Re: [PATCH] x86/sev: Make early_set_memory_decrypted() calls page aligned

[Tom Lendacky]

On 8/18/23 18:34, Steve Rutherford wrote:
> early_set_memory_decrypted() assumes its parameters are page aligned.
> Non-page aligned calls result in additional pages being marked as
> decrypted via the encryption status hypercall, which results in
> consistent corruption of pages during live migration. Live
> migration requires accurate encryption status information to avoid
> migrating pages from the wrong perspective.

Hmmm... I'm not sure this is the proper fix. The code is actually doing 
the right thing from a encyrption/decryption point of view by checking the 
c-bit for the PTE associated with the virtual address and the size 
(possibly crossing page boundaries).

I think the problem is on the call to early_set_mem_enc_dec_hypercall() 
where it doesn't take into account the possible crossing of page 
boundaries and so can under-count the number of pages, right?

Thanks,
Tom

> Fixes: 4716276184ec ("X86/KVM: Decrypt shared per-cpu variables when SEV is active")
> Signed-off-by: Steve Rutherford <srutherford@xxxxxxxxxx>
> ---
>   arch/x86/kernel/kvm.c | 14 +++++++++++++-
>   1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 6a36db4f79fd..a0c072d3103c 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -419,7 +419,14 @@ static u64 kvm_steal_clock(int cpu)
>   
>   static inline void __set_percpu_decrypted(void *ptr, unsigned long size)
>   {
> -	early_set_memory_decrypted((unsigned long) ptr, size);
> +	/*
> +	 * early_set_memory_decrypted() requires page aligned parameters, but
> +	 * this function needs to handle ptrs offset into a page.
> +	 */
> +	unsigned long start = PAGE_ALIGN_DOWN((unsigned long) ptr);
> +	unsigned long end = (unsigned long) ptr + size;
> +
> +	early_set_memory_decrypted(start, end - start);
>   }
>   
>   /*
> @@ -438,6 +445,11 @@ static void __init sev_map_percpu_data(void)
>   		return;
>   
>   	for_each_possible_cpu(cpu) {
> +		/*
> +		 * Calling __set_percpu_decrypted() for each per-cpu variable is
> +		 * inefficent, since it may decrypt the same page multiple times.
> +		 * That said, it avoids the need for more complicated logic.
> +		 */
>   		__set_percpu_decrypted(&per_cpu(apf_reason, cpu), sizeof(apf_reason));
>   		__set_percpu_decrypted(&per_cpu(steal_time, cpu), sizeof(steal_time));
>   		__set_percpu_decrypted(&per_cpu(kvm_apic_eoi, cpu), sizeof(kvm_apic_eoi));