From: Sathyanarayanan Kuppuswamy <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx> Sent: Tuesday, May 30, 2023 6:22 AM > > Hi, > > On 5/30/23 5:57 AM, Tom Lendacky wrote: > > On 5/29/23 19:57, Kirill A. Shutemov wrote: > >> On Fri, May 26, 2023 at 03:10:56PM -0700, Sathyanarayanan Kuppuswamy wrote: > >>> > >>> > >>> On 5/26/23 5:02 AM, Kirill A. Shutemov wrote: > >>>> Touching privately mapped GPA that is not properly converted to private > >>>> with MapGPA and accepted leads to unrecoverable exit to VMM. > >>>> > >>>> load_unaligned_zeropad() can touch memory that is not owned by the > >>>> caller, but just happened to next after the owned memory. > >>> > >>> /s/to/to be ? > >> > >> Yep, my bad. > >> > >>>> This load_unaligned_zeropad() behaviour makes it important when kernel > >>>> asks VMM to convert a GPA from shared to private or back. Kernel must > >>>> never have a page mapped into direct mapping (and aliases) as private > >>>> when the GPA is already converted to shared or when GPA is not yet > >>>> converted to private. > >>> > >>> I am wondering whether this issue exist in the AMD code? > >>> > >>> IMO, you can add some info on the window in set_memory_encrypted() > >>> where this race exists. > >> > >> I don't think AMD affected by load_unaligned_zeropad() the same way as > >> Intel does. But I'm not sure. > >> > >> Tom, do you have any comments? > > > > Right, shouldn't be an issue for SNP. > > Thanks for confirming. > Tom -- For my education, could you elaborate on why this problem can't occur in an SEV-SNP guest? There's still a window where the direct map PTE and the RMP as maintained by the hypervisor are out-of-sync. If load_unaligned_zeropad() does a read using the direct map PTE during this out-of-sync window, isn't that going to trap to the hypervisor? How is the scenario is handled from there to provide the zeros to load_unaligned_zeropad()? I need to make sure Hyper-V is doing whatever is needed. :-) Thanks, Michael
