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Miller" , sameehj@amazon.com Subject: Re: [PATCH] net-xdp: netdev attribute to control xdpgeneric skb linearization In-Reply-To: References: <20200122203253.20652-1-lrizzo@google.com> <875zh2bis0.fsf@toke.dk> <953c8fee-91f0-85e7-6c7b-b9a2f8df5aa6@iogearbox.net> <87blqui1zu.fsf@toke.dk> X-Clacks-Overhead: GNU Terry Pratchett Date: Thu, 23 Jan 2020 19:01:11 +0100 Message-ID: <875zh2hx20.fsf@toke.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org Luigi Rizzo writes: > On Thu, Jan 23, 2020 at 8:14 AM Toke H=C3=B8iland-J=C3=B8rgensen wrote: >> >> Daniel Borkmann writes: >> >> > On 1/23/20 10:53 AM, Toke H=C3=B8iland-J=C3=B8rgensen wrote: >> >> Luigi Rizzo writes: >> >> >> >>> Add a netdevice flag to control skb linearization in generic xdp mod= e. >> >>> Among the various mechanism to control the flag, the sysfs >> >>> interface seems sufficiently simple and self-contained. >> >>> The attribute can be modified through >> >>> /sys/class/net//xdp_linearize >> >>> The default is 1 (on) >> > >> > Needs documentation in Documentation/ABI/testing/sysfs-class-net. >> > >> >> Erm, won't turning off linearization break the XDP program's ability = to >> >> do direct packet access? >> > >> > Yes, in the worst case you only have eth header pulled into linear >> > section. :/ >> >> In which case an eBPF program could read/write out of bounds since the >> verifier only verifies checks against xdp->data_end. Right? > > Why out of bounds? Without linearization we construct xdp_buff as follows: > > mac_len =3D skb->data - skb_mac_header(skb); > hlen =3D skb_headlen(skb) + mac_len; > xdp->data =3D skb->data - mac_len; > xdp->data_end =3D xdp->data + hlen; > xdp->data_hard_start =3D skb->data - skb_headroom(skb); > > so we shouldn't go out of bounds. Hmm, right, as long as it's guaranteed that the bit up to hlen is already linear; is it? :) -Toke