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Date: Wed, 7 Jun 2023 18:51:18 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Tom Lendacky <thomas.lendacky@....com>
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 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?
> 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.
--- 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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