Re: [PATCH v4 1/2] x86/sev: Clear RMPOPT when segmented RMP is disabled by firmware

[Pankaj Gupta]

> From: Ashish Kalra <ashish.kalra@xxxxxxx>
>
> Segmented RMP has two independent states: the CPU *advertises* support via
> the X86_FEATURE_SEGMENTED_RMP CPUID bit, and the firmware *enables* it via
> MSR_AMD64_RMP_CFG[SEG_RMP_ENABLED].  A platform can advertise segmented RMP
> (CPUID bit set) while the firmware leaves it disabled (SEG_RMP_ENABLED
> clear) -- e.g. through a BIOS option to disable Segmented RMP -- in which
> case the system runs with a contiguous RMP.
>
> snp_probe_rmptable_info() clears X86_FEATURE_RMPOPT only when the CPU does
> not advertise segmented RMP, or when the segmented RMP table probe fails.
> It misses the advertised-but-firmware-disabled case: that path falls
> through to the contiguous probe with X86_FEATURE_RMPOPT still set.
>
> X86_FEATURE_RMPOPT is a CPUID bit advertised independently of whether
> segmented RMP is enabled, but RMPOPT requires an enabled segmented RMP.
> With the capability left set, RMPOPT_BASE is programmed and the RMPOPT
> instruction issued, both of which #GP on a contiguous RMP:
>
>   unchecked MSR access error: WRMSR to 0xc0010139 ...
>   general protection fault ... snp_enable_rmpopt
>
> Clear X86_FEATURE_RMPOPT on the common contiguous fall-through so it covers
> both "segmented RMP not supported" and "segmented RMP supported but
> disabled by firmware".
>
> Fixes: 15d974be0a3b ("x86/sev: Initialize RMPOPT configuration MSRs")
> Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>

Reviewed-by: Pankaj Gupta <pankaj.gupta@xxxxxxx>
Tested-by: Pankaj Gupta <pankaj.gupta@xxxxxxx>

> ---
>  arch/x86/virt/svm/sev.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c
> index bd70c4d0b774..cd0412b9d251 100644
> --- a/arch/x86/virt/svm/sev.c
> +++ b/arch/x86/virt/svm/sev.c
> @@ -827,10 +827,10 @@ bool snp_probe_rmptable_info(void)
>                         setup_clear_cpu_cap(X86_FEATURE_RMPOPT);
>                         return false;
>                 }
> -       } else {
> -               setup_clear_cpu_cap(X86_FEATURE_RMPOPT);
>         }
>
> +       setup_clear_cpu_cap(X86_FEATURE_RMPOPT);
> +
>         return probe_contiguous_rmptable_info();
>  }
>
> --
> 2.43.0
>
>