On Tue, Oct 10, 2023 at 08:02:18PM +0000, John Allen wrote:
> When a guest issues a cpuid instruction for Fn0000000D_x0B (CetUserOffset), the
> hypervisor may intercept and access the guest XSS value. For SEV-ES, this is
> encrypted and needs to be included in the GHCB to be visible to the hypervisor.
> The rdmsr instruction needs to be called directly as the code may be used in
> early boot in which case the rdmsr wrappers should be avoided as they are
> incompatible with the decompression boot phase.
>
> Signed-off-by: John Allen <john.allen@xxxxxxx>
> ---
> arch/x86/kernel/sev-shared.c | 15 +++++++++++++++
> 1 file changed, 15 insertions(+)
>
> diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
> index 2eabccde94fb..e38a1d049bc1 100644
> --- a/arch/x86/kernel/sev-shared.c
> +++ b/arch/x86/kernel/sev-shared.c
> @@ -890,6 +890,21 @@ static enum es_result vc_handle_cpuid(struct ghcb *ghcb,
> /* xgetbv will cause #GP - use reset value for xcr0 */
> ghcb_set_xcr0(ghcb, 1);
>
> + if (has_cpuflag(X86_FEATURE_SHSTK) && regs->ax == 0xd && regs->cx <= 1) {
> + unsigned long lo, hi;
> + u64 xss;
> +
> + /*
> + * Since vc_handle_cpuid may be used during early boot, the
> + * rdmsr wrappers are incompatible and should not be used.
> + * Invoke the instruction directly.
> + */
> + asm volatile("rdmsr" : "=a" (lo), "=d" (hi)
> + : "c" (MSR_IA32_XSS));
Does __rdmsr() work too?
I know it has exception handling but a SEV-ES guest should not fault
when accessing MSR_IA32_XSS anyway, especially if it has shadow stack
enabled. And if it does fault, your version would explode too but
__rdmsr() would be at least less code. :)
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
