Re: [PATCH 1/2] nfsd: don't replace page in rq_pages if it's a continuation of last page

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2023-03-17 at 06:56 -0400, Jeff Layton wrote:
> The splice read calls nfsd_splice_actor to put the pages containing file
> data into the svc_rqst->rq_pages array. It's possible however to get a
> splice result that only has a partial page at the end, if (e.g.) the
> filesystem hands back a short read that doesn't cover the whole page.
> 
> nfsd_splice_actor will plop the partial page into its rq_pages array and
> return. Then later, when nfsd_splice_actor is called again, the
> remainder of the page may end up being filled out. At this point,
> nfsd_splice_actor will put the page into the array _again_ corrupting
> the reply. If this is done enough times, rq_next_page will overrun the
> array and corrupt the trailing fields -- the rq_respages and
> rq_next_page pointers themselves.
> 
> If we've already added the page to the array in the last pass, don't add
> it to the array a second time when dealing with a splice continuation.
> This was originally handled properly in nfsd_splice_actor, but commit
> 91e23b1c3982 removed the check for it, and started universally replacing
> pages.
> 
> Fixes: 91e23b1c3982 ("NFSD: Clean up nfsd_splice_actor()")
> Reported-by: Dario Lesca <d.lesca@solinos.it>
> Tested-by: David Critch <dcritch@redhat.com>
> Link: https://bugzilla.redhat.com/show_bug.cgi?id=2150630
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
>  fs/nfsd/vfs.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index 502e1b7742db..3709ef57d96e 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -941,8 +941,11 @@ nfsd_splice_actor(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
>  	struct page *last_page;
>  
>  	last_page = page + (offset + sd->len - 1) / PAGE_SIZE;
> -	for (page += offset / PAGE_SIZE; page <= last_page; page++)
> -		svc_rqst_replace_page(rqstp, page);
> +	for (page += offset / PAGE_SIZE; page <= last_page; page++) {
> +		/* Only replace page if we haven't already done so */

Note that I think that this was probably the real rationale for the pp[-
1] check that 91e23b1c3982 removed. Given that, maybe we should flesh
this comment out a bit more for posterity?

		/*
		 * When we're splicing from a pipe, it's possible that
		 * we'll get an incomplete page that may be updated on
		 * a later call. Only splice it into rq_pages once.
		 */

> +		if (page != *(rqstp->rq_next_page - 1))
> +			svc_rqst_replace_page(rqstp, page);
> +	}
>  	if (rqstp->rq_res.page_len == 0)	// first call
>  		rqstp->rq_res.page_base = offset % PAGE_SIZE;
>  	rqstp->rq_res.page_len += sd->len;

-- 
Jeff Layton <jlayton@kernel.org>