linux-kernel - Re: [PATCH][v3] x86/bugs: Fix GDS mitigation selecting when mitigation is off

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Message-ID: <20250819052807.mpqdtauhs33uxnay@desk>
Date: Mon, 18 Aug 2025 22:28:07 -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][v3] x86/bugs: Fix GDS mitigation selecting when
 mitigation is off

On Tue, Aug 19, 2025 at 10:33:56AM +0800, lirongqing wrote:
> From: Li RongQing <lirongqing@...du.com>
> 
> The current GDS mitigation logic incorrectly returns early when the
> attack vector mitigation is turned off, which leads to two problems:
> 
> 1. CPUs without ARCH_CAP_GDS_CTRL support are incorrectly marked with
>    GDS_MITIGATION_OFF when they should be marked as
>    GDS_MITIGATION_UCODE_NEEDED.
> 
> 2. The mitigation state checks and locking verification that follow are
>    skipped, which means:
>    - fail to detect if the mitigation was locked
>    - miss the warning when trying to disable a locked mitigation
> 
> Remove the early return to ensure proper mitigation state handling. This
> allows:
> - Proper mitigation classification for non-ARCH_CAP_GDS_CTRL CPUs
> - Complete mitigation state verification
> 
> The change fixes the following runtime issues because mitigation is off
> in non-ARCH_CAP_GDS_CTRL CPUs:
> 
> [    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")
> Suggested-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
> Reviewed-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
> Signed-off-by: Li RongQing <lirongqing@...du.com>
> ---
> Diff with v2: refine the commit header and Reviewed-by: Pawan Gupta
> Diff with v1: directly remove return 
> 
>  arch/x86/kernel/cpu/bugs.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index b74bf93..9e0b9bf 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -1071,7 +1071,6 @@ static void __init gds_select_mitigation(void)
>  			gds_mitigation = GDS_MITIGATION_FULL;
>  		else {
>  			gds_mitigation = GDS_MITIGATION_OFF;
> -			return;
>  		}

With the "return" gone, braces are not needed in the "else" block (although
it is a bit strange to have braces only in the "else" block to begin with).

Sorry, I missed to spot that in the previous review.