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Subject[PATCH 5/7] x86/boot/compressed/64: Add CPUID sanity check to 32-bit boot-path
Date
From: Joerg Roedel <jroedel@suse.de>

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@suse.de>
---
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
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