Re: [FYI PATCH 0/7] Mitigation for CVE-2018-12207

[Paolo Bonzini <pbonzini@redhat.com>]

On 13/11/19 19:10, Nadav Amit wrote:
> 
>> On Nov 12, 2019, at 1:21 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>>
>> CVE-2018-12207 is a microarchitectural implementation issue
>> that could allow an unprivileged local attacker to cause system wide
>> denial-of-service condition.
>>
>> Privileged software may change the page size (ex. 4KB, 2MB, 1GB) in the
>> paging structures, without following such paging structure changes with
>> invalidation of the TLB entries corresponding to the changed pages. In
>> this case, the attacker could invoke instruction fetch, which will result
>> in the processor hitting multiple TLB entries, reporting a machine check
>> error exception, and ultimately hanging the system.
>>
>> The attached patches mitigate the vulnerability by making huge pages
>> non-executable. The processor will not be able to execute an instruction
>> residing in a large page (ie. 2MB, 1GB, etc.) without causing a trap into
>> the host kernel/hypervisor; KVM will then break the large page into 4KB
>> pages and gives executable permission to 4KB pages.
> 
> It sounds that this mitigation will trigger the “page fracturing” problem
> I once encountered [1], causing frequent full TLB flushes when invlpg
> runs. I wonder if VMs would benefit in performance from changing
> /sys/kernel/debug/x86/tlb_single_page_flush_ceiling to zero.
> 
> On a different note - I am not sure I fully understand the exact scenario.
> Any chance of getting a kvm-unit-test for this case?

No, for now I only have a test that causes lots of 2 MiB pages to be
shattered to 4 KiB.  But I wanted to get and post a test case in the
next week.

Paolo

> [1] https://patchwork.kernel.org/patch/9099311/
>