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Date: Wed, 7 Jun 2023 12:29:37 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
Evgeniy Baskov <baskov@...ras.ru>,
Borislav Petkov <bp@...en8.de>,
Andy Lutomirski <luto@...nel.org>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>,
Alexey Khoroshilov <khoroshilov@...ras.ru>,
Peter Jones <pjones@...hat.com>,
Gerd Hoffmann <kraxel@...hat.com>,
Dave Young <dyoung@...hat.com>,
Mario Limonciello <mario.limonciello@....com>,
Kees Cook <keescook@...omium.org>,
"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH v5 19/20] x86/efistub: Perform SNP feature test while
running in the firmware
On 6/7/23 11:51, Ard Biesheuvel wrote:
> On Wed, 7 Jun 2023 at 18:08, Tom Lendacky <thomas.lendacky@....com> wrote:
>>
>> On 6/7/23 02:23, Ard Biesheuvel wrote:
>>> Before refactoring the EFI stub boot flow to avoid the legacy bare metal
>>> decompressor, duplicate the SNP feature check in the EFI stub before
>>> handing over to the kernel proper.
>>>
>>> The SNP feature check can be performed while running under the EFI boot
>>> services, which means we can fail gracefully and return an error to the
>>> bootloader if the loaded kernel does not implement support for all the
>>> features that the hypervisor enabled.
>>>
>>> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
>>> ---
>>> arch/x86/boot/compressed/sev.c | 71 +++++++++++---------
>>> arch/x86/include/asm/sev.h | 4 ++
>>> drivers/firmware/efi/libstub/x86-stub.c | 17 +++++
>>> 3 files changed, 62 insertions(+), 30 deletions(-)
>>>
>>> diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
>>> index 09dc8c187b3cc752..9593bc80c9c6b89d 100644
>>> --- a/arch/x86/boot/compressed/sev.c
>>> +++ b/arch/x86/boot/compressed/sev.c
>>
>> ...
>>
>>> -void sev_enable(struct boot_params *bp)
>>> +u64 sev_get_status(void)
>>> {
>>> unsigned int eax, ebx, ecx, edx;
>>> struct msr m;
>>> +
>>> + /* Check for the SME/SEV support leaf */
>>> + eax = 0x80000000;
>>> + ecx = 0;
>>> + native_cpuid(&eax, &ebx, &ecx, &edx);
>>> + if (eax < 0x8000001f)
>>> + return 0;
>>> +
>>> + /*
>>> + * Check for the SME/SEV feature:
>>> + * CPUID Fn8000_001F[EAX]
>>> + * - Bit 0 - Secure Memory Encryption support
>>> + * - Bit 1 - Secure Encrypted Virtualization support
>>> + * CPUID Fn8000_001F[EBX]
>>> + * - Bits 5:0 - Pagetable bit position used to indicate encryption
>>> + */
>>> + eax = 0x8000001f;
>>> + ecx = 0;
>>> + native_cpuid(&eax, &ebx, &ecx, &edx);
>>> + /* Check whether SEV is supported */
>>> + if (!(eax & BIT(1)))
>>> + return 0;
>>> +
>>> + /* Set the SME mask if this is an SEV guest. */
>>> + sme_me_mask = BIT_ULL(ebx & 0x3f);
>>> +
>>> + boot_rdmsr(MSR_AMD64_SEV, &m);
>>> + return m.q;
>>> +}
>>> +
>>> +void sev_enable(struct boot_params *bp)
>>> +{
>>> bool snp;
>>>
>>> /*
>>> @@ -410,37 +447,13 @@ void sev_enable(struct boot_params *bp)
>>> */
>>> snp = snp_init(bp);
>>>
>>> - /* Check for the SME/SEV support leaf */
>>> - eax = 0x80000000;
>>> - ecx = 0;
>>> - native_cpuid(&eax, &ebx, &ecx, &edx);
>>> - if (eax < 0x8000001f)
>>> - return;
>>> -
>>> - /*
>>> - * Check for the SME/SEV feature:
>>> - * CPUID Fn8000_001F[EAX]
>>> - * - Bit 0 - Secure Memory Encryption support
>>> - * - Bit 1 - Secure Encrypted Virtualization support
>>> - * CPUID Fn8000_001F[EBX]
>>> - * - Bits 5:0 - Pagetable bit position used to indicate encryption
>>> - */
>>> - eax = 0x8000001f;
>>> - ecx = 0;
>>> - native_cpuid(&eax, &ebx, &ecx, &edx);
>>> - /* Check whether SEV is supported */
>>> - if (!(eax & BIT(1))) {
>>> + sev_status = sev_get_status();
>>> + if (!(sev_status & MSR_AMD64_SEV_ENABLED)) {
>>> if (snp)
>>> error("SEV-SNP support indicated by CC blob, but not CPUID.");
>>
>> This ends up checking the CPUID path because if SEV isn't advertised in
>> CPUID the returned status value is 0. But it also checks the SEV_STATUS
>> MSR as well. So I think you can remove the SNP / SEV_STATUS check at the
>> end of this function (since that check is identical to this now) and just
>> update the message to indicate not CPUID or SEV status MSR.
>>
>
> But that one checks for MSR_AMD64_SEV_SNP_ENABLED not
> MSR_AMD64_SEV_ENABLED. Does that matter at all?
Ugh, my bad, I misread that last check. Ignore my comment.
>
>> The sme_me_mask should probably be cleared at this point before returning,
>> too. Or, alternately, in sev_get_status(), you can update the setting of
>> sme_me_mask to based on MSR_AMD64_SEV_ENABLED being set in the SEV_STATUS MSR.
>>
>
> I'll go for the latter, seems cleaner not to touch it in that case.
Sounds good.
Thanks
Tom
>
> --- a/arch/x86/boot/compressed/sev.c
> +++ b/arch/x86/boot/compressed/sev.c
> @@ -422,10 +422,12 @@ u64 sev_get_status(void)
> if (!(eax & BIT(1)))
> return 0;
>
> - /* Set the SME mask if this is an SEV guest. */
> - sme_me_mask = BIT_ULL(ebx & 0x3f);
> -
> boot_rdmsr(MSR_AMD64_SEV, &m);
> +
> + /* Set the SME mask if this is an SEV guest. */
> + if (m.q & MSR_AMD64_SEV_ENABLED)
> + sme_me_mask = BIT_ULL(ebx & 0x3f);
> +
> return m.q;
> }
>
>
>>> /* Search for Confidential Computing blob in the EFI config table. */
>>> diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
>>> index 86e1296e87f513b7..081c39b0e8d0d208 100644
>>> --- a/arch/x86/include/asm/sev.h
>>> +++ b/arch/x86/include/asm/sev.h
>>> @@ -207,6 +207,8 @@ bool snp_init(struct boot_params *bp);
>>> void __init __noreturn snp_abort(void);
>>> int snp_issue_guest_request(u64 exit_code, struct snp_req_data *input, struct snp_guest_request_ioctl *rio);
>>> void snp_accept_memory(phys_addr_t start, phys_addr_t end);
>>> +u64 snp_get_unsupported_features(u64 status);
>>> +u64 sev_get_status(void);
>>> #else
>>> static inline void sev_es_ist_enter(struct pt_regs *regs) { }
>>> static inline void sev_es_ist_exit(void) { }
>>> @@ -232,6 +234,8 @@ static inline int snp_issue_guest_request(u64 exit_code, struct snp_req_data *in
>>> }
>>>
>>> static inline void snp_accept_memory(phys_addr_t start, phys_addr_t end) { }
>>> +static inline u64 snp_get_unsupported_features(u64 status) { return 0; }
>>> +static inline u64 sev_get_status(void) { return 0; }
>>> #endif
>>>
>>> #endif
>>> diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
>>> index abcd5703e9f3f980..1015ef883f5850a4 100644
>>> --- a/drivers/firmware/efi/libstub/x86-stub.c
>>> +++ b/drivers/firmware/efi/libstub/x86-stub.c
>>> @@ -15,6 +15,7 @@
>>> #include <asm/setup.h>
>>> #include <asm/desc.h>
>>> #include <asm/boot.h>
>>> +#include <asm/sev.h>
>>>
>>> #include "efistub.h"
>>> #include "x86-stub.h"
>>> @@ -790,6 +791,19 @@ static efi_status_t exit_boot(struct boot_params *boot_params, void *handle)
>>> return EFI_SUCCESS;
>>> }
>>>
>>> +static bool have_unsupported_snp_features(void)
>>> +{
>>> + u64 unsupported;
>>> +
>>> + unsupported = snp_get_unsupported_features(sev_get_status());
>>
>> This will also set sme_me_mask, but I think that is ok, since on error
>> things will terminate, otherwise sev_enable() should update appropriately
>> later.
>>
>
> OK
>
>>> + if (unsupported) {
>>> + efi_err("Unsupported SEV-SNP features detected: 0x%llx\n",
>>> + unsupported);
>>> + return true;
>>> + }
>>> + return false;
>>> +}
>>> +
>>> static void __noreturn enter_kernel(unsigned long kernel_addr,
>>> struct boot_params *boot_params)
>>> {
>>> @@ -820,6 +834,9 @@ void __noreturn efi_stub_entry(efi_handle_t handle,
>>> if (efi_system_table->hdr.signature != EFI_SYSTEM_TABLE_SIGNATURE)
>>> efi_exit(handle, EFI_INVALID_PARAMETER);
>>>
>>> + if (have_unsupported_snp_features())
>>> + efi_exit(handle, EFI_UNSUPPORTED);
>>> +
>>> if (IS_ENABLED(CONFIG_EFI_DXE_MEM_ATTRIBUTES)) {
>>> efi_dxe_table = get_efi_config_table(EFI_DXE_SERVICES_TABLE_GUID);
>>> if (efi_dxe_table &&
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