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

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Message-ID: <CA+icZUVhzsc+_PJr0RSwaVQTbz5TKa8wmyzgBNQEcody4YGesg@mail.gmail.com>
Date: Sat, 21 Dec 2024 18:06:06 +0100
From: Sedat Dilek <sedat.dilek@...il.com>
To: Andrew Cooper <andrew.cooper3@...rix.com>
Cc: Juergen Gross <jgross@...e.com>, Peter Zijlstra <peterz@...radead.org>, 
	Sami Tolvanen <samitolvanen@...gle.com>, Jan Beulich <jbeulich@...e.com>, 
	Josh Poimboeuf <jpoimboe@...hat.com>, Linus Torvalds <torvalds@...ux-foundation.org>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Sasha Levin <sashal@...nel.org>, 
	linux-kernel@...r.kernel.org, stable@...r.kernel.org, 
	Kees Cook <kees@...nel.org>, Nathan Chancellor <nathan@...nel.org>, llvm@...ts.linux.dev, 
	xen-devel <xen-devel@...ts.xenproject.org>
Subject: Re: [Linux-6.12.y] XEN: CVE-2024-53241 / XSA-466 and Clang-kCFI

On Fri, Dec 20, 2024 at 2:39 AM Andrew Cooper <andrew.cooper3@...rix.com> wrote:
>
> On 20/12/2024 12:27 am, Sedat Dilek wrote:
> > On Fri, Dec 20, 2024 at 12:26 AM Andrew Cooper
> > <andrew.cooper3@...rix.com> wrote:
> >> On 19/12/2024 11:10 pm, Sedat Dilek wrote:
> >>> On Thu, Dec 19, 2024 at 6:07 PM Sedat Dilek <sedat.dilek@...il.com> wrote:
> >>>> On Thu, Dec 19, 2024 at 5:44 PM Andrew Cooper <andrew.cooper3@...rix.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
> > Run on Debian 6.12.5 and my selfmade 6.12.5 and 6.12.6.
> > All tests lead to a reboot in case of Debian or in my kernels to a shutdown.
> >
> > Can you recommend a specific test?
>
> Oh, that's distinctly less good.
>
> Start with just "example".  It's literally a hello world microkernel,
> but the symptoms you're seeing is a dom0 crash, so it will likely
> provoke it.
>
> Do you have serial to the machine?  If so, boot Xen with `console=com1
> com1=115200,8n1` (or com2, as appropriate).
>
> If not and you've only got a regular screen, boot Xen with `vga=,keep
> noreboot` (comma is important) which might leave enough information on
> screen to get an idea of what's going on.
>
> Full command line docs at
> https://xenbits.xen.org/docs/unstable/misc/xen-command-line.html
>
> > dileks@...za:~/src/xtf/git$ sudo ./xtf-runner --list functional xsa | grep xsa-4
> > test-pv64-xsa-444
> > test-hvm64-xsa-451
> > test-hvm64-xsa-454
> >
> > Is there no xsa-466 test?
>
> No.  XSA-466 is really "well don't do that then if it matters".
>
> More generally, not all XSAs are amenable to testing in this way.
>
> ~Andrew

RUN example tests on Debian's 6.12.6 kernel.

$ cat /proc/version
Linux version 6.12.6-amd64 (debian-kernel@...ts.debian.org)
(x86_64-linux-gnu-gcc-14 (Debian 14.2.0-11) 14.2.0, GNU ld (GNU
Binutils for Debian) 2.43.50.20241215) #1 SMP PREEMPT_DYNAMIC Debian
6.12.6-1 (2024-12-21)

dileks@...za:~/src/xtf/git$ sudo ./xtf-runner --list example
test-hvm32-example
test-hvm32pae-example
test-hvm32pse-example
test-hvm64-example
test-pv32pae-example
test-pv64-example
dileks@...za:~/src/xtf/git$ sudo ./xtf-runner test-hvm32-example
Executing 'xl create -p tests/example/test-hvm32-example.cfg'
Executing 'xl console test-hvm32-example'
Executing 'xl unpause test-hvm32-example'
--- Xen Test Framework ---
Environment: HVM 32bit (No paging)
Hello World
Test result: SUCCESS

Combined test results:
test-hvm32-example                       SUCCESS
dileks@...za:~/src/xtf/git$ sudo ./xtf-runner test-hvm32pae-example
Executing 'xl create -p tests/example/test-hvm32pae-example.cfg'
Executing 'xl console test-hvm32pae-example'
Executing 'xl unpause test-hvm32pae-example'
--- Xen Test Framework ---
Environment: HVM 32bit (PAE 3 levels)
Hello World
Test result: SUCCESS

Combined test results:
test-hvm32pae-example                    SUCCESS
dileks@...za:~/src/xtf/git$ sudo ./xtf-runner test-hvm32pse-example
Executing 'xl create -p tests/example/test-hvm32pse-example.cfg'
Executing 'xl console test-hvm32pse-example'
Executing 'xl unpause test-hvm32pse-example'
--- Xen Test Framework ---
Environment: HVM 32bit (PSE 2 levels)
Hello World
Test result: SUCCESS

Combined test results:
test-hvm32pse-example                    SUCCESS
dileks@...za:~/src/xtf/git$ sudo ./xtf-runner test-hvm64-example
Executing 'xl create -p tests/example/test-hvm64-example.cfg'
Executing 'xl console test-hvm64-example'
Executing 'xl unpause test-hvm64-example'
--- Xen Test Framework ---
Environment: HVM 64bit (Long mode 4 levels)
Hello World
Test result: SUCCESS

Combined test results:
test-hvm64-example                       SUCCESS
dileks@...za:~/src/xtf/git$ sudo ./xtf-runner test-pv32pae-example
Combined test results:
test-pv32pae-example                     SKIP
dileks@...za:~/src/xtf/git$ sudo ./xtf-runner test-pv64-example
Executing 'xl create -p tests/example/test-pv64-example.cfg'
Executing 'xl console test-pv64-example'
Executing 'xl unpause test-pv64-example'
--- Xen Test Framework ---
Environment: PV 64bit (Long mode 4 levels)
Hello World
Test result: SUCCESS

Combined test results:
test-pv64-example                        SUCCESS

Thanks.

Best regards,
-Sedat-