Re: [Linux-6.12.y] XEN: CVE-2024-53241 / XSA-466 and Clang-kCFI

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 19/12/2024 11:10 pm, Sedat Dilek wrote:
> On Thu, Dec 19, 2024 at 6:07 PM Sedat Dilek <sedat.dilek@gmail.com> wrote:
>> On Thu, Dec 19, 2024 at 5:44 PM Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>>> On 19/12/2024 4:14 pm, Sedat Dilek wrote:
>>>> Hi,
>>>>
>>>> Linux v6.12.6 will include XEN CVE fixes from mainline.
>>>>
>>>> Here, I use Debian/unstable AMD64 and the SLIM LLVM toolchain 19.1.x
>>>> from kernel.org.
>>>>
>>>> What does it mean in ISSUE DESCRIPTION...
>>>>
>>>> Furthermore, the hypercall page has no provision for Control-flow
>>>> Integrity schemes (e.g. kCFI/CET-IBT/FineIBT), and will simply
>>>> malfunction in such configurations.
>>>>
>>>> ...when someone uses Clang-kCFI?
>>> The hypercall page has functions of the form:
>>>
>>>     MOV $x, %eax
>>>     VMCALL / VMMCALL / SYSCALL
>>>     RET
>>>
>>> There are no ENDBR instructions, and no prologue/epilogue for hash-based
>>> CFI schemes.
>>>
>>> This is because it's code provided by Xen, not code provided by Linux.
>>>
>>> The absence of ENDBR instructions will yield #CP when CET-IBT is active,
>>> and the absence of hash prologue/epilogue lets the function be used in a
>>> type-confused manor that CFI should have caught.
>>>
>>> ~Andrew
>> Thanks for the technical explanation, Andrew.
>>
>> Hope that helps the folks of "CLANG CONTROL FLOW INTEGRITY SUPPORT".
>>
>> I am not an active user of XEN in the Linux-kernel but I am willing to
>> test when Linux v6.12.6 is officially released and give feedback.
>>
> https://wiki.xenproject.org/wiki/Testing_Xen#Presence_test
> https://wiki.xenproject.org/wiki/Testing_Xen#Commands_for_presence_testing
>
> # apt install -t unstable xen-utils-4.17 -y
>
> # xl list
> Name                                        ID   Mem VCPUs      State   Time(s)
> Domain-0                                     0  7872     4     r-----     398.2
>
> Some basic tests LGTM - see also attached stuff.
>
> If you have any tests to recommend, let me know.

That itself is good enough as a smoke test.  Thankyou for trying it out.

If you want something a bit more thorough, try
https://xenbits.xen.org/docs/xtf/  (Xen's self-tests)

Grab and build it, and `./xtf-runner -aqq --host` will run a variety of
extra codepaths in dom0, without the effort of making/running full guests.

~Andrew