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x86/boot/compressed/64: Sanity-check CPUID results in the early #VC handler

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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
This commit is contained in:
Joerg Roedel 2020-10-28 17:46:56 +01:00 committed by Borislav Petkov
parent 3ad84246a4
commit ed7b895f3e
1 changed files with 26 additions and 0 deletions
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26
arch/x86/kernel/sev-es-shared.c
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@ -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;