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

[Chuck Lever III <chuck.lever@oracle.com>]

> On Mar 17, 2023, at 9:52 AM, Jeff Layton <jlayton@kernel.org> wrote:
> 
> On Fri, 2023-03-17 at 13:44 +0000, Chuck Lever III wrote:
>> 
>>> On Mar 17, 2023, at 6:56 AM, Jeff Layton <jlayton@kernel.org> wrote:
>>> 
>>> There's no good way to handle this gracefully, but if rq_next_page ends
>>> up pointing outside the array, we can at least crash the box before it
>>> scribbles over too much else.
>>> 
>>> Signed-off-by: Jeff Layton <jlayton@kernel.org>
>>> ---
>>> net/sunrpc/svc.c | 10 ++++++++++
>>> 1 file changed, 10 insertions(+)
>>> 
>>> diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
>>> index fea7ce8fba14..864e62945647 100644
>>> --- a/net/sunrpc/svc.c
>>> +++ b/net/sunrpc/svc.c
>>> @@ -845,6 +845,16 @@ EXPORT_SYMBOL_GPL(svc_set_num_threads);
>>> */
>>> void 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];
>>> +	BUG_ON(rqstp->rq_next_page < begin || rqstp->rq_next_page > end);
>> 
>> Linus has stated clearly that he does not want BUG_ON assertions
>> if the system is not actually in danger... and this is clearly
>> the result of a software bug, so a crash will occur anyway.
>> 
> 
> It'll crash, but only after we scribble over some memory.
> 
> Actually, it looks like the splice actor can return an error. We could
> return -EIO here or something without doing anything if we hit this case
> and then let that bubble back up to the read?

Yes, if it's possible to fail just the READ operation, that
would be best. Maybe a emitting a trace event would be better
than a pr_warn.


>> Can you make this a pr_warn_once() ?
>> 
>> 
>>> +
>>> 	if (*rqstp->rq_next_page) {
>>> 		if (!pagevec_space(&rqstp->rq_pvec))
>>> 			__pagevec_release(&rqstp->rq_pvec);
>>> -- 
>>> 2.39.2
>>> 
>> 
>> --
>> Chuck Lever
>> 
>> 
> 
> -- 
> Jeff Layton <jlayton@kernel.org>

--
Chuck Lever