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That address range is not part of the system RAM, so >>> it needed to be validated separately. >>> >>> Commit 0f4a1e80989a ("x86/sev: Skip ROM range scans and validation for >>> SEV-SNP guests") reverted those changes and instead chose to prevent the >>> guest from accessing the ROM region since SEV-SNP guests did not rely on >>> data from that region. However, while the kernel itself no longer >>> accessed the ROM region, there are userspace programs that probe this >>> region through /dev/mem and they started crashing due to this change. In >>> particular, fwupd (up until versions released last year that no longer >>> link against libsmbios) and smbios utilities such as smbios-sys-info >>> crash with a cryptic message in dmesg: >>> Wrong/unhandled opcode bytes: 0x8b, exit_code: 0x404, rIP: 0x7fe5404d3840 >>> SEV: Unsupported exit-code 0x404 in #VC exception (IP: 0x7fe5404d3840) >>> >>> Deny access to the BIOS region (rather than just the video ROM range) >>> via /dev/mem to address this. Restrict changes to CONFIG_STRICT_DEVMEM=y >>> which is enabled by default on x86. Add a new x86_platform_ops callback >>> so Intel can customize the address range to block. >>> >>> Fixes: 0f4a1e80989a ("x86/sev: Skip ROM range scans and validation for SEV-SNP guests") >>> Signed-off-by: Naveen N Rao (AMD) >>> --- >>> arch/x86/coco/sev/core.c | 13 +++++++++++++ >>> arch/x86/include/asm/sev.h | 2 ++ >>> arch/x86/include/asm/x86_init.h | 2 ++ >>> arch/x86/kernel/x86_init.c | 2 ++ >>> arch/x86/mm/init.c | 3 +++ >>> arch/x86/mm/mem_encrypt_amd.c | 1 + >>> 6 files changed, 23 insertions(+) >>> > >> >> Is there any driving need to allow devmem at all for TVM access at this >> point? >> >> I would be in favor of making this clearly tied to devmem, call it >> ".devmem_is_allowed" for symmetry with the mm/init.c helper, and make >> the default implementation be: >> >> static bool platform_devmem_is_allowed(unsigned long pfn) >> { >> return !cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT)); >> } >> >> ...if a TVM technology wants more leniency, it can override. > > I'm not fully aware of the history here, but I suppose a TVM should > appear as any other VM for userspace. For that reason, I didn't want to > block access to /dev/mem any more than was necessary. Admittedly, I have > limited insight into which utilities may be using /dev/mem today. > > Tom/Boris, do you see a problem blocking access to /dev/mem for SEV > guests? Not sure why we would suddenly not allow that. Thanks, Tom > > > - Naveen >