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

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

> 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.


> 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.

--
Chuck Lever