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.
--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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