Messages in this thread | | | Date | Mon, 21 Aug 2023 13:53:54 -0500 | Subject | Re: [PATCH] x86/sev: Make early_set_memory_decrypted() calls page aligned | From | Tom Lendacky <> |
| |
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));
| |