From: Tom Lendacky thomas.lendacky@amd.com Sent: Thursday, June 1, 2023 11:19 AM
On 5/31/23 15:00, Michael Kelley (LINUX) wrote:
From: Sathyanarayanan Kuppuswamy
sathyanarayanan.kuppuswamy@linux.intel.com
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. :-)
Ah, I think I misunderstood this when it was being talked about. The issue SNP would have would be between setting the c-bit but before the PVALIDATE is issued. Prior to the RMP being updated, referencing the page will generate an #NPF and automatically change the RMP over to private (in KVM). However, after the guest is resumed, the page will not have been validated resulting in a #VC with error code 0x404 being generated, causing the guest to terminate itself.
I suppose, when a 0x404 error code is encountered by the #VC handler, it could call search_exception_tables() and call ex_handler_zeropad() for the EX_TYPE_ZEROPAD type (ex_handler_zeropad is currently static, though).
Tom -- Does the above sequence *depend* on the hypervisor doing anything to make it work? I'm not clear on why KVM would automatically change the page over to private. If there's a dependency on the hypervisor doing something, then it seems like we'll need to standardize that "something" across hypervisors, lest we end up with per-hypervisor code in Linux to handle this scenario. And running SEV-SNP with multiple VMPLs probably makes it even more complicated.
Kirill -- Same question about TDX. Does making load_unaligned_zeropad() work in a TDX VM depend on the hypervisor doing anything? Or is the behavior seen by the guest dependent only on architected behavior of the TDX processor?
Looking at this problem from a slightly higher level, and thinking out loud a bit, load_unaligned_zeropad() functionality is provided only for certain architectures: x86/64, arm, arm64, and PowerPC 64 (little endian). There are fallbacks for architectures that don't support it. With two minor tweaks to Kconfig files, I've built x86 with load_unaligned_zeropad() disabled. Maybe with today's processors the performance benefits are past their prime, and running with it disabled in CoCo VMs is the better solution. Does anyone have a sense of whether the perf impact would be measureable?
If doing the load_unaligned_zeropad() enable/disable at build time is too limiting, maybe it could be runtime based on whether page private/shared state is being enforced. I haven't looked at the details.
Thoughts?
Michael