Date:   Tue, 23 Jan 2018 18:14:21 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     Guillaume Nault <g.nault@...halink.fr>
Cc:     network dev <netdev@...r.kernel.org>,
        Michal Ostrowski <mostrows@...thlink.net>,
        Johannes Berg <johannes@...solutions.net>
Subject: Re: [PATCH net] pppoe: take ->needed_headroom of lower device into
 account on xmit

On Tue, Jan 23, 2018 at 1:06 AM, Guillaume Nault <g.nault@...halink.fr> wrote:
> In pppoe_sendmsg(), reserving dev->hard_header_len bytes of headroom
> was probably fine before the introduction of ->needed_headroom in
> commit f5184d267c1a ("net: Allow netdevices to specify needed head/tailroom").
>
> But now, virtual devices typically advertise the size of their overhead
> in dev->needed_headroom, so we must also take it into account in
> skb_reserve().
> Allocation size of skb is also updated to take dev->needed_tailroom
> into account and replace the arbitrary 32 bytes with the real size of
> a PPPoE header.
>
> This issue was discovered by syzbot, who connected a pppoe socket to a
> gre device which had dev->header_ops->create == ipgre_header and
> dev->hard_header_len == 0. Therefore, PPPoE didn't reserve any
> headroom, and dev_hard_header() crashed when ipgre_header() tried to
> prepend its header to skb->data.
>
> skbuff: skb_under_panic: text:000000001d390b3a len:31 put:24
> head:00000000d8ed776f data:000000008150e823 tail:0x7 end:0xc0 dev:gre0
> ------------[ cut here ]------------
> kernel BUG at net/core/skbuff.c:104!
> invalid opcode: 0000 [#1] SMP KASAN
> Dumping ftrace buffer:
>     (ftrace buffer empty)
> Modules linked in:
> CPU: 1 PID: 3670 Comm: syzkaller801466 Not tainted
> 4.15.0-rc7-next-20180115+ #97
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 01/01/2011
> RIP: 0010:skb_panic+0x162/0x1f0 net/core/skbuff.c:100
> RSP: 0018:ffff8801d9bd7840 EFLAGS: 00010282
> RAX: 0000000000000083 RBX: ffff8801d4f083c0 RCX: 0000000000000000
> RDX: 0000000000000083 RSI: 1ffff1003b37ae92 RDI: ffffed003b37aefc
> RBP: ffff8801d9bd78a8 R08: 1ffff1003b37ae8a R09: 0000000000000000
> R10: 0000000000000001 R11: 0000000000000000 R12: ffffffff86200de0
> R13: ffffffff84a981ad R14: 0000000000000018 R15: ffff8801d2d34180
> FS:  00000000019c4880(0000) GS:ffff8801db300000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00000000208bc000 CR3: 00000001d9111001 CR4: 00000000001606e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>   skb_under_panic net/core/skbuff.c:114 [inline]
>   skb_push+0xce/0xf0 net/core/skbuff.c:1714
>   ipgre_header+0x6d/0x4e0 net/ipv4/ip_gre.c:879
>   dev_hard_header include/linux/netdevice.h:2723 [inline]
>   pppoe_sendmsg+0x58e/0x8b0 drivers/net/ppp/pppoe.c:890
>   sock_sendmsg_nosec net/socket.c:630 [inline]
>   sock_sendmsg+0xca/0x110 net/socket.c:640
>   sock_write_iter+0x31a/0x5d0 net/socket.c:909
>   call_write_iter include/linux/fs.h:1775 [inline]
>   do_iter_readv_writev+0x525/0x7f0 fs/read_write.c:653
>   do_iter_write+0x154/0x540 fs/read_write.c:932
>   vfs_writev+0x18a/0x340 fs/read_write.c:977
>   do_writev+0xfc/0x2a0 fs/read_write.c:1012
>   SYSC_writev fs/read_write.c:1085 [inline]
>   SyS_writev+0x27/0x30 fs/read_write.c:1082
>   entry_SYSCALL_64_fastpath+0x29/0xa0
>
> Admittedly PPPoE shouldn't be allowed to run on non Ethernet-like
> interfaces, but reserving space for ->needed_headroom is a more
> fundamental issue that needs to be addressed first.
>
> Same problem exists for __pppoe_xmit(), which also needs to take
> dev->needed_headroom into account in skb_cow_head().
>
> Fixes: f5184d267c1a ("net: Allow netdevices to specify needed head/tailroom")
> Reported-by: syzbot+ed0838d0fa4c4f2b528e20286e6dc63effc7c14d@...kaller.appspotmail.com
> Signed-off-by: Guillaume Nault <g.nault@...halink.fr>
> ---
>  drivers/net/ppp/pppoe.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/net/ppp/pppoe.c b/drivers/net/ppp/pppoe.c
> index 4e1da1645b15..5aa59f41bf8c 100644
> --- a/drivers/net/ppp/pppoe.c
> +++ b/drivers/net/ppp/pppoe.c
> @@ -842,6 +842,7 @@ static int pppoe_sendmsg(struct socket *sock, struct msghdr *m,
>         struct pppoe_hdr *ph;
>         struct net_device *dev;
>         char *start;
> +       int hlen;
>
>         lock_sock(sk);
>         if (sock_flag(sk, SOCK_DEAD) || !(sk->sk_state & PPPOX_CONNECTED)) {
> @@ -860,16 +861,16 @@ static int pppoe_sendmsg(struct socket *sock, struct msghdr *m,
>         if (total_len > (dev->mtu + dev->hard_header_len))
>                 goto end;
>
> -
> -       skb = sock_wmalloc(sk, total_len + dev->hard_header_len + 32,
> -                          0, GFP_KERNEL);
> +       hlen = LL_RESERVED_SPACE(dev);
> +       skb = sock_wmalloc(sk, hlen + sizeof(*ph) + total_len +
> +                          dev->needed_tailroom, 0, GFP_KERNEL);
>         if (!skb) {
>                 error = -ENOMEM;
>                 goto end;
>         }
>
>         /* Reserve space for headers. */
> -       skb_reserve(skb, dev->hard_header_len);
> +       skb_reserve(skb, hlen);
>         skb_reset_network_header(skb);
>
>         skb->dev = dev;
> @@ -930,7 +931,7 @@ static int __pppoe_xmit(struct sock *sk, struct sk_buff *skb)
>         /* Copy the data if there is no space for the header or if it's
>          * read-only.
>          */
> -       if (skb_cow_head(skb, sizeof(*ph) + dev->hard_header_len))
> +       if (skb_cow_head(skb, LL_RESERVED_SPACE(dev) + sizeof(*ph)))
>                 goto abort;
>
>         __skb_push(skb, sizeof(*ph));
Reviewed-by: Xin Long <lucien.xin@...il.com>

> --
> 2.15.1
>

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