linux-kernel - Re: [PATCH] x86/bugs: Fix GDS mitigation check for CPUs without ARCH_CAP_GDS

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Message-ID: <20250815050811.gm7nxcd7wn47lshy@desk>
Date: Thu, 14 Aug 2025 22:08:33 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: lirongqing <lirongqing@...du.com>
Cc: tglx@...utronix.de, bp@...en8.de, peterz@...radead.org,
	jpoimboe@...nel.org, mingo@...hat.com, dave.hansen@...ux.intel.com,
	x86@...nel.org, hpa@...or.com, david.kaplan@....com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/bugs: Fix GDS mitigation check for CPUs without
 ARCH_CAP_GDS_CTRL

On Fri, Aug 15, 2025 at 11:53:34AM +0800, lirongqing wrote:
> From: Li RongQing <lirongqing@...du.com>
> 
> The commit 8c7261abcb7ad("x86/bugs: Add attack vector controls for GDS")
> caused call traces during secondary CPU initialization because it didn't
> properly handle CPUs that lack the ARCH_CAP_GDS_CTRL capability.
> 
> For CPUs without ARCH_CAP_GDS_CTRL support, we should set the mitigation
> to GDS_MITIGATION_UCODE_NEEDED rather than GDS_MITIGATION_OFF, as these
> CPUs may still be vulnerable but cannot disable mitigation.
> 
> Add the missing check for ARCH_CAP_GDS_CTRL to properly determine the
> mitigation state for affected CPUs.
> 
> [    2.809147] unchecked MSR access error: RDMSR from 0x123 at rIP: 0xffffffffb3452807 (update_gds_msr+0x87/0xe0)
> (update_gds_msr+0x87/0xe0)
> [    2.809147] Call Trace:
> [    2.809147]  <TASK>
> [    2.809147]  identify_secondary_cpu+0x72/0x90
> [    2.809147]  start_secondary+0x7a/0x140
> [    2.809147]  common_startup_64+0x13e/0x141
> [    2.809147]  </TASK>
> [    2.809147] unchecked MSR access error: WRMSR to 0x123 (tried to write 0x0000000000000010) at rIP: 0xffffffffb34527b8
> (update_gds_msr+0x38/0xe0)
> [    2.809147] Call Trace:
> [    2.809147]  <TASK>
> [    2.809147]  identify_secondary_cpu+0x72/0x90
> [    2.809147]  start_secondary+0x7a/0x140
> [    2.809147]  common_startup_64+0x13e/0x141
> [    2.809147]  </TASK>
> [    2.809147] ------------[ cut here ]------------
> [    2.809147] WARNING: CPU: 1 PID: 0 at arch/x86/kernel/cpu/bugs.c:1053 update_gds_msr+0x9b/0xe0
> 
> Fixes: 8c7261abcb7ad ("x86/bugs: Add attack vector controls for GDS")
> Signed-off-by: Li RongQing <lirongqing@...du.com>
> ---
>  arch/x86/kernel/cpu/bugs.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index b74bf93..3af911c 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -1071,6 +1071,8 @@ static void __init gds_select_mitigation(void)
>  			gds_mitigation = GDS_MITIGATION_FULL;
>  		else {
>  			gds_mitigation = GDS_MITIGATION_OFF;
> +			if (!(x86_arch_cap_msr & ARCH_CAP_GDS_CTRL))

This check is already present few lines below.

> +				gds_mitigation = GDS_MITIGATION_UCODE_NEEDED;
>  			return;

To avoid duplicating, a better fix could be to not return here, and let the
next block DTRT:

         /* No microcode */
         if (!(x86_arch_cap_msr & ARCH_CAP_GDS_CTRL)) {
                 if (gds_mitigation != GDS_MITIGATION_FORCE)
                         gds_mitigation = GDS_MITIGATION_UCODE_NEEDED;
                 return;
         }