Re: [nf-next PATCH v2] netfilter: nft_exthdr: Fix for unsafe packet data read

[Florian Westphal <fw@strlen.de>]

Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> On Tue, Jun 08, 2021 at 11:40:57AM +0200, Phil Sutter wrote:
> > While iterating through an SCTP packet's chunks, skb_header_pointer() is
> > called for the minimum expected chunk header size. If (that part of) the
> > skbuff is non-linear, the following memcpy() may read data past
> > temporary buffer '_sch'. Use skb_copy_bits() instead which does the
> > right thing in this situation.
> > 
> > Fixes: 133dc203d77df ("netfilter: nft_exthdr: Support SCTP chunks")
> > Suggested-by: Florian Westphal <fw@strlen.de>
> > Signed-off-by: Phil Sutter <phil@nwl.cc>
> > ---
> > Changes since v1:
> > - skb_copy_bits() call error handling added
> > ---
> >  net/netfilter/nft_exthdr.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/net/netfilter/nft_exthdr.c b/net/netfilter/nft_exthdr.c
> > index 1b0579cb62d08..7f705b5c09de8 100644
> > --- a/net/netfilter/nft_exthdr.c
> > +++ b/net/netfilter/nft_exthdr.c
> > @@ -327,7 +327,9 @@ static void nft_exthdr_sctp_eval(const struct nft_expr *expr,
> >  				break;
> >  
> >  			dest[priv->len / NFT_REG32_SIZE] = 0;
> > -			memcpy(dest, (char *)sch + priv->offset, priv->len);
> > +			if (skb_copy_bits(pkt->skb, offset + priv->offset,
> > +					  dest, priv->len) < 0)
> > +				break;
> 
> Hm, it looks like tcp exthdt matching has the same problem?

Tcp exthdr either copies from skb linear area or a on-stack scratch space,
it looks ok to me.