Re: [RFC PATCH] x86/sev: Disallow userspace access to BIOS region for SEV-SNP guests

Kees Cook
2025-04-09T17:39:48Z

On Tue, Apr 08, 2025 at 04:55:08PM -0700, Dan Williams wrote:
> Dave Hansen wrote:
> > On 4/8/25 06:43, Tom Lendacky wrote:
> > >> Tom/Boris, do you see a problem blocking access to /dev/mem for SEV
> > >> guests?
> > > Not sure why we would suddenly not allow that.
> >
> > Both TDX and SEV-SNP have issues with allowing access to /dev/mem.
> > Disallowing access to the individually troublesome regions can fix
> > _part_ of the problem. But suddenly blocking access is guaranteed to fix
> > *ALL* the problems forever.
>
> ...or at least solicits practical use cases for why the kernel needs to
> poke holes in the policy.
>
> > Or, maybe we just start returning 0's for all reads and throw away all
> > writes. That is probably less likely to break userspace that doesn't
> > know what it's doing in the first place.
>
> Yes, and a bulk of the regression risk has already been pipe-cleaned by
> KERNEL_LOCKDOWN that shuts down /dev/mem and PCI resource file mmap in
> many scenarios.
>
> Here is an updated patch that includes some consideration for mapping
> zeros for known legacy compatibility use cases.
>
> -- 8< --
> From: Dan Williams <dan.j.williams@intel.com>
> Subject: [PATCH] x86: Restrict /dev/mem access for potentially unaccepted
> memory by default
>
> Nikolay reports [1] that accessing BIOS data (first 1MB of the physical
> address space) via /dev/mem results in an SEPT violation.
>
> The cause is ioremap() (via xlate_dev_mem_ptr()) establishes an
> unencrypted mapping where the kernel had established an encrypted
> mapping previously.
>
> An initial attempt to fix this revealed that TDX and SEV-SNP have
> different expectations about which and when address ranges can be mapped
> via /dev/mem.
>
> Rather than develop a precise set of allowed /dev/mem capable TVM
> address ranges, lean on the observation that KERNEL_LOCKDOWN is already
> blocking /dev/mem access in many cases to do the same by default for x86
> TVMs. This can still be later relaxed as specific needs arise, but in
> the meantime close off this source of mismatched IORES_MAP_ENCRYPTED
> expectations.
>
> Note that this is careful to map zeroes rather than reject mappings of
> the BIOS data space.
>
> Cc: <x86@kernel.org>
> Cc: Vishal Annapurve <vannapurve@google.com>
> Cc: Kirill Shutemov <kirill.shutemov@linux.intel.com>
> Reported-by: Nikolay Borisov <nik.borisov@suse.com>
> Closes: http://lore.kernel.org/20250318113604.297726-1-nik.borisov@suse.com [1]
> Fixes: 9aa6ea69852c ("x86/tdx: Make pages shared in ioremap()")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
> arch/x86/Kconfig | 2 ++
> arch/x86/include/asm/x86_init.h | 2 ++
> arch/x86/kernel/x86_init.c | 6 ++++++
> arch/x86/mm/init.c | 14 +++++++++++---
> 4 files changed, 21 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 15f346f02af0..6d4f94a79314 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -888,6 +888,7 @@ config INTEL_TDX_GUEST
> depends on X86_64 && CPU_SUP_INTEL
> depends on X86_X2APIC
> depends on EFI_STUB
> + depends on STRICT_DEVMEM
> select ARCH_HAS_CC_PLATFORM
> select X86_MEM_ENCRYPT
> select X86_MCE
> @@ -1507,6 +1508,7 @@ config AMD_MEM_ENCRYPT
> bool "AMD Secure Memory Encryption (SME) support"
> depends on X86_64 && CPU_SUP_AMD
> depends on EFI_STUB
> + depends on STRICT_DEVMEM
> select DMA_COHERENT_POOL
> select ARCH_USE_MEMREMAP_PROT
> select INSTRUCTION_DECODER
> diff --git a/arch/x86/include/asm/x86_init.h b/arch/x86/include/asm/x86_init.h
> index 213cf5379a5a..0ae436b34b88 100644
> --- a/arch/x86/include/asm/x86_init.h
> +++ b/arch/x86/include/asm/x86_init.h
> @@ -305,6 +305,7 @@ struct x86_hyper_runtime {
> * semantics.
> * @realmode_reserve: reserve memory for realmode trampoline
> * @realmode_init: initialize realmode trampoline
> + * @devmem_is_allowed restrict /dev/mem and PCI sysfs resource access
> * @hyper: x86 hypervisor specific runtime callbacks
> */
> struct x86_platform_ops {
> @@ -323,6 +324,7 @@ struct x86_platform_ops {
> void (*set_legacy_features)(void);
> void (*realmode_reserve)(void);
> void (*realmode_init)(void);
> + bool (*devmem_is_allowed)(unsigned long pfn);
> struct x86_hyper_runtime hyper;
> struct x86_guest guest;
> };
> diff --git a/arch/x86/kernel/x86_init.c b/arch/x86/kernel/x86_init.c
> index 0a2bbd674a6d..346301375bd4 100644
> --- a/arch/x86/kernel/x86_init.c
> +++ b/arch/x86/kernel/x86_init.c
> @@ -143,6 +143,11 @@ static void enc_kexec_begin_noop(void) {}
> static void enc_kexec_finish_noop(void) {}
> static bool is_private_mmio_noop(u64 addr) {return false; }
>
> +static bool platform_devmem_is_allowed(unsigned long pfn)
> +{
> + return !cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT);
> +}
> +
> struct x86_platform_ops x86_platform __ro_after_init = {
> .calibrate_cpu = native_calibrate_cpu_early,
> .calibrate_tsc = native_calibrate_tsc,
> @@ -156,6 +161,7 @@ struct x86_platform_ops x86_platform __ro_after_init = {
> .restore_sched_clock_state = tsc_restore_sched_clock_state,
> .realmode_reserve = reserve_real_mode,
> .realmode_init = init_real_mode,
> + .devmem_is_allowed = platform_devmem_is_allowed,
> .hyper.pin_vcpu = x86_op_int_noop,
> .hyper.is_private_mmio = is_private_mmio_noop,
>
> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
> index bfa444a7dbb0..c8679ae1bc8b 100644
> --- a/arch/x86/mm/init.c
> +++ b/arch/x86/mm/init.c
> @@ -867,6 +867,8 @@ void __init poking_init(void)
> */
> int devmem_is_allowed(unsigned long pagenr)
> {
> + bool platform_allowed = x86_platform.devmem_is_allowed(pagenr);
> +
> if (region_intersects(PFN_PHYS(pagenr), PAGE_SIZE,
> IORESOURCE_SYSTEM_RAM, IORES_DESC_NONE)
> != REGION_DISJOINT) {
> @@ -885,14 +887,20 @@ int devmem_is_allowed(unsigned long pagenr)
> * restricted resource under CONFIG_STRICT_DEVMEM.
> */
> if (iomem_is_exclusive(pagenr << PAGE_SHIFT)) {
> - /* Low 1MB bypasses iomem restrictions. */
> - if (pagenr < 256)
> + /*
> + * Low 1MB bypasses iomem restrictions unless the
> + * platform says "no", in which case map zeroes
> + */
> + if (pagenr < 256) {
> + if (!platform_allowed)
> + return 2;
> return 1;
> + }
>
> return 0;
> }
>
> - return 1;
> + return platform_allowed;
> }
>
> void free_init_pages(const char *what, unsigned long begin, unsigned long end)

I am reminded of this discussion:
https://lore.kernel.org/all/CAPcyv4iVt=peUAk1qx_EfKn7aGJM=XwRUpJftBhkUgQEti2bJA@mail.gmail.com/

As in, mmap will bypass this restriction, so if you really want the low
1MiB to be unreadable, a solution for mmap is still needed...

-Kees

--
Kees Cook