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Subject[PATCH v3 2/5] x86/boot/compressed/64: Add CPUID sanity check to early #VC handler
Date
From: Joerg Roedel <jroedel@suse.de>

The early #VC handler which doesn't have a GHCB 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 early #VC handlers to make sure the hypervisor
can not pretend that SEV is disabled.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
arch/x86/kernel/sev-es-shared.c | 26 ++++++++++++++++++++++++++
1 file changed, 26 insertions(+)

diff --git a/arch/x86/kernel/sev-es-shared.c b/arch/x86/kernel/sev-es-shared.c
index 5f83ccaab877..48bb14563dcd 100644
--- a/arch/x86/kernel/sev-es-shared.c
+++ b/arch/x86/kernel/sev-es-shared.c
@@ -178,6 +178,32 @@ void __init do_vc_no_ghcb(struct pt_regs *regs, unsigned long exit_code)
goto fail;
regs->dx = val >> 32;

+ /*
+ * This is a VC handler and it is only raised when SEV-ES is active,
+ * which means SEV must be active too. Do sanity checks on the CPUID
+ * results to make sure the hypervisor does not trick the kernel into
+ * the no-sev path. This could map sensitive data unencrypted and make
+ * it accessible to the hypervisor.
+ *
+ * In particular, check for:
+ * - Hypervisor CPUID bit
+ * - Availability of CPUID leaf 0x8000001f
+ * - SEV CPUID bit.
+ *
+ * The hypervisor might still report the wrong C-bit position, but this
+ * can't be checked here.
+ */
+
+ if ((fn == 1 && !(regs->cx & BIT(31))))
+ /* Hypervisor Bit */
+ goto fail;
+ else if (fn == 0x80000000 && (regs->ax < 0x8000001f))
+ /* SEV Leaf check */
+ goto fail;
+ else if ((fn == 0x8000001f && !(regs->ax & BIT(1))))
+ /* SEV Bit */
+ goto fail;
+
/* Skip over the CPUID two-byte opcode */
regs->ip += 2;

--
2.28.0
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