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bpf, lwt: Fix crash when using bpf_skb_set_tunnel_key() from bpf_xmit lwt hook 

Bugzilla: https://bugzilla.redhat.com/2120966

commit b02d196c44ead1a5949729be9ff08fe781c3e48a
Author: Eyal Birger <eyal.birger@gmail.com>
Date:   Wed Apr 20 19:52:19 2022 +0300

    bpf, lwt: Fix crash when using bpf_skb_set_tunnel_key() from bpf_xmit lwt hook

    xmit_check_hhlen() observes the dst for getting the device hard header
    length to make sure a modified packet can fit. When a helper which changes
    the dst - such as bpf_skb_set_tunnel_key() - is called as part of the
    xmit program the accessed dst is no longer valid.

    This leads to the following splat:

     BUG: kernel NULL pointer dereference, address: 00000000000000de
     #PF: supervisor read access in kernel mode
     #PF: error_code(0x0000) - not-present page
     PGD 0 P4D 0
     Oops: 0000 [#1] PREEMPT SMP PTI
     CPU: 0 PID: 798 Comm: ping Not tainted 5.18.0-rc2+ #103
     Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
     RIP: 0010:bpf_xmit+0xfb/0x17f
     Code: c6 c0 4d cd 8e 48 c7 c7 7d 33 f0 8e e8 42 09 fb ff 48 8b 45 58 48 8b 95 c8 00 00 00 48 2b 95 c0 00 00 00 48 83 e0 fe 48 8b 00 <0f> b7 80 de 00 00 00 39 c2 73 22 29 d0 b9 20 0a 00 00 31 d2 48 89
     RSP: 0018:ffffb148c0bc7b98 EFLAGS: 00010282
     RAX: 0000000000000000 RBX: 0000000000240008 RCX: 0000000000000000
     RDX: 0000000000000010 RSI: 00000000ffffffea RDI: 00000000ffffffff
     RBP: ffff922a828a4e00 R08: ffffffff8f1350e8 R09: 00000000ffffdfff
     R10: ffffffff8f055100 R11: ffffffff8f105100 R12: 0000000000000000
     R13: ffff922a828a4e00 R14: 0000000000000040 R15: 0000000000000000
     FS:  00007f414e8f0080(0000) GS:ffff922afdc00000(0000) knlGS:0000000000000000
     CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
     CR2: 00000000000000de CR3: 0000000002d80006 CR4: 0000000000370ef0
     Call Trace:
      <TASK>
      lwtunnel_xmit.cold+0x71/0xc8
      ip_finish_output2+0x279/0x520
      ? __ip_finish_output.part.0+0x21/0x130

    Fix by fetching the device hard header length before running the BPF code.

    Fixes: 3a0af8fd61 ("bpf: BPF for lightweight tunnel infrastructure")
    Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
    Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
    Link: https://lore.kernel.org/bpf/20220420165219.1755407-1-eyal.birger@gmail.com

Signed-off-by: Jiri Benc <jbenc@redhat.com>
This commit is contained in:
Jiri Benc 2022-09-08 11:58:07 +02:00 committed by Jerome Marchand
parent 6e9b7b2232
commit 9c1b425aa2
1 changed files with 3 additions and 4 deletions
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7
net/core/lwt_bpf.c
View File

@ -159,10 +159,8 @@ static int bpf_output(struct net *net, struct sock *sk, struct sk_buff *skb)
return dst->lwtstate->orig_output(net, sk, skb);
}
static int xmit_check_hhlen(struct sk_buff *skb)
static int xmit_check_hhlen(struct sk_buff *skb, int hh_len)
{
int hh_len = skb_dst(skb)->dev->hard_header_len;
if (skb_headroom(skb) < hh_len) {
int nhead = HH_DATA_ALIGN(hh_len - skb_headroom(skb));
@ -274,6 +272,7 @@ static int bpf_xmit(struct sk_buff *skb)
bpf = bpf_lwt_lwtunnel(dst->lwtstate);
if (bpf->xmit.prog) {
int hh_len = dst->dev->hard_header_len;
__be16 proto = skb->protocol;
int ret;
@ -291,7 +290,7 @@ static int bpf_xmit(struct sk_buff *skb)
/* If the header was expanded, headroom might be too
* small for L2 header to come, expand as needed.
*/
ret = xmit_check_hhlen(skb);
ret = xmit_check_hhlen(skb, hh_len);
if (unlikely(ret))
return ret;