linux-kernel - Re: [PATCH] bpf: Reject variable offset alu on PTR_TO_FLOW

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Date: Tue, 09 Jan 2024 18:21:01 +0200
From: Eduard Zingerman <eddyz87@...il.com>
To: Hao Sun <sunhao.th@...il.com>, bpf@...r.kernel.org
Cc: ppenkov@...gle.com, willemb@...gle.com, ast@...nel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bpf: Reject variable offset alu on PTR_TO_FLOW_KEYS

On Tue, 2024-01-09 at 16:36 +0100, Hao Sun wrote:
> For PTR_TO_FLOW_KEYS, check_flow_keys_access() only uses fixed off
> for validation. However, variable offset ptr alu is not prohibited
> for this ptr kind. So the variable offset is not checked.
> 
[...]
> 
> Fixes: d58e468b1112 ("flow_dissector: implements flow dissector BPF hook")
> Signed-off-by: Hao Sun <sunhao.th@...il.com>
> ---
>  kernel/bpf/verifier.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index adbf330d364b..65f598694d55 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -12826,6 +12826,10 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
>  	}
>  
>  	switch (base_type(ptr_reg->type)) {
> +	case PTR_TO_FLOW_KEYS:
> +		if (known)
> +			break;
> +		fallthrough;
>  	case CONST_PTR_TO_MAP:
>  		/* smin_val represents the known value */
>  		if (known && smin_val == 0 && opcode == BPF_ADD)

This change makes sense, could you please add a testcase?

Also, this switch is written to explicitly disallow and implicitly allow
pointer arithmetics, which might be a bit unsafe when new ptr types are added.
Would it make more sense to instead rewrite it to explicitly allow?
E.g. here is what it currently allows / disallows:

| Pointer type        | Arithmetics allowed |
|---------------------+---------------------|
| PTR_TO_CTX          | yes                 |
| CONST_PTR_TO_MAP    | conditionally       |
| PTR_TO_MAP_VALUE    | yes                 |
| PTR_TO_MAP_KEY      | yes                 |
| PTR_TO_STACK        | yes                 |
| PTR_TO_PACKET_META  | yes                 |
| PTR_TO_PACKET       | yes                 |
| PTR_TO_PACKET_END   | no                  |
| PTR_TO_FLOW_KEYS    | yes                 |
| PTR_TO_SOCKET       | no                  |
| PTR_TO_SOCK_COMMON  | no                  |
| PTR_TO_TCP_SOCK     | no                  |
| PTR_TO_TP_BUFFER    | yes                 |
| PTR_TO_XDP_SOCK     | no                  |
| PTR_TO_BTF_ID       | yes                 |
| PTR_TO_MEM          | yes                 |
| PTR_TO_BUF          | yes                 |
| PTR_TO_FUNC         | yes                 |
| CONST_PTR_TO_DYNPTR | yes                 |

Of these PTR_TO_FUNC and CONST_PTR_TO_DYNPTR (?) should not be allowed
as well, probably (not sure if that could be exploited).