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

Edgecombe, Rick P
2022-10-20T21:22:31Z

On Tue, 2022-10-04 at 13:50 -0700, Nathan Chancellor wrote:
> 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!

Indeed this was a SW bug and had nothing to do with the CPU model. The
altshstk selftest was not fully initializing the stack_t struct, and
getting lucky on some compilers. Thanks to Nathan for helping me debug
it.