linux-kernel - Re: [RFC][PATCH] x86,nospec: Simplify {JMP,CALL}

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Date:   Wed, 20 Jul 2022 11:12:04 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
Cc:     linux-kernel@...r.kernel.org, keescook@...omium.org,
        hjl.tools@...il.com, andrew.cooper3@...rix.com,
        mark.rutland@....com, will@...nel.org, ndesaulniers@...gle.com,
        x86@...nel.org, Ankur Arora <ankur.a.arora@...cle.com>,
        joao@...rdrivepizza.com
Subject: Re: [RFC][PATCH] x86,nospec: Simplify {JMP,CALL}_NOSPEC

On Wed, Jul 20, 2022 at 02:01:39AM +0200, Maciej S. Szmigiero wrote:
> > Obviously I meant: apply_retpolines() ...
> 
> Will apply_retpolines() actually patch in that trailing int3 in
> the X86_FEATURE_RETPOLINE_LFENCE case?
> 
> Looking at its code it uses just ordinary NOPs as fill:
> > 	/*
> > 	 * For RETPOLINE_LFENCE: prepend the indirect CALL/JMP with an LFENCE.
> > 	 */
> > 	if (cpu_feature_enabled(X86_FEATURE_RETPOLINE_LFENCE)) {
> > 		bytes[i++] = 0x0f;
> > 		bytes[i++] = 0xae;
> > 		bytes[i++] = 0xe8; /* LFENCE */
> > 	}
> > 
> > 	ret = emit_indirect(op, reg, bytes + i);
> > 	if (ret < 0)
> > 		return ret;
> > 	i += ret;
> > 
> > 	for (; i < insn->length;)
> > 		bytes[i++] = BYTES_NOP1;

There is no space for int3 in that case. You get 3 bytes for LFENCE and
{2,3} bytes for 'jmp *%reg', which fully consumes the {5,6} bytes
available.

There will be no nops added.

But this is what all regular retpolines get to look like.

The plan was; and that's still pending, to get the INT3 from the AMD BTC
mitigation that adds INT3 after regular JMPs but those compiler patches
still need to happen I think.