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SubjectRe: [PATCH] x86/sev: Make early_set_memory_decrypted() calls page aligned
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On 8/21/23 13:15, Steve Rutherford wrote:
> On Mon, Aug 21, 2023 at 6:10 AM Tom Lendacky <thomas.lendacky@amd.com> wrote:
>>
>> 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?
>
> Right now, if you request decryption of e.g. a non-page aligned 0x40
> byte structure, it rounds the 0x40 bytes up to one page, and then
> hypercalls to mark both the page it's on and the subsequent page as
> decrypted (since the rounding stretches the structure onto the next
> page spuriously). The arithmetic in the combination of
> early_set_memory_enc_dec() and early_set_mem_enc_dec_hypercall() are
> correct if they are called with page aligned vaddrs (non-page-aligned
> sizes are fine iiuc).

Ah, right, correct. It is still related to how the page count is
calculated for the hypercall, though, right? The encryption/decryption
operations function properly.

If another caller of early_set_memory_decrypted() gets added, it would
need to know to do the same thing. So I just wonder if this wouldn't be
better fixed in early_set_memory_enc_dec() by using a page aligned address
and proper number of pages when calling early_set_mem_enc_dec_hypercall()
or in early_set_mem_enc_dec_hypercall() where it would take a size
argument instead of a page count and does the proper work to get a page
aligned address and proper page count.

Also, if it is the hypercall that is causing the issue, should the Fixes
tag be 064ce6c550a0 ("mm: x86: Invoke hypercall when page encryption
status is changed") since the problem is around the hypercall.

Thanks,
Tom

>
> Thanks,
> Steve
>>
>> Thanks,
>> Tom
>>
>>>
>>> Fixes: 4716276184ec ("X86/KVM: Decrypt shared per-cpu variables when SEV is active")
>>> Signed-off-by: Steve Rutherford <srutherford@google.com>
>>> ---
>>> 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));

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