Re: [PATCH v2 33/39] x86/cpufeatures: Limit shadow stack to Intel CPUs

Nathan Chancellor
2022-10-04T20:50:35Z

On Tue, Oct 04, 2022 at 08:34:54PM +0000, Edgecombe, Rick P wrote:
> On Tue, 2022-10-04 at 14:43 -0500, John Allen wrote:
> > On 10/4/22 10:47 AM, Nathan Chancellor wrote:
> > > Hi Kees,
> > >
> > > On Mon, Oct 03, 2022 at 09:54:26PM -0700, Kees Cook wrote:
> > > > On Mon, Oct 03, 2022 at 05:09:04PM -0700, Dave Hansen wrote:
> > > > > On 10/3/22 16:57, Kees Cook wrote:
> > > > > > On Thu, Sep 29, 2022 at 03:29:30PM -0700, Rick Edgecombe
> > > > > > wrote:
> > > > > > > Shadow stack is supported on newer AMD processors, but the
> > > > > > > kernel
> > > > > > > implementation has not been tested on them. Prevent basic
> > > > > > > issues from
> > > > > > > showing up for normal users by disabling shadow stack on
> > > > > > > all CPUs except
> > > > > > > Intel until it has been tested. At which point the
> > > > > > > limitation should be
> > > > > > > removed.
> > > > > > >
> > > > > > > Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> > > > > >
> > > > > > So running the selftests on an AMD system is sufficient to
> > > > > > drop this
> > > > > > patch?
> > > > >
> > > > > Yes, that's enough.
> > > > >
> > > > > I _thought_ the AMD folks provided some tested-by's at some
> > > > > point in the
> > > > > past. But, maybe I'm confusing this for one of the other
> > > > > shared
> > > > > features. Either way, I'm sure no tested-by's were dropped on
> > > > > purpose.
> > > > >
> > > > > I'm sure Rick is eager to trim down his series and this would
> > > > > be a great
> > > > > patch to drop. Does anyone want to make that easy for Rick?
> > > > >
> > > > > <hint> <hint>
> > > >
> > > > Hey Gustavo, Nathan, or Nick! I know y'all have some fancy AMD
> > > > testing
> > > > rigs. Got a moment to spin up this series and run the selftests?
> > > > :)
> > >
> > > I do have access to a system with an EPYC 7513, which does have
> > > Shadow
> > > Stack support (I can see 'shstk' in the "Flags" section of lscpu
> > > with
> > > this series). As far as I understand it, AMD only added Shadow
> > > Stack
> > > with Zen 3; my regular AMD test system is Zen 2 (probably should
> > > look at
> > > procurring a Zen 3 or Zen 4 one at some point).
> > >
> > > I applied this series on top of 6.0 and reverted this change then
> > > booted
> > > it on that system. After building the selftest (which did require
> > > 'make headers_install' and a small addition to make it build beyond
> > > that, see below), I ran it and this was the result. I am not sure
> > > if
> > > that is expected or not but the other results seem promising for
> > > dropping this patch.
> > >
> > > $ ./test_shadow_stack_64
> > > [INFO] new_ssp = 7f8a36c9fff8, *new_ssp = 7f8a36ca0001
> > > [INFO] changing ssp from 7f8a374a0ff0 to 7f8a36c9fff8
> > > [INFO] ssp is now 7f8a36ca0000
> > > [OK] Shadow stack pivot
> > > [OK] Shadow stack faults
> > > [INFO] Corrupting shadow stack
> > > [INFO] Generated shadow stack violation successfully
> > > [OK] Shadow stack violation test
> > > [INFO] Gup read -> shstk access success
> > > [INFO] Gup write -> shstk access success
> > > [INFO] Violation from normal write
> > > [INFO] Gup read -> write access success
> > > [INFO] Violation from normal write
> > > [INFO] Gup write -> write access success
> > > [INFO] Cow gup write -> write access success
> > > [OK] Shadow gup test
> > > [INFO] Violation from shstk access
> > > [OK] mprotect() test
> > > [OK] Userfaultfd test
> > > [FAIL] Alt shadow stack test
> >
> > The selftest is looking OK on my system (Dell PowerEdge R6515 w/ EPYC
> > 7713). I also just pulled a fresh 6.0 kernel and applied the series
> > including the fix Nathan mentions below.
> >
> > $ tools/testing/selftests/x86/test_shadow_stack_64
> > [INFO] new_ssp = 7f30cccc5ff8, *new_ssp = 7f30cccc6001
> > [INFO] changing ssp from 7f30cd4c6ff0 to 7f30cccc5ff8
> > [INFO] ssp is now 7f30cccc6000
> > [OK] Shadow stack pivot
> > [OK] Shadow stack faults
> > [INFO] Corrupting shadow stack
> > [INFO] Generated shadow stack violation successfully
> > [OK] Shadow stack violation test
> > [INFO] Gup read -> shstk access success
> > [INFO] Gup write -> shstk access success
> > [INFO] Violation from normal write
> > [INFO] Gup read -> write access success
> > [INFO] Violation from normal write
> > [INFO] Gup write -> write access success
> > [INFO] Cow gup write -> write access success
> > [OK] Shadow gup test
> > [INFO] Violation from shstk access
> > [OK] mprotect() test
> > [OK] Userfaultfd test
> > [OK] Alt shadow stack test.
>
> Thanks for the testing. Based on the test, I wonder if this could be a
> SW bug. Nathan, could I send you a tweaked test with some more debug
> information?

Yes, more than happy to help you look into this further!

> John, are we sure AMD and Intel systems behave the same with respect to
> CPUs not creating Dirty=1,Write=0 PTEs in rare situations? Or any other
> CET related differences we should hash out? Otherwise I'll drop the
> patch for the next version. (and assuming the issue Nathan hit doesn't
> turn up anything HW related).