linux-kernel - Re: [PATCH v1 1/1] x86/fred: Clear the WFE bit in missing-ENDBRANCH #CP

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Message-ID: <20240912075714.GT4723@noisy.programming.kicks-ass.net>
Date: Thu, 12 Sep 2024 09:57:14 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Xin Li (Intel)" <xin@...or.com>
Cc: linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
	bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
	hpa@...or.com, andrew.cooper3@...rix.com
Subject: Re: [PATCH v1 1/1] x86/fred: Clear the WFE bit in missing-ENDBRANCH
 #CP

On Wed, Sep 11, 2024 at 04:19:29PM -0700, Xin Li (Intel) wrote:

> +static void ibt_clear_fred_wfe(struct pt_regs *regs)
> +{
> +	if (cpu_feature_enabled(X86_FEATURE_FRED))
> +		regs->fred_cs.wfe = 0;
> +}
> +
>  static void do_kernel_cp_fault(struct pt_regs *regs, unsigned long error_code)
>  {
>  	if ((error_code & CP_EC) != CP_ENDBR) {
> @@ -90,6 +107,7 @@ static void do_kernel_cp_fault(struct pt_regs *regs, unsigned long error_code)
>  
>  	if (unlikely(regs->ip == (unsigned long)&ibt_selftest_noendbr)) {
>  		regs->ax = 0;
> +		ibt_clear_fred_wfe(regs);
>  		return;
>  	}
>  
> @@ -97,6 +115,7 @@ static void do_kernel_cp_fault(struct pt_regs *regs, unsigned long error_code)
>  	if (!ibt_fatal) {
>  		printk(KERN_DEFAULT CUT_HERE);
>  		__warn(__FILE__, __LINE__, (void *)regs->ip, TAINT_WARN, regs, NULL);
> +		ibt_clear_fred_wfe(regs);
>  		return;
>  	}
>  	BUG();

So, why not clear the bit the moment we know this is CP_ENDBR?

In the fatal case, we'll hit that BUG and die anyway, nobody cares about
the tracker state in that case.

diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
index d2c732a34e5d..fde4bdd25a73 100644
--- a/arch/x86/kernel/cet.c
+++ b/arch/x86/kernel/cet.c
@@ -88,6 +88,8 @@ static void do_kernel_cp_fault(struct pt_regs *regs, unsigned long error_code)
 		return;
 	}
 
+	ibt_clear_fred_wfe(regs);
+
 	if (unlikely(regs->ip == (unsigned long)&ibt_selftest_noendbr)) {
 		regs->ax = 0;
 		return;