linux-kernel - Re: [PATCH v2 18/39] x86/ibt: Add IBT feature, MSR and #CP handling

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Message-ID: <Yhi0lY8w3E5VKquU@hirez.programming.kicks-ass.net>
Date:   Fri, 25 Feb 2022 11:51:01 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com, mbenes@...e.cz, rostedt@...dmis.org,
        mhiramat@...nel.org, alexei.starovoitov@...il.com
Subject: Re: [PATCH v2 18/39] x86/ibt: Add IBT feature, MSR and #CP handling

On Thu, Feb 24, 2022 at 03:55:16PM -0800, Josh Poimboeuf wrote:
> On Thu, Feb 24, 2022 at 03:51:56PM +0100, Peter Zijlstra wrote:
> > +static __always_inline void setup_cet(struct cpuinfo_x86 *c)
> > +{
> > +	u64 msr = CET_ENDBR_EN;
> > +
> > +	if (!HAS_KERNEL_IBT ||
> > +	    !cpu_feature_enabled(X86_FEATURE_IBT))
> > +		return;
> 
> If you add X86_FEATURE_BIT to arch/x86/include/asm/disabled-features.h,
> the HAS_KERNEL_IBT check becomes redundant.

Cute.

> > +bool ibt_selftest(void)
> > +{
> > +	unsigned long ret;
> > +
> > +	asm ("1: lea 2f(%%rip), %%rax\n\t"
> > +	     ANNOTATE_RETPOLINE_SAFE
> > +	     "   jmp *%%rax\n\t"
> > +	     ASM_REACHABLE
> > +	     ANNOTATE_NOENDBR
> > +	     "2: nop\n\t"
> > +
> > +	     /* unsigned ibt_selftest_ip = 2b */
> > +	     ".pushsection .rodata,\"a\"\n\t"
> > +	     ".align 8\n\t"
> > +	     ".type ibt_selftest_ip, @object\n\t"
> > +	     ".size ibt_selftest_ip, 8\n\t"
> > +	     "ibt_selftest_ip:\n\t"
> > +	     ".quad 2b\n\t"
> > +	     ".popsection\n\t"
> 
> It's still seems silly to make this variable in asm.
> 
> Also .rodata isn't going to work for CPU hotplug.

It's just the IP, that stays invariant. I'm not sure how else to match
regs->ip to 2 in #CP.