[PATCH 5.4 47/84] bpf, lwt: Fix crash when using bpf_skb_set_tunnel_key() from bpf_xmit lwt hook

Greg Kroah-Hartman
2022-05-04T16:52:45Z

From: Eyal Birger <eyal.birger@gmail.com>

[ Upstream commit b02d196c44ead1a5949729be9ff08fe781c3e48a ]

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xmit program the accessed dst is no longer valid.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: 3a0af8fd61f9 ("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: Sasha Levin <sashal@kernel.org>
---
net/core/lwt_bpf.c | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/net/core/lwt_bpf.c b/net/core/lwt_bpf.c
index a5502c5aa44e..bf270b6a99b4 100644
--- a/net/core/lwt_bpf.c
+++ b/net/core/lwt_bpf.c
@@ -158,10 +158,8 @@ 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));

@@ -273,6 +271,7 @@ 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;

@@ -290,7 +289,7 @@ 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;

--
2.35.1