[PATCH bpf] lwt_bpf: 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which changes the dst - such as bpf_skb_set_tunnel_key() - is called as part of the xmit program the accessed dst is no longer valid.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.
Cc: stable@vger.kernel.org Fixes: commit 3a0af8fd61f9 ("bpf: BPF for lightweight tunnel infrastructure") Signed-off-by: Eyal Birger eyal.birger@gmail.com --- 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 349480ef68a5..8b6b5e72b217 100644 --- a/net/core/lwt_bpf.c +++ b/net/core/lwt_bpf.c @@ -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;
On 4/20/22 6:52 PM, Eyal Birger wrote:
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.
Cc: stable@vger.kernel.org Fixes: commit 3a0af8fd61f9 ("bpf: BPF for lightweight tunnel infrastructure") Signed-off-by: Eyal Birger eyal.birger@gmail.com
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 349480ef68a5..8b6b5e72b217 100644 --- a/net/core/lwt_bpf.c +++ b/net/core/lwt_bpf.c @@ -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;@@ -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);
Ok, makes sense given for BPF_OK the dst->dev shouldn't change here (e.g. as opposed to BPF_REDIRECT). Applied, please also follow-up with a BPF selftest for test_progs so that this won't break in future when it's running as part of BPF CI.
if (unlikely(ret)) return ret;
Thanks, Daniel
On 4/22/22 5:48 PM, Daniel Borkmann wrote:
On 4/20/22 6:52 PM, Eyal Birger wrote:
[...]
Ok, makes sense given for BPF_OK the dst->dev shouldn't change here (e.g. as opposed to BPF_REDIRECT). Applied, please also follow-up with a BPF selftest for test_progs so that this won't break in future when it's running as part of BPF CI.
(Coverage for lwt BPF flavor from test_progs is afaik non-existent aside from some section name tests. Would be great in general to have runtime tests asserting lwt behavior there if you have a chance.)
Hello:
This patch was applied to bpf/bpf.git (master) by Daniel Borkmann daniel@iogearbox.net:
On Wed, 20 Apr 2022 19:52:19 +0300 you wrote:
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:
[...]
Here is the summary with links: - [bpf] lwt_bpf: fix crash when using bpf_skb_set_tunnel_key() from bpf_xmit lwt hook https://git.kernel.org/bpf/bpf/c/b02d196c44ea
You are awesome, thank you!
linux-stable-mirror@lists.linaro.org
-
Daniel Borkmann -
Eyal Birger -
patchwork-bot+netdevbpf＠kernel.org