Re: Is _PAGE_PROTNONE set only for user mappings?

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Tue, May 24, 2022 at 08:22:30PM +0000, Sean Christopherson wrote:
> On Sun, May 22, 2022, Hyeonggon Yoo wrote:
> > On Mon, May 16, 2022 at 07:04:32AM -0700, Dave Hansen wrote:
> > > I was thinking of something more along the lines of taking the
> > > set_memory.c code and ensuring that it never sets (or even observes)
> > > _PAGE_BIT_GLOBAL on a _PAGE_USER mapping.
> > 
> > Yeah that would be a bit more explicit solution.
> > 
> > > There was also a question of
> > > if set_memory.c is ever used on userspace mappings.  It would be good to
> > > validate whether it's possible in-tree today and if not, enforce that
> > > _PAGE_USER PTEs should never even be observed with set_memory.c.
> > 
> > Simply adding dump_stack() tells me my kernel on my machine does not use
> > set_memory.c for userspace mappings but Hmm I'll take a look.
> 
> vc_slow_virt_to_phys() uses lookup_address_in_pgd() with user mappings, but that
> code is all but guaranteed to be buggy, e.g. doesn't guard against concurrent
> modifications to user mappings.
> 
> show_fault_oops() can also call into lookup_address_in_pgd() with a user mapping,
> though at that point the kernel has bigger problems since it's executing from user
> memory.
> 
> And up until commits 44187235cbcc ("KVM: x86/mmu: fix potential races when walking
> host page table") and 643d95aac59a ("Revert "x86/mm: Introduce lookup_address_in_mm()""),
> KVM had a similar bug.

Thanks for your helpful insight.

I was curious if set_memory*() helpers are used for user mappings.
with some quick look ptrace() and uprobes (where updating application's text is needed)
use kmap + memcpy or replace_page() instead of set_memory*() API.

_lookup_address_cpa() uses init_mm.pgd when cpa.pgd is not specified
and the only place that passes pgd is efi subsystem (efi_mm.pgd), which is not a
userspace.

So it is *obvious* that set_memory*() functions should not be used
for user mappings. because that will only result in updating only init_mm's
page table.

Therefore answering to the first question ('do we really need to unset _PAGE_GLOBAL when we're
clearing _PAGE_PRESENT in set_memory.c to avoid confusing it as
_PAGE_PROTNONE?'):

	we don't need to consider PROT_NONE semantics in set_memory.c
	because we don't (shouldn't) change user mappings in it and _PAGE_PROTNONE is not used
	for kernel mappings.

> Generally speaking, set_memory.c is not equipped to play nice with user mappings.
> It mostly "works", but there are races galore.  IMO, hardening set_memory.c to scream
> if it's handed a user address or encounters _PAGE_USER PTEs would be a very good thing.

I agree that would be a good thing too.

Thanks,
Hyeonggon