Re: [RFC Part1 PATCH v3 17/17] X86/KVM: Clear encryption attribute when SEV is active

[Borislav Petkov]

On Mon, Jul 24, 2017 at 02:07:57PM -0500, Brijesh Singh wrote:
> The guest physical memory area holding the struct pvclock_wall_clock and
> struct pvclock_vcpu_time_info are shared with the hypervisor. Hypervisor
> periodically updates the contents of the memory. When SEV is active, we
> must clear the encryption attributes from the shared memory pages so that
> both hypervisor and guest can access the data.
> 
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> ---
>  arch/x86/entry/vdso/vma.c  |  5 ++--
>  arch/x86/kernel/kvmclock.c | 64 +++++++++++++++++++++++++++++++++++++++-------
>  2 files changed, 58 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/entry/vdso/vma.c b/arch/x86/entry/vdso/vma.c
> index 726355c..ff50251 100644
> --- a/arch/x86/entry/vdso/vma.c
> +++ b/arch/x86/entry/vdso/vma.c
> @@ -114,10 +114,11 @@ static int vvar_fault(const struct vm_special_mapping *sm,
>  		struct pvclock_vsyscall_time_info *pvti =
>  			pvclock_pvti_cpu0_va();
>  		if (pvti && vclock_was_used(VCLOCK_PVCLOCK)) {
> -			ret = vm_insert_pfn(
> +			ret = vm_insert_pfn_prot(
>  				vma,
>  				vmf->address,
> -				__pa(pvti) >> PAGE_SHIFT);
> +				__pa(pvti) >> PAGE_SHIFT,
> +				pgprot_decrypted(vma->vm_page_prot));
>  		}
>  	} else if (sym_offset == image->sym_hvclock_page) {
>  		struct ms_hyperv_tsc_page *tsc_pg = hv_get_tsc_page();

Yuck, that vvar_fault() function is one unreadable mess.

> diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
> index d889676..f3a8101 100644
> --- a/arch/x86/kernel/kvmclock.c
> +++ b/arch/x86/kernel/kvmclock.c
> @@ -27,6 +27,7 @@
>  #include <linux/sched.h>
>  #include <linux/sched/clock.h>
>  
> +#include <asm/mem_encrypt.h>
>  #include <asm/x86_init.h>
>  #include <asm/reboot.h>
>  #include <asm/kvmclock.h>
> @@ -45,7 +46,7 @@ early_param("no-kvmclock", parse_no_kvmclock);
>  
>  /* The hypervisor will put information about time periodically here */
>  static struct pvclock_vsyscall_time_info *hv_clock;
> -static struct pvclock_wall_clock wall_clock;
> +static struct pvclock_wall_clock *wall_clock;
>  
>  struct pvclock_vsyscall_time_info *pvclock_pvti_cpu0_va(void)
>  {
> @@ -64,15 +65,18 @@ static void kvm_get_wallclock(struct timespec *now)
>  	int low, high;
>  	int cpu;
>  
> -	low = (int)__pa_symbol(&wall_clock);
> -	high = ((u64)__pa_symbol(&wall_clock) >> 32);
> +	if (!wall_clock)
> +		return;

Hmm, so if you return here, @now will remain unchanged so how is the
caller to know that ->get_wallclock() failed?

Maybe a WARN_ON_ONCE() at least...?

Dunno, what's the policy in kvm if the kvmclock init fails?

Paolo? Radim?

Because it does say:

        printk(KERN_INFO "kvm-clock: Using msrs %x and %x",
                msr_kvm_system_time, msr_kvm_wall_clock);

too early. We can error out later and users will still think it is using
kvmclock ...

Hmmm.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix ImendÃrffer, Jane Smithard, Graham Norton, HRB 21284 (AG NÃrnberg)
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