Re: [PATCH Part2 v5 26/45] KVM: SVM: Mark the private vma unmerable for SEV-SNP guests

[Brijesh Singh]

On 10/12/21 11:46 AM, Sean Christopherson wrote:
> On Fri, Aug 20, 2021, Brijesh Singh wrote:
>> When SEV-SNP is enabled, the guest private pages are added in the RMP
>> table; while adding the pages, the rmp_make_private() unmaps the pages
>> from the direct map. If KSM attempts to access those unmapped pages then
>> it will trigger #PF (page-not-present).
>>
>> Encrypted guest pages cannot be shared between the process, so an
>> userspace should not mark the region mergeable but to be safe, mark the
>> process vma unmerable before adding the pages in the RMP table.
> To be safe from what?  Does the !PRESENT #PF crash the kernel?

Yes, kernel crashes when KSM attempts to access to an unmaped pfn.

[...]
>> +	mmap_write_lock(kvm->mm);
>> +	ret = snp_mark_unmergable(kvm, params.uaddr, params.len);
>> +	mmap_write_unlock(kvm->mm);
> This does not, and practically speaking cannot, work.  There are multiple TOCTOU
> bugs, here and in __snp_handle_page_state_change().  Userspace can madvise() the
> range at any later point, munmap()/mmap() the entire range, mess with the memslots
> in the PSC case, and so on and so forth.  Relying on MADV_UNMERGEABLE for functional
> correctness simply cannot work in KVM, barring mmu_notifier and a big pile of code.

AFAICT, ksm does not exclude the unmapped pfn from its scan list. We
need to tell ksm somehow to exclude the unmapped pfn from its scan list.
I understand that if userspace is messing with us, we have an issue, but
it's a userspace bug ;) To fix it right, we need to enhance ksm to
exclude the pfn when it is getting unmapped from the direct map. I
believe that work can be done outside of the SNP series. I am okay to
drop snp_mark_unmerable(), and until then, we just run with KSM
disabled. Thoughts?

thanks