Re: [PATCH] sunrpc: Avoid a KASAN slab-out-of-bounds bug in xdr_set_page_base()

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

On Thu, Aug 26, 2021 at 03:44:32PM -0400, Anna Schumaker wrote:
> On Thu, Aug 12, 2021 at 4:32 PM J. Bruce Fields <bfields@fieldses.org> wrote:
> >
> > On Mon, Jun 14, 2021 at 07:14:40PM -0400, bfields wrote:
> > > On Wed, Jun 09, 2021 at 05:07:29PM -0400, schumaker.anna@gmail.com wrote:
> > > > From: Anna Schumaker <Anna.Schumaker@Netapp.com>
> > > >
> > > > This seems to happen fairly easily during READ_PLUS testing on NFS v4.2.
> > >
> > > Yep, I hit a KASAN warning here every time, and this fixes it,
> > > thanks.--b.
> >
> > By the way, config NFS_V4_2_READ_PLUS still says:
> >
> >         This is intended for developers only. The READ_PLUS operation
> >         has been shown to have issues under specific conditions and
> >         should not be used in production.
> >
> > But this warning was the only thing I was seeing.  Is there another
> > known issue remaining?
> 
> I think it was an issue around using lseek to generate the reply. The
> file contents could change between each call, leading to inconsistent
> results (and a new failing xfstest that previously passed)

OK, thanks, I see now that you mentioned in 21e31401fc45 "NFS: Disable
READ_PLUS by default" that there were generic/091 and generic/263
failures.

Looks like they're both testing concurrent direct and buffered IO.  I
don't know what we try to guarantee in that case.

--b.

> 
> Anna
> 
> >
> > --b.
> >
> > >
> > > > I found that we could end up accessing xdr->buf->pages[pgnr] with a pgnr
> > > > greater than the number of pages in the array. So let's just return
> > > > early if we're setting base to a point at the end of the page data and
> > > > let xdr_set_tail_base() handle setting up the buffer pointers instead.
> > > >
> > > > Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
> > > > ---
> > > >  net/sunrpc/xdr.c | 7 +++----
> > > >  1 file changed, 3 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/net/sunrpc/xdr.c b/net/sunrpc/xdr.c
> > > > index 3964ff74ee51..ca10ba2626f2 100644
> > > > --- a/net/sunrpc/xdr.c
> > > > +++ b/net/sunrpc/xdr.c
> > > > @@ -1230,10 +1230,9 @@ static unsigned int xdr_set_page_base(struct xdr_stream *xdr,
> > > >     void *kaddr;
> > > >
> > > >     maxlen = xdr->buf->page_len;
> > > > -   if (base >= maxlen) {
> > > > -           base = maxlen;
> > > > -           maxlen = 0;
> > > > -   } else
> > > > +   if (base >= maxlen)
> > > > +           return 0;
> > > > +   else
> > > >             maxlen -= base;
> > > >     if (len > maxlen)
> > > >             len = maxlen;
> > > > --
> > > > 2.32.0