From: Joerg Roedel <jroedel@xxxxxxx> The 32-bit #VC handler has no GHCB and can only handle CPUID exit codes. It is needed by the early boot code to handle #VC exceptions raised in verify_cpu() and to get the position of the C bit. But the CPUID information comes from the hypervisor, which is untrusted and might return results which trick the guest into the no-SEV boot path with no C bit set in the page-tables. All data written to memory would then be unencrypted and could leak sensitive data to the hypervisor. Add sanity checks to the 32-bit boot #VC handler to make sure the hypervisor does not pretend that SEV is not enabled. Signed-off-by: Joerg Roedel <jroedel@xxxxxxx> --- arch/x86/boot/compressed/mem_encrypt.S | 36 ++++++++++++++++++++++++++ 1 file changed, 36 insertions(+) diff --git a/arch/x86/boot/compressed/mem_encrypt.S b/arch/x86/boot/compressed/mem_encrypt.S index 350ecb56c7e4..091502cde070 100644 --- a/arch/x86/boot/compressed/mem_encrypt.S +++ b/arch/x86/boot/compressed/mem_encrypt.S @@ -126,6 +126,34 @@ SYM_CODE_START(startup32_vc_handler) SEV_ES_REQ_CPUID fn=%ebx reg=3 movl %edx, (%esp) + /* + * Sanity check CPUID results from the Hypervisor. See comment in + * do_vc_no_ghcb() for more details on why this is necessary. + */ + + /* Fail if Hypervisor bit not set in CPUID[1].ECX[31] */ + cmpl $1, %ebx + jne .Lcheck_leaf + btl $31, 4(%esp) + jnc .Lfail + jmp .Ldone + +.Lcheck_leaf: + /* Fail if SEV leaf not available in CPUID[0x80000000].EAX */ + cmpl $0x80000000, %ebx + jne .Lcheck_sev + cmpl $0x8000001f, 12(%esp) + jb .Lfail + jmp .Ldone + +.Lcheck_sev: + /* Fail if SEV bit not set in CPUID[0x8000001f].EAX[1] */ + cmpl $0x8000001f, %ebx + jne .Ldone + btl $1, 12(%esp) + jnc .Lfail + +.Ldone: popl %edx popl %ecx popl %ebx @@ -139,6 +167,14 @@ SYM_CODE_START(startup32_vc_handler) iret .Lfail: + /* Send terminate request to Hypervisor */ + movl $0x100, %eax + xorl %edx, %edx + movl $MSR_AMD64_SEV_ES_GHCB, %ecx + wrmsr + rep; vmmcall + + /* If request fails, go to hlt loop */ hlt jmp .Lfail SYM_CODE_END(startup32_vc_handler) -- 2.30.0
