Unchecked sock pointer causes panic in RAW_TP

[Yan Zhai <yan@cloudflare.com>]

Hello,

We encountered a panic when tracing kfree_skb with RAW_TP. The problematic
argument was introduced in commit ba8de796baf4 ("net: introduce
sk_skb_reason_drop function"). It turns out that the verifier still accepted
the program despite it didn't test sk == NULL. And this caused kernel panic. I
attached a small reproducer and panic trace at the end. It's stably
reproducible when packets are dropped without a receiver (e.g. run iperf2 UDP
test toward localhost), in both 6.12.11 release and a recent bpf-next master
snapshot (I was using commit c03320a6768c).

As a contrast, for another tracepoint like tcp_send_reset, if sk is not checked
before dereferencing, the verifier will complain and reject the program as
expected. So this feels like some annotation is missing? Appreciate if someone
could help me figure out.

thanks
Yan

----- Reproducer and panic trace ----
#include "vmlinux.h"
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_tracing.h>

char _license[] SEC("license") = "GPL";

SEC("tp_btf/tcp_send_reset")
int BPF_PROG(tcp_send_reset, struct sock *sk, struct sk_buff *skb)
{
        if (skb && sk && sk->__sk_common.skc_state == TCP_LISTEN) {
                bpf_printk("sk: %d, %d\n", sk, sk->__sk_common.skc_family);
        }
        return 0;
}

SEC("tp_btf/kfree_skb")
int BPF_PROG(drop, struct sk_buff *skb, void *location,
             enum skb_drop_reason reason, struct sock *sk)
{
        bpf_printk("sk: %d, %d\n", sk, sk->__sk_common.skc_family);
        return 0;
}

Byte code:
int drop(unsigned long long * ctx):
; int BPF_PROG(drop, struct sk_buff *skb, void *location,
   0: (79) r3 = *(u64 *)(r1 +24)
; bpf_printk("sk: %d, %d\n", sk, sk->__sk_common.skc_family);
   1: (69) r4 = *(u16 *)(r3 +16)
   2: (18) r1 = map[id:7][0]+12
   4: (b7) r2 = 12
   5: (85) call bpf_trace_printk#-63104
; int BPF_PROG(drop, struct sk_buff *skb, void *location,
   6: (b7) r0 = 0
   7: (95) exit

Trace:
[   29.982295][  T348] BUG: kernel NULL pointer
dereference, address: 0000000000000010
[   29.983487][  T348] #PF: supervisor read access in kernel mode
[   29.984326][  T348] #PF: error_code(0x0000) - not-present page
[   29.985138][  T348] PGD 0 P4D 0
[   29.985654][  T348] Oops: Oops: 0000 [#1] PREEMPT SMP
[   29.986351][  T348] CPU: 6 UID: 0 PID: 348 Comm: sshd Not tainted
6.12.11 #206
[   29.987309][  T348] Hardware name: QEMU Standard PC (i440FX + PIIX,
1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
[   29.988678][  T348] RIP:
0010:bpf_prog_5e21a6db8fcff1aa_drop+0x10/0x2d
[   29.989553][  T348] 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
[   29.992008][  T348] RSP: 0018:ffffa86640b53da8 EFLAGS: 00010202
[   29.992811][  T348] RAX: 0000000000000001 RBX: ffffa866402d1000
RCX: 0000000000000002
[   29.993852][  T348] RDX: 0000000000000000 RSI: ffffa866402d1048
RDI: ffffa86640b53dc8
[   29.994929][  T348] RBP: ffffa86640b53da8 R08: 0000000000000000
R09: 9c908cd09b9c8c91
[   29.995991][  T348] R10: ffff90adc056b540 R11: 0000000000000002
R12: 0000000000000000
[   29.997043][  T348] R13: ffffa86640b53e88 R14: 0000000000000800
R15: fffffffffffffffe
[   29.998097][  T348] FS:  00007f2a27c2b480(0000)
GS:ffff90b0efd00000(0000) knlGS:0000000000000000
[   29.999279][  T348] CS:  0010 DS: 0000 ES: 0000 CR0:
0000000080050033
[   30.000161][  T348] CR2: 0000000000000010 CR3: 0000000100e69004
CR4: 00000000001726f0
[   30.001217][  T348] Call Trace:
[   30.001724][  T348]  <TASK>
[   30.002145][  T348]  ? __die+0x1f/0x60
[   30.002694][  T348]  ? page_fault_oops+0x148/0x420
[   30.003386][  T348]  ? search_bpf_extables+0x5b/0x70
[   30.004082][  T348]  ? fixup_exception+0x27/0x2c0
[   30.004748][  T348]  ? exc_page_fault+0x75/0x170
[   30.005416][  T348]  ? asm_exc_page_fault+0x22/0x30
[   30.006104][  T348]  ? bpf_prog_5e21a6db8fcff1aa_drop+0x10/0x2d
[   30.006923][  T348]  bpf_trace_run4+0x68/0xd0
[   30.007566][  T348]  ? unix_stream_connect+0x1f4/0x6f0
[   30.008274][  T348]  sk_skb_reason_drop+0x90/0x120
[   30.008960][  T348]  unix_stream_connect+0x1f4/0x6f0
[   30.009662][  T348]  __sys_connect+0x7f/0xb0
[   30.010267][  T348]  __x64_sys_connect+0x14/0x20
[   30.010927][  T348]  do_syscall_64+0x47/0xc30
[   30.011567][  T348]  entry_SYSCALL_64_after_hwframe+0x4b/0x53
[   30.012371][  T348] RIP: 0033:0x7f2a27f296a0
[   30.012998][  T348] 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
[   30.015491][  T348] RSP: 002b:00007ffe29274f58 EFLAGS: 00000202
ORIG_RAX: 000000000000002a