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

[Dan Williams]

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@xxxxxxxxxxxxxx/
> 
> 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.