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From:   Eric Dumazet <edumazet@google.com>
Date:   Thu, 30 Nov 2023 15:05:38 +0100
Message-ID: <CANn89i+u6tFJQKESV9DH-HypezVV7Ux+XhnyFGLd833PR9Qpyw@mail.gmail.com>
Subject: Re: [PATCH net] ipv4: ip_gre: Handle skb_pull() failure in ipgre_xmit()
To:     Shigeru Yoshida <syoshida@redhat.com>
Cc:     willemdebruijn.kernel@gmail.com, pabeni@redhat.com,
        davem@davemloft.net, dsahern@kernel.org, kuba@kernel.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
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On Thu, Nov 30, 2023 at 3:03=E2=80=AFPM Shigeru Yoshida <syoshida@redhat.co=
m> wrote:
>
> On Wed, 29 Nov 2023 10:56:47 +0100, Eric Dumazet wrote:
> > On Wed, Nov 29, 2023 at 2:51=E2=80=AFAM Shigeru Yoshida <syoshida@redha=
t.com> wrote:
> >>
> >> On Mon, 27 Nov 2023 10:55:01 -0500, Willem de Bruijn wrote:
> >> > Shigeru Yoshida wrote:
> >> >> In ipgre_xmit(), skb_pull() may fail even if pskb_inet_may_pull() r=
eturns
> >> >> true. For example, applications can create a malformed packet that =
causes
> >> >> this problem with PF_PACKET.
> >> >
> >> > It may fail because because pskb_inet_may_pull does not account for
> >> > tunnel->hlen.
> >> >
> >> > Is that what you are referring to with malformed packet? Can you
> >> > eloborate a bit on in which way the packet has to be malformed to
> >> > reach this?
> >>
> >> Thank you very much for your prompt feedback.
> >>
> >> Actually, I found this problem by running syzkaller. Syzkaller
> >> reported the following uninit-value issue (I think the root cause of
> >> this issue is the same as the one Eric mentioned):
> >
> > Yes, I also have a similar syzbot report (but no repro yet) I am
> > releasing it right now.
> >
> >>
> >> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D
> >> BUG: KMSAN: uninit-value in __gre_xmit net/ipv4/ip_gre.c:469 [inline]
> >> BUG: KMSAN: uninit-value in ipgre_xmit+0xdf4/0xe70 net/ipv4/ip_gre.c:6=
62
> >>  __gre_xmit net/ipv4/ip_gre.c:469 [inline]
> >>
> >
> >
> >
> >> The simplified version of the repro is shown below:
> >>
> >> #include <linux/if_ether.h>
> >> #include <sys/ioctl.h>
> >> #include <netinet/ether.h>
> >> #include <net/if.h>
> >> #include <sys/socket.h>
> >> #include <netinet/in.h>
> >> #include <string.h>
> >> #include <unistd.h>
> >> #include <stdio.h>
> >> #include <stdlib.h>
> >> #include <linux/if_packet.h>
> >>
> >> int main(void)
> >> {
> >>         int s, s1, s2, data =3D 0;
> >>         struct ifreq ifr;
> >>         struct sockaddr_ll addr =3D { 0 };
> >>         unsigned char mac_addr[] =3D {0x1, 0x2, 0x3, 0x4, 0x5, 0x6};
> >>
> >>         s =3D socket(AF_PACKET, SOCK_DGRAM, 0x300);
> >>         s1 =3D socket(AF_PACKET, SOCK_RAW, 0x300);
> >>         s2 =3D socket(AF_NETLINK, SOCK_RAW, 0);
> >>
> >>         strcpy(ifr.ifr_name, "gre0");
> >>         ioctl(s2, SIOCGIFINDEX, &ifr);
> >>
> >>         addr.sll_family =3D AF_PACKET;
> >>         addr.sll_ifindex =3D ifr.ifr_ifindex;
> >>         addr.sll_protocol =3D htons(0);
> >>         addr.sll_hatype =3D ARPHRD_ETHER;
> >>         addr.sll_pkttype =3D PACKET_HOST;
> >>         addr.sll_halen =3D ETH_ALEN;
> >>         memcpy(addr.sll_addr, mac_addr, ETH_ALEN);
> >>
> >>         sendto(s1, &data, 1, 0, (struct sockaddr *)&addr, sizeof(addr)=
);
> >>
> >>         return 0;
> >> }
> >>
> >> The repro sends a 1-byte packet that doesn't have the correct IP
> >> header. I meant this as "malformed pachet", but that might be a bit
> >> confusing, sorry.
> >>
> >> I think the cause of the uninit-value access is that ipgre_xmit()
> >> reallocates the skb with skb_cow_head() and copies only the 1-byte
> >> data, so any IP header access through `tnl_params` can cause the
> >> problem.
> >>
> >> At first I tried to modify pskb_inet_may_pull() to detect this type of
> >> packet, but I ended up doing this patch.
> >
> > Even after your patch, __skb_pull() could call BUG() and crash.
> >
> > I would suggest using this fix instead.
>
> Thank you for your comment.
>
> Your patch ensures that skb_pull() can pull the required size, so it
> looks good to me. Also, I have tested your suggested patch with the
> repro and confirmed that it fixes the issue.
>

This is great, please cook/send a V2 with this updated patch.

I will add a 'Reviewed-by: Eric Dumazet <edumazet@google.com>' then.

Thanks.