On Tue, Apr 20, 2010 at 03:39:44PM -0400, J. Bruce Fields wrote:
> On Tue, Apr 20, 2010 at 03:24:59PM -0400, William A. (Andy) Adamson wrote:
> > On Tue, Apr 20, 2010 at 12:51 PM, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote:
> > > On Tue, Apr 20, 2010 at 12:16:52PM +1000, Neil Brown wrote:
> > >>
> > >> Surely this can never have worked... which implies that the code has
> > >> never been used?
> > >>
> > >> When read_buf is called to move over to the next page in the pagelist
> > >> of an NFSv4 request, it sets argp->end to essentially a random
> > >> number, certainly not an address within the page which argp->p now
> > >> points to. So subsequent calls to READ_BUF will think there is much
> > >> more than a page of spare space (the cast to u32 ensures an unsigned
> > >> comparison) so we can expect to fall off the end of the second
> > >> page.
> > >
> > > Yipes, thanks.
> > >
> > >> I guess we never ever receive requests with any operation starting
> > >> beyond the first page!
> > >
> > > putfh-write-getattr, for example, is common enough. The write decoding
> > > should leave arg->end set correctly. But there are two read_buf()'s in
> > > decode_getattr(), and I can't see why we don't hit this bug on a write
> > > that leaves that final getattr exactly straddling a page boundary.
> >
> > The write data is dumped into the rq_vec which has non-contiguous
> > pages. So the xdr_buf head only holds the putfh result, the short
> > write response header (v4 stateid, offset, how, length, etc), and then
> > the getattr. so there is plenty of space.
>
> This is the server-side write-decoding, so you could see:
>
>
> rpc header | putfh | write ... data ... | getattr
> ^
> |
> page boundary here
Hm, I guess even when argp->end is wrong, argp->p is always set to
something sane; so on the next READ_BUF(), when you hit the
nbytes <= (u32)((char *)argp->end - (char *)argp->p
case, you do
p = argp->p;
argp->p += XDR_QUADLEN(nbytes);
and p is something reasonable. "end" stays wrong, but that won't be a
problem until you run past the end of the *next* page, which it would
take a very unusual compound to do.
--b.
>
> --b.
>
> >
> > -->Andy
> >
> > >
> > > --b.
> > >
> > >> [[
> > >> I found this while looking at why fsstress over NFS over RDMA caused
> > >> a bad memory dereference in READ32, suggesting that 'p' had a bad
> > >> value. However it was ffff8801299188f0, which is not an "I've fallen
> > >> off the end of the page" sort of value. So I think it must be a
> > >> different bug :-( It is as if the page is being unmapped underneath
> > >> us...
> > >> ]]
> > >> NeilBrown
> > >>
> > >>
> > >>
> > >>
> > >> diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
> > >> index e170317..34ccf81 100644
> > >> --- a/fs/nfsd/nfs4xdr.c
> > >> +++ b/fs/nfsd/nfs4xdr.c
> > >> @@ -161,10 +161,10 @@ static __be32 *read_buf(struct nfsd4_compoundargs *argp, u32 nbytes)
> > >> argp->p = page_address(argp->pagelist[0]);
> > >> argp->pagelist++;
> > >> if (argp->pagelen < PAGE_SIZE) {
> > >> - argp->end = p + (argp->pagelen>>2);
> > >> + argp->end = argp->p + (argp->pagelen>>2);
> > >> argp->pagelen = 0;
> > >> } else {
> > >> - argp->end = p + (PAGE_SIZE>>2);
> > >> + argp->end = argp->p + (PAGE_SIZE>>2);
> > >> argp->pagelen -= PAGE_SIZE;
> > >> }
> > >> memcpy(((char*)p)+avail, argp->p, (nbytes - avail));
> > >> @@ -1426,10 +1426,10 @@ nfsd4_decode_compound(struct nfsd4_compoundargs *argp)
> > >> argp->p = page_address(argp->pagelist[0]);
> > >> argp->pagelist++;
> > >> if (argp->pagelen < PAGE_SIZE) {
> > >> - argp->end = p + (argp->pagelen>>2);
> > >> + argp->end = argp->p + (argp->pagelen>>2);
> > >> argp->pagelen = 0;
> > >> } else {
> > >> - argp->end = p + (PAGE_SIZE>>2);
> > >> + argp->end = argp->p + (PAGE_SIZE>>2);
> > >> argp->pagelen -= PAGE_SIZE;
> > >> }
> > >> }
> > >>
> > >>
> > > --
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> > >
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