> On Aug 22, 2017, at 5:45 PM, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote:
>
> On Mon, Aug 21, 2017 at 06:08:15PM -0400, Chuck Lever wrote:
>>
>>> On Aug 21, 2017, at 5:21 PM, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote:
>>>
>>> On Mon, Aug 21, 2017 at 05:15:38PM -0400, Chuck Lever wrote:
>>>>
>>>>> On Aug 21, 2017, at 5:13 PM, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote:
>>>>>
>>>>> On Fri, Aug 18, 2017 at 11:12:19AM -0400, Chuck Lever wrote:
>>>>>> When processing an NFSv4 WRITE operation, argp->end should never
>>>>>> point past the end of the data in the final page of the page list.
>>>>>> Otherwise, nfsd4_decode_compound can walk into uninitialized memory.
>>>>>>
>>>>>> Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
>>>>>> ---
>>>>>> fs/nfsd/nfs4xdr.c | 6 ++----
>>>>>> 1 file changed, 2 insertions(+), 4 deletions(-)
>>>>>>
>>>>>> diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
>>>>>> index 51e729a..7c48d68 100644
>>>>>> --- a/fs/nfsd/nfs4xdr.c
>>>>>> +++ b/fs/nfsd/nfs4xdr.c
>>>>>> @@ -144,7 +144,7 @@ static void next_decode_page(struct nfsd4_compoundargs *argp)
>>>>>> argp->p = page_address(argp->pagelist[0]);
>>>>>> argp->pagelist++;
>>>>>> if (argp->pagelen < PAGE_SIZE) {
>>>>>> - argp->end = argp->p + (argp->pagelen>>2);
>>>>>> + argp->end = argp->p + XDR_QUADLEN(argp->pagelen);
>>
>> ^^^^^^^^^^^^^^ A
>>
>>>>>> argp->pagelen = 0;
>>>>>> } else {
>>>>>> argp->end = argp->p + (PAGE_SIZE>>2);
>>>>>> @@ -1279,9 +1279,7 @@ static __be32 nfsd4_decode_opaque(struct nfsd4_compoundargs *argp, struct xdr_ne
>>>>>> argp->pagelen -= pages * PAGE_SIZE;
>>>>>> len -= pages * PAGE_SIZE;
>>>>>>
>>>>>> - argp->p = (__be32 *)page_address(argp->pagelist[0]);
>>>>>> - argp->pagelist++;
>>>>>> - argp->end = argp->p + XDR_QUADLEN(PAGE_SIZE);
>>
>> ^^^^^^^^^^^^^^ B
>>
>>>>>> + next_decode_page(argp);
>>>>>
>>>>> I think there's no change in behavior here *except* for adding a new
>>>>> argp->pagelen=0 (or argp->pagelen -= PAGE_SIZE).
>>>>
>>>> The code around this change is currently working correctly,
>>>> so there is no change in behavior AFAICT. This is a defensive
>>>> change, but it also replaces duplicate code.
>>>
>>> I don't understand. I'm saying that by calling next_decode_page() there
>>> you've added a new argp->pagelen assignment. I don't understand how
>>> that can't change behavior, unless there's another bug in our bounds
>>> checking someplace.
>>
>> Because of line B above, argp->end always points to the
>> end of the final page in the page list. However, the
>> buffer might end somewhere in the middle of that page,
>> in which case, the transport hasn't initialized any of
>> the bytes between the end of the buffer and the end of
>> the page.
>>
>> As long as the other fields in the xdr_buf are set up
>> properly, the XDR decoder will not walk into that uninit-
>> ialized section of the last page. But there's nothing
>> preventing a decoder or transport bug from causing it
>> to walk into the uninitialized area. And always setting
>> to the end of the page is confusing when the buffer
>> itself is actually shorter.
>>
>> The key is to replace line B above with line A. argp->end
>> is advanced by the remaining part of the final page rather
>> than by a whole page.
>
> Got it, I agree with that part of the change, it's the pagelen change I
> was having trouble with.
>
> But looking at it more, I think your patch is a fix and the current code
> is wrong.
>
>> The next patch uses this new behavior to signal precisely
>> when it has to move from the page list to the tail iovec.
>>
>>
>>> Most likely it could cause subsequent op parsers to believe there's less
>>> space in the argument buffer than there really is, so it might fail to
>>> parse a compound with a write plus some other ops, if that puts the
>>> total call close to the maximum size?
>>
>> Where is argp->pagelen used after the final next_decode_page
>> call?
>
> Well, it's checked in every read_buf and next_decode_page to decide how
> much space is left.
Right, and I didn't change the pagelen adjustment that occurs
in the loop. Just the final adjustment should be different.
> It looks to me like the current code is wrong not to be decreasing
> page_len at the end there. So I wonder if there's a bug right now.
> E.g. maybe a compound with multiple writes could leave the xdr decoding
> thinking it has more space than it does and allow someone to write
> unrelated memory to some file.
I believe that's possible.
--
Chuck Lever
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