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

[Li,Rongqing]

> -----Original Message-----
> From: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
> Sent: 2025年8月15日 13:09
> To: Li,Rongqing <lirongqing@xxxxxxxxx>
> Cc: tglx@xxxxxxxxxxxxx; bp@xxxxxxxxx; peterz@xxxxxxxxxxxxx;
> jpoimboe@xxxxxxxxxx; mingo@xxxxxxxxxx; dave.hansen@xxxxxxxxxxxxxxx;
> x86@xxxxxxxxxx; hpa@xxxxxxxxx; david.kaplan@xxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx
> 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@xxxxxxxxx>
> >
> > 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@xxxxxxxxx>
> > ---
> >  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:

But if cpu has ARCH_CAP_GDS_CTRL, the next block will be skipped, and the codes after checking ARCH_CAP_GDS_CTRL block will be run, this is not expected

So I add a duplicating check

Br

-Li


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