[PATCH][v3] x86/bugs: Fix GDS mitigation selecting when mitigation is off

[PATCH][v3] x86/bugs: Fix GDS mitigation selecting when mitigation is off

[lirongqing]

From: Li RongQing <lirongqing@baidu.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@linux.intel.com>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Li RongQing <lirongqing@baidu.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;
 		}
 	}
 
-- 
2.9.4

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

[Pawan Gupta]

On Tue, Aug 19, 2025 at 10:33:56AM +0800, lirongqing wrote:
> From: Li RongQing <lirongqing@baidu.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@linux.intel.com>
> Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> Signed-off-by: Li RongQing <lirongqing@baidu.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.

[tip: x86/urgent] x86/bugs: Fix GDS mitigation selecting when mitigation is off

[tip-bot2 for Li RongQing]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     d4932a1b148bb6121121e56bad312c4339042d70
Gitweb:        https://git.kernel.org/tip/d4932a1b148bb6121121e56bad312c4339042d70
Author:        Li RongQing <lirongqing@baidu.com>
AuthorDate:    Tue, 19 Aug 2025 10:33:56 +08:00
Committer:     Borislav Petkov (AMD) <bp@alien8.de>
CommitterDate: Tue, 19 Aug 2025 10:38:04 +02:00

x86/bugs: Fix GDS mitigation selecting when mitigation is off

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

This also addresses the failed MSR 0x123 write attempt at boot on
non-ARCH_CAP_GDS_CTRL CPUs:

  unchecked MSR access error: WRMSR to 0x123 (tried to write 0x0000000000000010) at rIP: ... (update_gds_msr)
  Call Trace:
   identify_secondary_cpu
   start_secondary
   common_startup_64
   WARNING: CPU: 1 PID: 0 at arch/x86/kernel/cpu/bugs.c:1053 update_gds_msr

  [ bp: Massage, zap superfluous braces. ]

Fixes: 8c7261abcb7ad ("x86/bugs: Add attack vector controls for GDS")
Suggested-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Link: https://lore.kernel.org/20250819023356.2012-1-lirongqing@baidu.com
---
 arch/x86/kernel/cpu/bugs.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 2186a77..49ef1b8 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1068,10 +1068,8 @@ static void __init gds_select_mitigation(void)
 	if (gds_mitigation == GDS_MITIGATION_AUTO) {
 		if (should_mitigate_vuln(X86_BUG_GDS))
 			gds_mitigation = GDS_MITIGATION_FULL;
-		else {
+		else
 			gds_mitigation = GDS_MITIGATION_OFF;
-			return;
-		}
 	}
 
 	/* No microcode */