Re: [PATCH] x86/CPU/AMD: Make sure EFER[AIBRSE] is set

[Pawan Gupta]

On Sat, Feb 25, 2023 at 01:20:24AM +0100, Borislav Petkov wrote:
> On Fri, Feb 24, 2023 at 04:09:31PM -0800, Josh Poimboeuf wrote:
> > Ah, I had to stare it that for a bit to figure out how it works.
> 
> Yeah, it is a bit "hidden". :)
> 
> > setup_real_mode() reads MSR_EFER from the boot CPU and stores it in
> > trampoline_header->efer.  Then the other CPUs read that stored value in
> > startup_32() and write it into their MSR.
> 
> Exactly.
> 
> > Yeah, I think that would be good.  Otherwise it's rather magical.
> 
> Yap, see below.
> 
> > That EFER MSR is a surprising place to put that bit.
> 
> That MSR is very important on AMD. Consider it AMD's CR4. :-)
> 
> Thx.
> 
> ---
> From: "Borislav Petkov (AMD)" <bp@xxxxxxxxx>
> Date: Sat, 25 Feb 2023 01:11:31 +0100
> Subject: [PATCH] x86/CPU/AMD: Make sure EFER[AIBRSE] is set
> 
> The AutoIBRS bit gets set only on the BSP as part of determining which
> mitigation to enable on AMD. Setting on the APs relies on the
> circumstance that the APs get booted through the trampoline and EFER
> - the MSR which contains that bit - gets replicated on every AP from the
> BSP.
> 
> However, this can change in the future and considering the security
> implications of this bit not being set on every CPU, make sure it is set
> by verifying EFER later in the boot process and on every AP.
> 
> Reported-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
> Link: https://lore.kernel.org/r/20230224185257.o3mcmloei5zqu7wa@treble
> ---
>  arch/x86/kernel/cpu/amd.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> index 380753b14cab..de624c1442c2 100644
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -996,6 +996,16 @@ static void init_amd(struct cpuinfo_x86 *c)
>  		msr_set_bit(MSR_K7_HWCR, MSR_K7_HWCR_IRPERF_EN_BIT);
>  
>  	check_null_seg_clears_base(c);
> +
> +	/*
> +	 * Make sure EFER[AIBRSE - Automatic IBRS Enable] is set. The APs are brought up
> +	 * using the trampoline code and as part of it, EFER gets prepared there in order
> +	 * to be replicated onto them. Regardless, set it here again, if not set, to protect
> +	 * against any future refactoring/code reorganization which might miss setting
> +	 * this important bit.
> +	 */
> +	if (cpu_has(c, X86_FEATURE_AUTOIBRS))
> +		msr_set_bit(MSR_EFER, _EFER_AUTOIBRS);

Is it intended to be set regardless of the spectre_v2 mitigation status?