net: Add rx_skb of kfree_skb to raw_tp_null_args[].

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 kfree_skb to raw_tp_null_args[] 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 838a10bd2ebf ("bpf: Augment raw_tp
arguments with PTR_MAYBE_NULL").

[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
RIP: 0010:bpf_prog_5e21a6db8fcff1aa_drop+0x10/0x2d
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

Fixes: c53795d48ee8 ("net: add rx_sk to trace_kfree_skb")
Reported-by: Yan Zhai <yan@cloudflare.com>
Closes: https://lore.kernel.org/netdev/Z50zebTRzI962e6X@debian.debian/
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Tested-by: Yan Zhai <yan@cloudflare.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20250201030142.62703-1-kuniyu@amazon.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 9de6acddd479b4f5e32a5e6ba43cf369de4cee29..c3223e0db2f51834caf31a420d51f6ea3da71c63 100644 (file)
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -6507,6 +6507,8 @@ static const struct bpf_raw_tp_null_args raw_tp_null_args[] = {
        /* rxrpc */
        { "rxrpc_recvdata", 0x1 },
        { "rxrpc_resend", 0x10 },
+       /* skb */
+       {"kfree_skb", 0x1000},
        /* sunrpc */
        { "xs_stream_read_data", 0x1 },
        /* ... from xprt_cong_event event class */