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

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

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?

--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