On 06/13/2016 08:51 AM, Matt Fleming wrote: > On Thu, 09 Jun, at 01:33:30PM, Tom Lendacky wrote: >> >> I was trying to play it safe here, but as you say, the firmware should >> be using our page tables so we can get rid of this call. The problem >> will actually be if we transition to a 32-bit efi. The encryption bit >> will be lost in cr3 and so the pgd table will have to be un-encrypted. >> The entries in the pgd can have the encryption bit set so I would only >> need to worry about the pgd itself. I'll have to update the >> efi_alloc_page_tables routine. > > Interesting, I hadn't expected 32-bit EFI to be an option for > platforms with the SME technology. I'd assumed we could just ignore > that. We may be able to do that. > > Are you saying that the encryption bit isn't supported in 32-bit > compatibility mode? We don't do a "full" switch to 32-bit protected > mode when in mixed mode, just load a 32-bit code segment descriptor. > The page tables are not modified at all. The encryption bit is supported in 32-bit compatibility mode and since we're not doing the "full" switch the cr3 register will remain as a 64-bit register so we can leave the pgd table encrypted. > >> The encryption bit in the cr3 register will indicate if the pgd table >> is encrypted or not. Based on my comment above about the pgd having >> to be un-encrypted in case we have to transition to 32-bit efi, this >> can be removed. > > I'm not (yet) sure that the pgd needs to be unencrypted for 32-bit EFI > when running a 64-bit kernel. In the AMD Programmer's Manual, Section > 7.10.3 Operating Modes seems to indicate that running encrypted should > work fine. > >> I'll look into this a bit more. From looking at it I don't want the >> _PAGE_ENC bit set for the memmap unless it gets re-allocated (which >> I missed in these patches). Let me see what I can do with this. > > I don't understand your comment about re-allocating the memmap. > > The kernel builds its own EFI memory map at runtime, initially based > on the memory map provided by the firmware. We always allocate a new > memory map. Sorry, I mis-interpreted the efi_map_regions function/loop and see that the memmap is always allocated by the kernel. > > In efi_setup_page_tables() we're building our own page tables, which > should be encrypted, and mapping EFI regions described by the memmap > into those page tables. > > So unless we're mapping an MMIO region (in which case _PAGE_PCD is set > in @flags for kernel_map_pages_in_pgd()) I would expect _PAGE_ENC to > be set. > >> I'll look further into this, but I saw that this area of virtual memory >> was mapped un-encrypted and after freeing the boot services the >> mappings were somehow reused as un-encrypted for DMA which assumes >> (unless using swiotlb) encrypted. This resulted in DMA data being >> transferred in as encrypted and then accessed un-encrypted. > > That the mappings were re-used isn't a surprise. > > efi_free_boot_services() lifts the reservation that was put in place > during efi_reserve_boot_services() and releases the pages to the > kernel's memory allocators. > > What is surprising is that they were marked unencrypted at all. > There's nothing special about these pages as far as the __va() region > is concerned. Right, let me keep looking into this to see if I can pin down what was (or is) happening. Thanks, Tom > -- To unsubscribe from this list: send the line "unsubscribe linux-efi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
