Messages in this thread Patch in this message | | | From | Joerg Roedel <> | Subject | [PATCH v4 2/5] x86/boot/compressed/64: Add CPUID sanity check to early #VC handler | Date | Wed, 28 Oct 2020 17:46:56 +0100 |
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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..56d16c405b03 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 the #VC 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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