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

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Date: Wed, 17 Jan 2024 10:42:51 +0100
From: Hao Sun <sunhao.th@...il.com>
To: bpf@...r.kernel.org
Cc: ast@...nel.org, andrii@...nel.org, daniel@...earbox.net, eddyz87@...il.com, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bpf: Refactor ptr alu checking rules to allow alu explicitly

On Wed, Jan 17, 2024 at 10:40 AM Hao Sun <sunhao.th@...il.com> 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>
> ---

Not specifying bpf-next as the target repo as my previous patch is not
in it yet.