Re: [PATCH v2] x86/kernel: skip ROM range scans and validation for SEV-SNP guests

[Kevin Loughlin]

On Mon, Feb 26, 2024 at 2:16 PM Mike Stunes <mike.stunes@xxxxxxxxxxxx> wrote:
>
> Hi,
>
> > On Feb 22, 2024, at 12:24 PM, Kevin Loughlin <kevinloughlin@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> > 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. I will update this in v3.