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

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Message-ID: <CAO3-PbpS=YrWnFCr1K8vdD50pyBamHysRQfcZRBn1Q=icwCW8w@mail.gmail.com>
Date: Fri, 31 Jan 2025 20:40:17 -0600
From: Yan Zhai <yan@...udflare.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>
Cc: Martin KaFai Lau <martin.lau@...ux.dev>, 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 6:14 PM Kuniyuki Iwashima <kuniyu@...zon.com> 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").
>
> [0]:
> BUG: kernel NULL pointer dereference, address: 0000000000000010
>  PF: supervisor read access in kernel mode
>  PF: error_code(0x0000) - not-present page
> PGD 0 P4D 0
> PREEMPT SMP
> CPU: 6 UID: 0 PID: 348 Comm: sshd Not tainted 6.12.11 #206
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> RIP: 0010:bpf_prog_5e21a6db8fcff1aa_drop+0x10/0x2d
> Code: cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc 0f 1f 44 00 00 0f 1f 00 55 48 89 e5 48 8b 57 18 <48> 0f b7 4a 10 48 bf 0c 4f e2 c1 ad 90 ff ff be 0c 00 00 00 e8 0f
> RSP: 0018:ffffa86640b53da8 EFLAGS: 00010202
> RAX: 0000000000000001 RBX: ffffa866402d1000 RCX: 0000000000000002
> RDX: 0000000000000000 RSI: ffffa866402d1048 RDI: ffffa86640b53dc8
> RBP: ffffa86640b53da8 R08: 0000000000000000 R09: 9c908cd09b9c8c91
> R10: ffff90adc056b540 R11: 0000000000000002 R12: 0000000000000000
> R13: ffffa86640b53e88 R14: 0000000000000800 R15: fffffffffffffffe
> FS:  00007f2a27c2b480(0000) GS:ffff90b0efd00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000010 CR3: 0000000100e69004 CR4: 00000000001726f0
> Call Trace:
>  <TASK>
>  ? __die+0x1f/0x60
>  ? page_fault_oops+0x148/0x420
>  ? search_bpf_extables+0x5b/0x70
>  ? fixup_exception+0x27/0x2c0
>  ? exc_page_fault+0x75/0x170
>  ? asm_exc_page_fault+0x22/0x30
>  ? bpf_prog_5e21a6db8fcff1aa_drop+0x10/0x2d
>  bpf_trace_run4+0x68/0xd0
>  ? unix_stream_connect+0x1f4/0x6f0
>  sk_skb_reason_drop+0x90/0x120
>  unix_stream_connect+0x1f4/0x6f0
>  __sys_connect+0x7f/0xb0
>  __x64_sys_connect+0x14/0x20
>  do_syscall_64+0x47/0xc30
>  entry_SYSCALL_64_after_hwframe+0x4b/0x53
> RIP: 0033:0x7f2a27f296a0
> Code: 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 80 3d 41 ff 0c 00 00 74 17 b8 2a 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 58 c3 0f 1f 80 00 00 00 00 48 83 ec 18 89 54
> RSP: 002b:00007ffe29274f58 EFLAGS: 00000202 ORIG_RAX: 000000000000002a
>
> Fixes: c53795d48ee8 ("net: add rx_sk to trace_kfree_skb")
> Reported-by: Yan Zhai <yan@...udflare.com>
> Closes: https://lore.kernel.org/netdev/Z50zebTRzI962e6X@debian.debian/
> Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> ---
>  include/trace/events/skb.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/include/trace/events/skb.h b/include/trace/events/skb.h
> index b877133cd93a..8bf0e61b8549 100644
> --- a/include/trace/events/skb.h
> +++ b/include/trace/events/skb.h
> @@ -24,9 +24,9 @@ DEFINE_DROP_REASON(FN, FN)
>  TRACE_EVENT(kfree_skb,
>
>         TP_PROTO(struct sk_buff *skb, void *location,
> -                enum skb_drop_reason reason, struct sock *rx_sk),
> +                enum skb_drop_reason reason, struct sock *rx_sk__nullable),
>
> -       TP_ARGS(skb, location, reason, rx_sk),
> +       TP_ARGS(skb, location, reason, rx_sk__nullable),
>
>         TP_STRUCT__entry(
>                 __field(void *,         skbaddr)
> @@ -39,7 +39,7 @@ TRACE_EVENT(kfree_skb,
>         TP_fast_assign(
>                 __entry->skbaddr = skb;
>                 __entry->location = location;
> -               __entry->rx_sk = rx_sk;
> +               __entry->rx_sk = rx_sk__nullable;
>                 __entry->protocol = ntohs(skb->protocol);
>                 __entry->reason = reason;
>         ),
> --
> 2.39.5 (Apple Git-154)
>

Tested-by: Yan Zhai <yan@...udflare.com>