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 20:55 +0000, Chuck Lever III wrote:
> 
> > On Mar 17, 2023, at 2:59 PM, Jeff Layton <jlayton@kernel.org> wrote:
> > 
> > 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.
> 
> nfsd_splice_actor() has never returned an error, so IMO it is
> necessary to confirm that when svc_rqst_replace_page() returns
> an error, it doesn't create further problems. I don't see how
> we can avoid some kind of simple fault injection while developing
> the fix.
> 
> Tell you what, I can take it from here if you'd like.
> 

Sure! All yours!

> 
> > If you think handling it as a short read is overblown,
> > then tell me what you would like see here.
> 
> It's not the short reads that bugs me, it's the additional
> code in a hot path that is worrisome.
> 

I wouldn't think tracking some extra stuff on the stack would show up
much in profiles, but it's your call.

-- 
Jeff Layton <jlayton@kernel.org>