x86/boot/compressed/64: Sanity-check CPUID results in the early #VC handler (ed7b895f) · Commits · 戴 / test

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 handler to make sure the hypervisor can not pretend that SEV is disabled. [ bp: Massage a bit. ] Signed-off-by:

Joerg Roedel <jroedel@suse.de> Signed-off-by:

Borislav Petkov <bp@suse.de> Reviewed-by:

Tom Lendacky <thomas.lendacky@amd.com> Link: https://lkml.kernel.org/r/20201028164659.27002-3-joro@8bytes.org

arch/x86/kernel/sev-es-shared.c

+26 −0

Original line number	Diff line number	Diff line
		@@ -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;

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