netdev - Re: [Patch net] net: remove the bogus overflow debug check in pskb_may

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Date: Sun, 9 Jun 2024 10:01:14 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Eric Dumazet <eric.dumazet@...il.com>
Cc: Kuniyuki Iwashima <kuniyu@...zon.com>, xiyou.wangcong@...il.com, bpf@...r.kernel.org, 
	cong.wang@...edance.com, fw@...len.de, netdev@...r.kernel.org, 
	syzbot+0c4150bff9fff3bf023c@...kaller.appspotmail.com
Subject: Re: [Patch net] net: remove the bogus overflow debug check in pskb_may_pull()

Hello Eric,

On Sat, Jun 8, 2024 at 4:01 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
> On 6/7/24 23:32, Kuniyuki Iwashima wrote:
> > From: Cong Wang <xiyou.wangcong@...il.com>
> > Date: Fri, 7 Jun 2024 09:14:04 -0700
> >> On Fri, Jun 07, 2024 at 01:27:47AM +0200, Florian Westphal wrote:
> >>> Cong Wang <xiyou.wangcong@...il.com> wrote:
> >>>> From: Cong Wang <cong.wang@...edance.com>
> >>>>
> >>>> Commit 219eee9c0d16 ("net: skbuff: add overflow debug check to pull/push
> >>>> helpers") introduced an overflow debug check for pull/push helpers.
> >>>> For __skb_pull() this makes sense because its callers rarely check its
> >>>> return value. But for pskb_may_pull() it does not make sense, since its
> >>>> return value is properly taken care of. Remove the one in
> >>>> pskb_may_pull(), we can continue rely on its return value.
> >>> See 025f8ad20f2e3264d11683aa9cbbf0083eefbdcd which would not exist
> >>> without this check, I would not give up yet.
> >> What's the point of that commit?
> > 4b911a9690d7 would be better example.  The warning actually found a
> > bug in NSH GSO.
> >
> > Here's splats triggered by syzkaller using NSH over various tunnels.
> > https://lore.kernel.org/netdev/20240415222041.18537-2-kuniyu@amazon.com/
>
>
> Right. We discussed this before. I guess I forgot to send the fix.
>
> Florian could you submit the suggestion I made before ?
>
> diff --git a/net/core/filter.c b/net/core/filter.c
> index
> 358870408a51e61f3cbc552736806e4dfee1ec39..da7aae6fd8ba557c66699d1cfebd47f18f442aa2
> 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -1662,6 +1662,11 @@ static DEFINE_PER_CPU(struct bpf_scratchpad, bpf_sp);
>   static inline int __bpf_try_make_writable(struct sk_buff *skb,
>                         unsigned int write_len)
>   {
> +#if defined(CONFIG_DEBUG_NET)

I wonder why we want to avoid printing warning information especially
when CONFIG_DEBUG_NET is on? Any reasons?

> +    /* Avoid a splat in pskb_may_pull_reason() */
> +    if (write_len > INT_MAX)

I guess: if the purpose is to skip pskb_may_pull_reason and then
return a proper value (-EINVAL) to its caller when the above statement
is true, can we add the warning here on top of you patch:
DEBUG_NET_WARN_ON_ONCE(len > INT_MAX);
?

After this, we can print useful information and let the caller catch
the -EINVAL return value.

Thanks,
Jason

> +        return -EINVAL;
> +#endif
>       return skb_ensure_writable(skb, write_len);
>   }
>
>
>