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

Nikolay Borisov
2025-04-10T12:03:59Z

On 9.04.25 г. 21:39 ч., Dan Williams wrote:
> Kees Cook wrote:
>> 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.
> [..]
>>> 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...
>
> Glad you remembered that!
>
> This needs a self-test to verify the assumptions here. I can circle back
> next week or so take a look at turning this into a bigger series. If
> someone has cycles to take this on before that I would not say no to
> some help.

Can't we simply treat return value of 2 for range_is_allowed the same
way as if 0 was returned in mmap_mem and simply fail the call with -EPERM?