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| author | Joerg Roedel <jroedel@suse.de> | 2021-03-12 13:38:18 +0100 |
|---|---|---|
| committer | Borislav Petkov <bp@suse.de> | 2021-03-18 16:44:40 +0100 |
| commit | eab696d8e8b9c9d600be6fad8dd8dfdfaca6ca7c (patch) | |
| tree | cba158f57355578cfa92b336fc2961a9b087e90a /arch/x86/boot/compressed | |
| parent | x86/boot/compressed/64: Cleanup exception handling before booting kernel (diff) | |
| download | linux-eab696d8e8b9c9d600be6fad8dd8dfdfaca6ca7c.tar.xz linux-eab696d8e8b9c9d600be6fad8dd8dfdfaca6ca7c.zip | |
x86/sev: Do not require Hypervisor CPUID bit for SEV guests
A malicious hypervisor could disable the CPUID intercept for an SEV or
SEV-ES guest and trick it into the no-SEV boot path, where it could
potentially reveal secrets. This is not an issue for SEV-SNP guests,
as the CPUID intercept can't be disabled for those.
Remove the Hypervisor CPUID bit check from the SEV detection code to
protect against this kind of attack and add a Hypervisor bit equals zero
check to the SME detection path to prevent non-encrypted guests from
trying to enable SME.
This handles the following cases:
1) SEV(-ES) guest where CPUID intercept is disabled. The guest
will still see leaf 0x8000001f and the SEV bit. It can
retrieve the C-bit and boot normally.
2) Non-encrypted guests with intercepted CPUID will check
the SEV_STATUS MSR and find it 0 and will try to enable SME.
This will fail when the guest finds MSR_K8_SYSCFG to be zero,
as it is emulated by KVM. But we can't rely on that, as there
might be other hypervisors which return this MSR with bit
23 set. The Hypervisor bit check will prevent that the guest
tries to enable SME in this case.
3) Non-encrypted guests on SEV capable hosts with CPUID intercept
disabled (by a malicious hypervisor) will try to boot into
the SME path. This will fail, but it is also not considered
a problem because non-encrypted guests have no protection
against the hypervisor anyway.
[ bp: s/non-SEV/non-encrypted/g ]
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lkml.kernel.org/r/20210312123824.306-3-joro@8bytes.org
Diffstat (limited to 'arch/x86/boot/compressed')
| -rw-r--r-- | arch/x86/boot/compressed/mem_encrypt.S | 6 |
1 files changed, 0 insertions, 6 deletions
diff --git a/arch/x86/boot/compressed/mem_encrypt.S b/arch/x86/boot/compressed/mem_encrypt.S index aa561795efd1..a6dea4e8a082 100644 --- a/arch/x86/boot/compressed/mem_encrypt.S +++ b/arch/x86/boot/compressed/mem_encrypt.S @@ -23,12 +23,6 @@ SYM_FUNC_START(get_sev_encryption_bit) push %ecx push %edx - /* Check if running under a hypervisor */ - movl $1, %eax - cpuid - bt $31, %ecx /* Check the hypervisor bit */ - jnc .Lno_sev - movl $0x80000000, %eax /* CPUID to check the highest leaf */ cpuid cmpl $0x8000001f, %eax /* See if 0x8000001f is available */ |