linux-kernel - Re: [PATCH 1/2] x86/kprobes: Fix kprobes instruction boudary check with CONFIG

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Date:   Wed, 7 Sep 2022 15:05:13 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Suleiman Souhlal <suleiman@...gle.com>,
        bpf <bpf@...r.kernel.org>, linux-kernel@...r.kernel.org,
        Borislav Petkov <bp@...e.de>,
        Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org
Subject: Re: [PATCH 1/2] x86/kprobes: Fix kprobes instruction boudary check
 with CONFIG_RETHUNK

On Wed, Sep 07, 2022 at 10:02:41AM +0200, Peter Zijlstra wrote:

> 	struct queue q;
> 
> 	start = paddr - offset;
> 	end = start + size;
> 	push(&q, paddr - offset);
> 
> 	while (start = pop(&q)) {
> 		for_each_insn(&insn, start, end, buf) {
> 			if (insn.kaddr == paddr)
> 				return 1;
> 
> 			target = insn_get_branch_addr(&insn);
> 			if (target)
> 				push(&q, target);
> 
> 			if (dead_end_insn(&insn))
> 				break;
> 		}
> 	}

There is the very rare case of intra-function-calls; but I *think*
they're all in noinstr/nokprobe code anyway.

For instance we have RSB stuffing code like:

	.rept 16
	call 1f;
	int3
	1:
	.endr
	add $(BITS_PER_LONG/8) * 16, %_ASM_SP

And the proposed will be horribly confused by that. But like said; it
should also never try and untangle it.