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Subject: Re: [PATCH v2] x86/kernel: skip ROM range scans and validation for
 SEV-SNP guests
From: Mike Stunes <mike.stunes@broadcom.com>
In-Reply-To: <20240222202404.36206-1-kevinloughlin@google.com>
Date: Mon, 26 Feb 2024 11:15:54 -0800
Cc: acdunlap@google.com,
 alexander.shishkin@linux.intel.com,
 andrisaar@google.com,
 ardb@kernel.org,
 bhe@redhat.com,
 Borislav Petkov <bp@alien8.de>,
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 "Huang, Kai" <kai.huang@intel.com>,
 linux-kernel@vger.kernel.org,
 Michael Roth <michael.roth@amd.com>,
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 Peter Gonda <pgonda@google.com>,
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References: <CAGdbjmK9WoOQSbKUvcTdLJDW-RB=qe2tHFDZ-MeD266xZOxh7w@mail.gmail.com>
 <20240222202404.36206-1-kevinloughlin@google.com>
To: Kevin Loughlin <kevinloughlin@google.com>
Content-Type: text/plain; charset="ISO-8859-1"

Hi,

> On Feb 22, 2024, at 12:24 PM, Kevin Loughlin <kevinloughlin@google.com> wrote:
> 
> SEV-SNP requires encrypted memory to be validated before access.
> Because the ROM memory range is not part of the e820 table, it is not
> pre-validated by the BIOS. Therefore, if a SEV-SNP guest kernel wishes
> to access this range, the guest must first validate the range.
> 
> The current SEV-SNP code does indeed scan the ROM range during early
> boot and thus attempts to validate the ROM range in probe_roms().
> However, this behavior is neither necessary nor sufficient.
> 
> With regards to sufficiency, if EFI_CONFIG_TABLES are not enabled and
> CONFIG_DMI_SCAN_MACHINE_NON_EFI_FALLBACK is set, the kernel will
> attempt to access the memory at SMBIOS_ENTRY_POINT_SCAN_START (which
> falls in the ROM range) prior to validation. The specific problematic
> call chain occurs during dmi_setup() -> dmi_scan_machine() and results
> in a crash during boot if SEV-SNP is enabled under these conditions.
> 
> With regards to necessity, SEV-SNP guests currently read garbage (which
> changes across boots) from the ROM range, meaning these scans are
> unnecessary. The guest reads garbage because the legacy ROM range
> is unencrypted data but is accessed via an encrypted PMD during early
> boot (where the PMD is marked as encrypted due to potentially mapping
> actually-encrypted data in other PMD-contained ranges).
> 
> While one solution would be to overhaul the early PMD mapping to treat
> the ROM region of the PMD as unencrypted, SEV-SNP guests do not rely on
> data from the legacy ROM region during early boot (nor can they
> currently, since the data would be garbage that changes across boots).
> As such, this patch opts for the simpler approach of skipping the ROM
> range scans (and the otherwise-necessary range validation) during
> SEV-SNP guest early boot.
> 
> Ultimatly, the potential SEV-SNP guest crash due to lack of ROM range
> validation is avoided by simply not accessing the ROM range.
> 
> Fixes: 9704c07bf9f7 ("x86/kernel: Validate ROM memory before accessing when SEV-SNP is active")
> Signed-off-by: Kevin Loughlin <kevinloughlin@google.com>
> ---
> arch/x86/include/asm/sev.h   |  2 --
> arch/x86/kernel/mpparse.c    |  7 +++++++
> arch/x86/kernel/probe_roms.c | 11 ++++-------
> arch/x86/kernel/sev.c        | 15 ---------------
> drivers/firmware/dmi_scan.c  |  7 ++++++-
> 5 files changed, 17 insertions(+), 25 deletions(-)
> 
> diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
> index 5b4a1ce3d368..474c24ba0f6f 100644
> --- a/arch/x86/include/asm/sev.h
> +++ b/arch/x86/include/asm/sev.h
> @@ -203,7 +203,6 @@ void __init early_snp_set_memory_private(unsigned long vaddr, unsigned long padd
> unsigned long npages);
> void __init early_snp_set_memory_shared(unsigned long vaddr, unsigned long paddr,
> unsigned long npages);
> -void __init snp_prep_memory(unsigned long paddr, unsigned int sz, enum psc_op op);
> void snp_set_memory_shared(unsigned long vaddr, unsigned long npages);
> void snp_set_memory_private(unsigned long vaddr, unsigned long npages);
> void snp_set_wakeup_secondary_cpu(void);
> @@ -227,7 +226,6 @@ static inline void __init
> early_snp_set_memory_private(unsigned long vaddr, unsigned long paddr, unsigned long npages) { }
> static inline void __init
> early_snp_set_memory_shared(unsigned long vaddr, unsigned long paddr, unsigned long npages) { }
> -static inline void __init snp_prep_memory(unsigned long paddr, unsigned int sz, enum psc_op op) { }
> static inline void snp_set_memory_shared(unsigned long vaddr, unsigned long npages) { }
> static inline void snp_set_memory_private(unsigned long vaddr, unsigned long npages) { }
> static inline void snp_set_wakeup_secondary_cpu(void) { }
> diff --git a/arch/x86/kernel/mpparse.c b/arch/x86/kernel/mpparse.c
> index b223922248e9..39ea771e2d4c 100644
> --- a/arch/x86/kernel/mpparse.c
> +++ b/arch/x86/kernel/mpparse.c
> @@ -553,6 +553,13 @@ static int __init smp_scan_config(unsigned long base, unsigned long length)
>    base, base + length - 1);
> BUILD_BUG_ON(sizeof(*mpf) != 16);
> 
> + /*
> + * Skip scan in SEV-SNP guest if it would touch the legacy ROM region,
> + * as this memory is not pre-validated and would thus cause a crash.
> + */
> + if (cc_platform_has(CC_ATTR_GUEST_SEV_SNP) && base < 0x100000 && base + length >= 0xC0000)
> + return 0;
> +
> while (length > 0) {
> bp = early_memremap(base, length);
> mpf = (struct mpf_intel *)bp;
> diff --git a/arch/x86/kernel/probe_roms.c b/arch/x86/kernel/probe_roms.c
> index 319fef37d9dc..84ff4b052fc1 100644
> --- a/arch/x86/kernel/probe_roms.c
> +++ b/arch/x86/kernel/probe_roms.c
> @@ -204,14 +204,11 @@ void __init probe_roms(void)
> int i;
> 
> /*
> - * The ROM memory range is not part of the e820 table and is therefore not
> - * pre-validated by BIOS. The kernel page table maps the ROM region as encrypted
> - * memory, and SNP requires encrypted memory to be validated before access.
> - * Do that here.
> + * These probes are skipped in SEV-SNP guests because the ROM range
> + * is not pre-validated, meaning access would cause a crash.
> */
> - snp_prep_memory(video_rom_resource.start,
> - ((system_rom_resource.end + 1) - video_rom_resource.start),
> - SNP_PAGE_STATE_PRIVATE);
> + if (cc_platform_has(CC_ATTR_GUEST_SEV_SNP))
> + return;
> 
> /* video rom */
> upper = adapter_rom_resources[0].start;
> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index c67285824e82..d2362631da91 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -774,21 +774,6 @@ void __init early_snp_set_memory_shared(unsigned long vaddr, unsigned long paddr
> early_set_pages_state(vaddr, paddr, npages, SNP_PAGE_STATE_SHARED);
> }
> 
> -void __init snp_prep_memory(unsigned long paddr, unsigned int sz, enum psc_op op)
> -{
> - unsigned long vaddr, npages;
> -
> - vaddr = (unsigned long)__va(paddr);
> - npages = PAGE_ALIGN(sz) >> PAGE_SHIFT;
> -
> - if (op == SNP_PAGE_STATE_PRIVATE)
> - early_snp_set_memory_private(vaddr, paddr, npages);
> - else if (op == SNP_PAGE_STATE_SHARED)
> - early_snp_set_memory_shared(vaddr, paddr, npages);
> - else
> - WARN(1, "invalid memory op %d\n", op);
> -}
> -
> static unsigned long __set_pages_state(struct snp_psc_desc *data, unsigned long vaddr,
>       unsigned long vaddr_end, int op)
> {
> diff --git a/drivers/firmware/dmi_scan.c b/drivers/firmware/dmi_scan.c
> index 015c95a825d3..22e27087eb5b 100644
> --- a/drivers/firmware/dmi_scan.c
> +++ b/drivers/firmware/dmi_scan.c
> @@ -703,7 +703,12 @@ static void __init dmi_scan_machine(void)
> dmi_available = 1;
> return;
> }
> - } else if (IS_ENABLED(CONFIG_DMI_SCAN_MACHINE_NON_EFI_FALLBACK)) {
> + } else if (IS_ENABLED(CONFIG_DMI_SCAN_MACHINE_NON_EFI_FALLBACK) &&
> + !cc_platform_has(CC_ATTR_GUEST_SEV_SNP)) {
> + /*
> + * This scan is skipped in SEV-SNP guests because the ROM range
> + * is not pre-validated, meaning access would cause a crash.
> + */
> p = dmi_early_remap(SMBIOS_ENTRY_POINT_SCAN_START, 0x10000);
> if (p == NULL)
> goto error;
> -- 
> 2.44.0.rc0.258.g7320e95886-goog
> 
> 

In addition to these changes, I also had to skip pirq_find_routing_table if SEV-SNP is active.

Thanks!
Mike


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