[PATCH] x86/bugs: Fix GDS mitigation check for CPUs without ARCH_CAP_GDS_CTRL

[lirongqing <lirongqing@baidu.com>]

From: Li RongQing <lirongqing@baidu.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@baidu.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))
+				gds_mitigation = GDS_MITIGATION_UCODE_NEEDED;
 			return;
 		}
 	}
-- 
2.9.4