netdev - Re: [PATCH v1 bpf] net: Annotate rx_sk with __nullable for trace_kfree

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Message-ID: <CAO3-Pbr6dRto6LcACKsBDcn1s+gzbF4Kb-6WWeKgyNSAnvvkoQ@mail.gmail.com>
Date: Fri, 31 Jan 2025 20:41:24 -0600
From: Yan Zhai <yan@...udflare.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: Kuniyuki Iwashima <kuniyu@...zon.com>, Kumar Kartikeya Dwivedi <memxor@...il.com>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Kuniyuki Iwashima <kuni1840@...il.com>, bpf@...r.kernel.org, 
	netdev@...r.kernel.org
Subject: Re: [PATCH v1 bpf] net: Annotate rx_sk with __nullable for trace_kfree_skb.

On Fri, Jan 31, 2025 at 8:19 PM Martin KaFai Lau <martin.lau@...ux.dev> wrote:
>
> On 1/31/25 4:14 PM, Kuniyuki Iwashima wrote:
> > Yan Zhai reported a BPF prog could trigger a null-ptr-deref [0]
> > in trace_kfree_skb if the prog does not check if rx_sk is NULL.
> >
> > Commit c53795d48ee8 ("net: add rx_sk to trace_kfree_skb") added
> > rx_sk to trace_kfree_skb, but rx_sk is optional and could be NULL.
> >
> > Let's add __nullable suffix to rx_sk to let the BPF verifier
> > validate such a prog and prevent the issue.
> >
> > Now we fail to load such a prog:
> >
> >    libbpf: prog 'drop': -- BEGIN PROG LOAD LOG --
> >    0: R1=ctx() R10=fp0
> >    ; int BPF_PROG(drop, struct sk_buff *skb, void *location, @ kfree_skb_sk_null.bpf.c:21
> >    0: (79) r3 = *(u64 *)(r1 +24)
> >    func 'kfree_skb' arg3 has btf_id 5253 type STRUCT 'sock'
> >    1: R1=ctx() R3_w=trusted_ptr_or_null_sock(id=1)
> >    ; bpf_printk("sk: %d, %d\n", sk, sk->__sk_common.skc_family); @ kfree_skb_sk_null.bpf.c:24
> >    1: (69) r4 = *(u16 *)(r3 +16)
> >    R3 invalid mem access 'trusted_ptr_or_null_'
> >    processed 2 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
> >    -- END PROG LOAD LOG --
> >
> > Note this fix requires commit 8aeaed21befc ("bpf: Support
> > __nullable argument suffix for tp_btf").
>
> I believe the current way is to add kfree_skb to the raw_tp_null_args[],
> https://lore.kernel.org/all/20241213221929.3495062-3-memxor@gmail.com/
>
Nice to learn the trick. Thanks Martin!

Yan

> cc: Kumar
>