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

Nikolay Borisov
2025-04-10T16:40:01Z

On 10.04.25 г. 19:32 ч., Kees Cook wrote:
> On Thu, Apr 10, 2025 at 03:03:55PM +0300, Nikolay Borisov wrote:
>>
>>
>> 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?
>
> The historical concern was that EPERM would break old tools. I don't
> have any current evidence either way, though.

Right, but we are only about to return 2 in a TVM context, so chances of
running old tools are slim to none. Also it's perfectly valid to have
mmap fail for a number of reasons, so old tools should be equipped with
handling it returning -EPERM, no ?

>