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authorJoerg Roedel <jroedel@suse.de>2020-10-28 17:46:56 +0100
committerBorislav Petkov <bp@suse.de>2020-10-29 13:48:49 +0100
commited7b895f3efb5df184722f5a30f8164fcaffceb1 (patch)
tree7748da27da985bd96ca5cb2175aa5b200d1c99a9 /arch
parent3ad84246a4097010f3ae3d6944120c0be00e9e7a (diff)
downloadlinux-stericsson-ed7b895f3efb5df184722f5a30f8164fcaffceb1.tar.gz
x86/boot/compressed/64: Sanity-check CPUID results in the early #VC handler
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
Diffstat (limited to 'arch')
-rw-r--r--arch/x86/kernel/sev-es-shared.c26
1 files changed, 26 insertions, 0 deletions
diff --git a/arch/x86/kernel/sev-es-shared.c b/arch/x86/kernel/sev-es-shared.c
index 5f83ccaab877..7d04b356d44d 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;