Re: [PATCH] nfsd: free nfsd_file by gc after adding it to lru list

[Jeff Layton]

On Mon, 2025-01-13 at 10:59 +0800, Li Lingfeng wrote:
> In nfsd_file_put, after inserting the nfsd_file into the nfsd_file_lru
> list, gc may be triggered in another thread and immediately release this
> nfsd_file, which will lead to a UAF when accessing this nfsd_file again.
> 
> All the places where unhash is done will also perform lru_remove, so there
> is no need to do lru_remove separately here. After inserting the nfsd_file
> into the nfsd_file_lru list, it can be released by relying on gc.
> 
> Fixes: 4a0e73e635e3 ("NFSD: Leave open files out of the filecache LRU")
> Signed-off-by: Li Lingfeng <lilingfeng3@xxxxxxxxxx>
> ---
>  fs/nfsd/filecache.c | 12 ++----------
>  1 file changed, 2 insertions(+), 10 deletions(-)
> 
> diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
> index a1cdba42c4fa..37b65cb1579a 100644
> --- a/fs/nfsd/filecache.c
> +++ b/fs/nfsd/filecache.c
> @@ -372,18 +372,10 @@ nfsd_file_put(struct nfsd_file *nf)
>  		/* Try to add it to the LRU.  If that fails, decrement. */
>  		if (nfsd_file_lru_add(nf)) {
>  			/* If it's still hashed, we're done */
> -			if (test_bit(NFSD_FILE_HASHED, &nf->nf_flags)) {
> +			if (list_lru_count(&nfsd_file_lru))
>  				nfsd_file_schedule_laundrette();
> -				return;
> -			}
>  
> -			/*
> -			 * We're racing with unhashing, so try to remove it from
> -			 * the LRU. If removal fails, then someone else already
> -			 * has our reference.
> -			 */
> -			if (!nfsd_file_lru_remove(nf))
> -				return;
> +			return;
>  		}
>  	}
>  	if (refcount_dec_and_test(&nf->nf_ref))

I think this looks OK. Filecache bugs are particularly nasty though, so
let's run this through a nice long testing cycle.

Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>