On 5/3/21 8:05 AM, Erdem Aktas wrote: > Hi Brijesh, > I have few naive questions inlined: > > On Fri, Apr 30, 2021 at 4:52 AM Brijesh Singh <brijesh.si...@amd.com> wrote: >> + ; Use PVALIDATE instruction to invalidate the page >> + mov eax, GHCB_BASE >> + mov ecx, 0 >> + mov edx, 0 >> + DB 0xF2, 0x0F, 0x01, 0xFF >> + cmp eax, 0 >> + jnz TerminateSevGuestLaunch > Any reason why the PVALIDATE return value (EFLAGS.CF) is not checked > here? IMO, this might lead some page replay attacks.
Ah, good catch. I will add this in next rev. > >> + ; >> + ; The page table built above cleared the memory encryption mask from the >> + ; GHCB_BASE (aka made it shared). When SEV-SNP is enabled, to maintain >> + ; the security guarantees, the page state transition from private to >> + ; shared must go through the page invalidation steps. Invalidate the >> + ; memory range before loading the page table below. >> + ; >> + ; NOTE: the invalidation must happen after zeroing the GHCB memory. This >> + ; is because, in the 32-bit mode all the access are considered >> private. >> + ; The invalidation before the zero'ing will cause a #VC. >> + ; >> + OneTimeCall InvalidateGHCBPage > I am not sure if this is a great idea. > 1. Zeroing page content before paging is enabled. We are actually > writing 0s encrypted with a guest key. > 2. invalidating the page and making it shared. > Doesn't this reveal a mapping of what 0's look like when a specific > page is encrypted? And when the page is marked as shared, from the > guest and host perspective, it is not zeroed but filled with some data > that looks random. So what is the purpose of zeroing the page before > invalidation? I don't know why zeroing of the GHCB page is done here. The code to zero'ing was added in ES. I will check with Tom to get a bit of histroy. The GHCB is not established until we reach to Sec/SecMain.c, so, theoretically we can remove the zero'ing and push it to Sec/SecMain.c. -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent to this group. View/Reply Online (#74708): https://edk2.groups.io/g/devel/message/74708 Mute This Topic: https://groups.io/mt/82479067/21656 Group Owner: devel+ow...@edk2.groups.io Unsubscribe: https://edk2.groups.io/g/devel/unsub [arch...@mail-archive.com] -=-=-=-=-=-=-=-=-=-=-=-
