Re: [PATCH v1 1/3] nfsd: Limit end of page list when decoding NFSv4 WRITE

["J. Bruce Fields" <bfields@fieldses.org>]

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@fieldses.org> 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@fieldses.org> 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@oracle.com>
> >>>> ---
> >>>> 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.

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.

--b.