linux-kernel - Re: [PATCH] bpf: Refactor ptr alu checking rules to allow alu explicitly

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Date: Thu, 18 Jan 2024 16:38:38 +0200
From: Eduard Zingerman <eddyz87@...il.com>
To: Hao Sun <sunhao.th@...il.com>, bpf@...r.kernel.org
Cc: ast@...nel.org, andrii@...nel.org, daniel@...earbox.net, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bpf: Refactor ptr alu checking rules to allow alu
 explicitly

On Wed, 2024-01-17 at 10:40 +0100, Hao Sun wrote:
> Current checking rules are structured to disallow alu on particular ptr
> types explicitly, so default cases are allowed implicitly. This may lead
> to newly added ptr types being allowed unexpectedly. So restruture it to
> allow alu explicitly. The tradeoff is mainly a bit more cases added in
> the switch. The following table from Eduard summarizes the rules:
> 
>         | 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    | conditionally       |
>         | 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                 |
> 
> The refactored rules are equivalent to the original one. Note that
> PTR_TO_FUNC and CONST_PTR_TO_DYNPTR are not reject here because: (1)
> check_mem_access() rejects load/store on those ptrs, and those ptrs
> with offset passing to calls are rejected check_func_arg_reg_off();
> (2) someone may rely on the verifier not rejecting programs earily.
> 
> Signed-off-by: Hao Sun <sunhao.th@...il.com>
> ---

Tried this on top of "Reject variable offset alu on PTR_TO_FLOW_KEYS",
all seems to be ok.

Acked-by: Eduard Zingerman <eddyz87@...il.com>