Re: [RFC PATCH v2] bpf: introduce TAINT_UNSAFE_BPF for mutating helpers

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] The kernel is prematurely tainted during the BPF verifier's static analysis phase, causing false-positive taints and potential panics for rejected programs.
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commit 58605e7e9b9266affb08aa0c20744ecff2dd01d4
Author: Aaron Tomlin <atomlin@atomlin.com>

bpf: introduce TAINT_UNSAFE_BPF for mutating helpers

This commit introduces a new kernel taint flag (TAINT_UNSAFE_BPF) to indicate
when an eBPF program utilizing mutating helpers like bpf_probe_write_user()
or bpf_override_return() is loaded.

> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 69d75515ed3f..cf0634d8901f 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c

[ ... ]

> @@ -10287,6 +10287,14 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
>  		return err;
>  	}
>  
> +	/*
> +	 * Taint the kernel if the program attempts to use mutating
> +	 * helpers.
> +	 */
> +	if (func_id == BPF_FUNC_probe_write_user ||
> +	    func_id == BPF_FUNC_override_return)
> +		add_taint(TAINT_UNSAFE_BPF, LOCKDEP_STILL_OK);

Is it possible for the kernel to be tainted here even if the program is
ultimately rejected by the verifier?

check_helper_call() is executed during the verifier's static analysis phase.
If the program fails verification later (for example, failing the
GPL-compatibility check immediately following this code, or encountering an
invalid memory access), the program is safely rejected and never loaded.

Because add_taint() synchronously modifies global state, wouldn't the kernel
remain permanently tainted in this scenario, falsely indicating a compromised
state? 

Furthermore, if panic_on_taint is enabled, could a user trigger a panic
merely by attempting to load a malformed program that contains these helpers?

Could this be addressed by recording the intent to taint within the program's
auxiliary data during verification, and only invoking add_taint() once the
program has successfully passed all checks and is definitively loaded?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260503153730.541685-1-atomlin@atomlin.com?part=1