> the folio once it is locked, using an inferred reference on the folio.
> Previously we held a reference on the folio for the entire duration of
> the readpages call.
> 
> This is fine, however I failed to catch the case for splice pipe
> responses where we will remove the old folio and splice in the new
> folio.  Here we assumed that there is a reference held on the folio for
> ap->folios, which is no longer the case.
> 
> To fix this, simply drop the extra put to keep us consistent with the
> non-splice variation.  This will fix the UAF bug that was reported.
> 
> Link: https://lore.kernel.org/linux-fsdevel/2f681f48-00f5-4e09-8431-2b3dbfaa881e@xxxxxxxxx/
> Fixes: 3eab9d7bc2f4 ("fuse: convert readahead to use folios")
> Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
> ---
>  fs/fuse/dev.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
> index 5b5f789b37eb..5bd6e2e184c0 100644
> --- a/fs/fuse/dev.c
> +++ b/fs/fuse/dev.c
> @@ -918,8 +918,6 @@ static int fuse_try_move_page(struct fuse_copy_state *cs, struct page **pagep)
>  	}
>  
>  	folio_unlock(oldfolio);
> -	/* Drop ref for ap->pages[] array */
> -	folio_put(oldfolio);
>  	cs->len = 0;

But aren't we now leaking a reference to newfolio?  ie shouldn't
we also:

-	folio_get(newfolio);

a few lines earlier?