* [net/sched] Question about possible misuse checksum in tcf_csum_ipv6_icmp()
@ 2024-03-18 19:31 Chenyuan Yang
2024-03-18 19:46 ` Eric Dumazet
0 siblings, 1 reply; 3+ messages in thread
From: Chenyuan Yang @ 2024-03-18 19:31 UTC (permalink / raw)
To: jhs, xiyou.wangcong, jiri; +Cc: davem, edumazet, kuba, pabeni, netdev, zzjas98
Dear TC subsystem maintainers,
We are curious whether the function `tcf_csum_ipv6_icmp()` would have a misuse of `csum_partial()` leading to an out-of-bounds access.
The function `tcf_csum_ipv6_icmp` is https://elixir.bootlin.com/linux/v6.8/source/net/sched/act_csum.c#L183 and the relevant code is
```
static int tcf_csum_ipv6_icmp(struct sk_buff *skb, unsigned int ihl,
unsigned int ipl)
{
...
ip6h = ipv6_hdr(skb);
icmp6h->icmp6_cksum = 0;
skb->csum = csum_partial(icmp6h, ipl - ihl, 0);
icmp6h->icmp6_cksum = csum_ipv6_magic(&ip6h->saddr, &ip6h->daddr,
ipl - ihl, IPPROTO_ICMPV6,
skb->csum);
...
}
```
Based on this patch: https://lore.kernel.org/netdev/20240201083817.12774-1-atenart@kernel.org/T/, it seems that the `skb` here for ICMPv6 could be non-linear, and `csum_partial` is not suitable for non-linear SKBs, which could lead to an out-of-bound access. The correct approach is to use `skb_checksum` which properly handles non-linear SKBs.
Based on the above information, a possible fix would be
```
- skb->csum = csum_partial(icmp6h, ipl - ihl, 0);
+ skb->csum = skb_checksum(skb, skb_transport_offset(skb), ipl - ihl, 0);
```
Please kindly correct us if we missed any key information. Looking forward to your response!
Best,
Chenyuan
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [net/sched] Question about possible misuse checksum in tcf_csum_ipv6_icmp()
2024-03-18 19:31 [net/sched] Question about possible misuse checksum in tcf_csum_ipv6_icmp() Chenyuan Yang
@ 2024-03-18 19:46 ` Eric Dumazet
2024-03-18 19:54 ` Chenyuan Yang
0 siblings, 1 reply; 3+ messages in thread
From: Eric Dumazet @ 2024-03-18 19:46 UTC (permalink / raw)
To: Chenyuan Yang
Cc: jhs, xiyou.wangcong, jiri, davem, kuba, pabeni, netdev, zzjas98
On Mon, Mar 18, 2024 at 8:31 PM Chenyuan Yang <chenyuan0y@gmail.com> wrote:
>
> Dear TC subsystem maintainers,
>
> We are curious whether the function `tcf_csum_ipv6_icmp()` would have a misuse of `csum_partial()` leading to an out-of-bounds access.
>
> The function `tcf_csum_ipv6_icmp` is https://elixir.bootlin.com/linux/v6.8/source/net/sched/act_csum.c#L183 and the relevant code is
> ```
> static int tcf_csum_ipv6_icmp(struct sk_buff *skb, unsigned int ihl,
> unsigned int ipl)
> {
> ...
> ip6h = ipv6_hdr(skb);
> icmp6h->icmp6_cksum = 0;
> skb->csum = csum_partial(icmp6h, ipl - ihl, 0);
> icmp6h->icmp6_cksum = csum_ipv6_magic(&ip6h->saddr, &ip6h->daddr,
> ipl - ihl, IPPROTO_ICMPV6,
> skb->csum);
> ...
> }
> ```
>
> Based on this patch: https://lore.kernel.org/netdev/20240201083817.12774-1-atenart@kernel.org/T/, it seems that the `skb` here for ICMPv6 could be non-linear, and `csum_partial` is not suitable for non-linear SKBs, which could lead to an out-of-bound access. The correct approach is to use `skb_checksum` which properly handles non-linear SKBs.
>
> Based on the above information, a possible fix would be
> ```
> - skb->csum = csum_partial(icmp6h, ipl - ihl, 0);
> + skb->csum = skb_checksum(skb, skb_transport_offset(skb), ipl - ihl, 0);
> ```
Why would this code be an issue only in tcf_csum_ipv6_icmp(), and not
in other functions ?
It seems we have proper pskb_may_pull() calls.
If you have a syzbot/syzkaller trace, please share it.
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [net/sched] Question about possible misuse checksum in tcf_csum_ipv6_icmp()
2024-03-18 19:46 ` Eric Dumazet
@ 2024-03-18 19:54 ` Chenyuan Yang
0 siblings, 0 replies; 3+ messages in thread
From: Chenyuan Yang @ 2024-03-18 19:54 UTC (permalink / raw)
To: Eric Dumazet
Cc: jhs, xiyou.wangcong, jiri, davem, kuba, pabeni, netdev, zzjas98
Thank you very much for your swift response.
> It seems we have proper pskb_may_pull() calls.
I realized that I had overlooked verifying the suitability of the SKB
in this context. Your observation is indeed correct, and I now
understand that this should not present any concerns.
Thank you once again for your valuable feedback.
Best,
Chenyuan
On Mon, Mar 18, 2024 at 2:46 PM Eric Dumazet <edumazet@google.com> wrote:
>
> On Mon, Mar 18, 2024 at 8:31 PM Chenyuan Yang <chenyuan0y@gmail.com> wrote:
> >
> > Dear TC subsystem maintainers,
> >
> > We are curious whether the function `tcf_csum_ipv6_icmp()` would have a misuse of `csum_partial()` leading to an out-of-bounds access.
> >
> > The function `tcf_csum_ipv6_icmp` is https://elixir.bootlin.com/linux/v6.8/source/net/sched/act_csum.c#L183 and the relevant code is
> > ```
> > static int tcf_csum_ipv6_icmp(struct sk_buff *skb, unsigned int ihl,
> > unsigned int ipl)
> > {
> > ...
> > ip6h = ipv6_hdr(skb);
> > icmp6h->icmp6_cksum = 0;
> > skb->csum = csum_partial(icmp6h, ipl - ihl, 0);
> > icmp6h->icmp6_cksum = csum_ipv6_magic(&ip6h->saddr, &ip6h->daddr,
> > ipl - ihl, IPPROTO_ICMPV6,
> > skb->csum);
> > ...
> > }
> > ```
> >
> > Based on this patch: https://lore.kernel.org/netdev/20240201083817.12774-1-atenart@kernel.org/T/, it seems that the `skb` here for ICMPv6 could be non-linear, and `csum_partial` is not suitable for non-linear SKBs, which could lead to an out-of-bound access. The correct approach is to use `skb_checksum` which properly handles non-linear SKBs.
> >
> > Based on the above information, a possible fix would be
> > ```
> > - skb->csum = csum_partial(icmp6h, ipl - ihl, 0);
> > + skb->csum = skb_checksum(skb, skb_transport_offset(skb), ipl - ihl, 0);
> > ```
>
> Why would this code be an issue only in tcf_csum_ipv6_icmp(), and not
> in other functions ?
>
> It seems we have proper pskb_may_pull() calls.
>
> If you have a syzbot/syzkaller trace, please share it.
^ permalink raw reply [flat|nested] 3+ messages in thread
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2024-03-18 19:31 [net/sched] Question about possible misuse checksum in tcf_csum_ipv6_icmp() Chenyuan Yang
2024-03-18 19:46 ` Eric Dumazet
2024-03-18 19:54 ` Chenyuan Yang
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