Re: [PATCH v2 2/2] sunrpc: add bounds checking to svc_rqst_replace_page

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2023-03-17 at 18:08 +0000, Chuck Lever III wrote:
> 
> > On Mar 17, 2023, at 2:04 PM, Jeff Layton <jlayton@kernel.org> wrote:
> > 
> > On Fri, 2023-03-17 at 17:51 +0000, Chuck Lever III wrote:
> > > > On Mar 17, 2023, at 1:13 PM, Jeff Layton <jlayton@kernel.org> wrote:
> > > > 
> > > > If rq_next_page ends up pointing outside the array, then we can
> > > > corrupt
> > > > memory when we go to change its value. Ensure that it hasn't strayed
> > > > outside the array, and have svc_rqst_replace_page return -EIO
> > > > without
> > > > changing anything if it has.
> > > > 
> > > > Fix up nfsd_splice_actor (the only caller) to handle this case by
> > > > either
> > > > returning an error or a short splice when this happens.
> > > 
> > > IMO it's not worth the extra complexity to return a short splice.
> > > This is a "should never happen" scenario in a hot I/O path. Let's
> > > keep this code as simple as possible, and use unlikely() for the
> > > error cases in both nfsd_splice_actor and svc_rqst_replace_page().
> > > 
> > 
> > Are there any issues with just returning an error even though we have
> > successfully spliced some of the data into the buffer? I guess the
> > caller will just see an EIO or whatever instead of the short read in
> > that case?
> 
> NFSv4 READ is probably going to truncate the XDR buffer. I'm not
> sure NFSv3 is so clever, so you should test it.
> 

Honestly, I don't have the cycles to do that sort of fault injection
testing for this. If you think handling it as a short read is overblown,
then tell me what you would like see here.

> > > Also, since "nfsd_splice_actor ... [is] the only caller", a WARN_ON
> > > stack trace is not adding value. I still think a tracepoint is more
> > > appropriate. I suggest:
> > > 
> > >   trace_svc_replace_page_err(rqst);
> > 
> > Ok, I can look at adding a conditional tracepoint.
> 
> I thought about that: it doesn't help much, since you have to
> explicitly test anyway to see whether to return an error.
> 

Fair point. I can just make it a regular tracepoint that fires inside
the conditional.

> 
> > > > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > > > ---
> > > > fs/nfsd/vfs.c              | 23 +++++++++++++++++------
> > > > include/linux/sunrpc/svc.h |  2 +-
> > > > net/sunrpc/svc.c           | 14 +++++++++++++-
> > > > 3 files changed, 31 insertions(+), 8 deletions(-)
> > > > 
> > > > diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> > > > index 97b38b47c563..0ebd7a65a9f0 100644
> > > > --- a/fs/nfsd/vfs.c
> > > > +++ b/fs/nfsd/vfs.c
> > > > @@ -939,6 +939,7 @@ nfsd_splice_actor(struct pipe_inode_info *pipe,
> > > > struct pipe_buffer *buf,
> > > > 	struct page *page = buf->page;	// may be a compound one
> > > > 	unsigned offset = buf->offset;
> > > > 	struct page *last_page;
> > > > +	int ret = 0, consumed = 0;
> > > > 
> > > > 	last_page = page + (offset + sd->len - 1) / PAGE_SIZE;
> > > > 	for (page += offset / PAGE_SIZE; page <= last_page; page++)
> > > > {
> > > > @@ -946,13 +947,23 @@ nfsd_splice_actor(struct pipe_inode_info
> > > > *pipe, struct pipe_buffer *buf,
> > > > 		 * Skip page replacement when extending the
> > > > contents
> > > > 		 * of the current page.
> > > > 		 */
> > > > -		if (page != *(rqstp->rq_next_page - 1))
> > > > -			svc_rqst_replace_page(rqstp, page);
> > > > +		if (page != *(rqstp->rq_next_page - 1)) {
> > > > +			ret = svc_rqst_replace_page(rqstp, page);
> > > > +			if (ret)
> > > > +				break;
> > > > +		}
> > > > +		consumed += min_t(int,
> > > > +				  PAGE_SIZE -
> > > > offset_in_page(offset),
> > > > +				  sd->len - consumed);
> > > > +		offset = 0;
> > > > 	}
> > > > -	if (rqstp->rq_res.page_len == 0)	// first call
> > > > -		rqstp->rq_res.page_base = offset % PAGE_SIZE;
> > > > -	rqstp->rq_res.page_len += sd->len;
> > > > -	return sd->len;
> > > > +	if (consumed) {
> > > > +		if (rqstp->rq_res.page_len == 0)	// first
> > > > call
> > > > +			rqstp->rq_res.page_base = offset %
> > > > PAGE_SIZE;
> > > > +		rqstp->rq_res.page_len += consumed;
> > > > +		return consumed;
> > > > +	}
> > > > +	return ret;
> > > > }
> > > > 
> > > > static int nfsd_direct_splice_actor(struct pipe_inode_info *pipe,
> > > > diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
> > > > index 877891536c2f..9ea52f143f49 100644
> > > > --- a/include/linux/sunrpc/svc.h
> > > > +++ b/include/linux/sunrpc/svc.h
> > > > @@ -422,7 +422,7 @@ struct svc_serv *svc_create(struct svc_program
> > > > *, unsigned int,
> > > > 			    int (*threadfn)(void *data));
> > > > struct svc_rqst *svc_rqst_alloc(struct svc_serv *serv,
> > > > 					struct svc_pool *pool, int
> > > > node);
> > > > -void		   svc_rqst_replace_page(struct svc_rqst *rqstp,
> > > > +int		   svc_rqst_replace_page(struct svc_rqst *rqstp,
> > > > 					 struct page *page);
> > > > void		   svc_rqst_free(struct svc_rqst *);
> > > > void		   svc_exit_thread(struct svc_rqst *);
> > > > diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
> > > > index fea7ce8fba14..d624c02f09be 100644
> > > > --- a/net/sunrpc/svc.c
> > > > +++ b/net/sunrpc/svc.c
> > > > @@ -843,8 +843,19 @@ EXPORT_SYMBOL_GPL(svc_set_num_threads);
> > > >  * When replacing a page in rq_pages, batch the release of the
> > > >  * replaced pages to avoid hammering the page allocator.
> > > >  */
> > > > -void svc_rqst_replace_page(struct svc_rqst *rqstp, struct page
> > > > *page)
> > > > +int svc_rqst_replace_page(struct svc_rqst *rqstp, struct page
> > > > *page)
> > > > {
> > > > +	struct page **begin, **end;
> > > > +
> > > > +	/*
> > > > +	 * Bounds check: make sure rq_next_page points into the
> > > > rq_respages
> > > > +	 * part of the array.
> > > > +	 */
> > > > +	begin = rqstp->rq_pages;
> > > > +	end = &rqstp->rq_pages[RPCSVC_MAXPAGES];
> > > > +	if (WARN_ON_ONCE(rqstp->rq_next_page < begin || rqstp-
> > > > > rq_next_page > end))
> > > > +		return -EIO;
> > > > +
> > > > 	if (*rqstp->rq_next_page) {
> > > > 		if (!pagevec_space(&rqstp->rq_pvec))
> > > > 			__pagevec_release(&rqstp->rq_pvec);
> > > > @@ -853,6 +864,7 @@ void svc_rqst_replace_page(struct svc_rqst
> > > > *rqstp, struct page *page)
> > > > 
> > > > 	get_page(page);
> > > > 	*(rqstp->rq_next_page++) = page;
> > > > +	return 0;
> > > > }
> > > > EXPORT_SYMBOL_GPL(svc_rqst_replace_page);
> > > > 
> > > > -- 
> > > > 2.39.2
> > > > 
> > > 
> > > --
> > > Chuck Lever
> > > 
> > > 
> > 
> > -- 
> > Jeff Layton <jlayton@kernel.org>
> 
> --
> Chuck Lever
> 
> 

-- 
Jeff Layton <jlayton@kernel.org>