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

John Allen
2022-10-04T19:43:36Z

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.

>
> $ echo $?
> 1
>
> I am happy to provide any information that would be useful for exploring
> this failure and test further iterations of this series as necessary.
>
> Cheers,
> Nathan
>
> test_shadow_stack.c: In function ‘create_shstk’:
> test_shadow_stack.c:86:70: error: ‘SHADOW_STACK_SET_TOKEN’ undeclared (first use in this function)
> 86 | return (void *)syscall(__NR_map_shadow_stack, addr, SS_SIZE, SHADOW_STACK_SET_TOKEN);
> | ^~~~~~~~~~~~~~~~~~~~~~
> test_shadow_stack.c:86:70: note: each undeclared identifier is reported only once for each function it appears in
> test_shadow_stack.c:87:1: warning: control reaches end of non-void function [-Wreturn-type]
> 87 | }
> | ^
>
> diff --git a/tools/testing/selftests/x86/test_shadow_stack.c b/tools/testing/selftests/x86/test_shadow_stack.c
> index 22b856de5cdd..958dbb248518 100644
> --- a/tools/testing/selftests/x86/test_shadow_stack.c
> +++ b/tools/testing/selftests/x86/test_shadow_stack.c
> @@ -11,6 +11,7 @@
> #define _GNU_SOURCE
>
> #include <sys/syscall.h>
> +#include <asm/mman.h>
> #include <sys/mman.h>
> #include <sys/stat.h>
> #include <sys/wait.h>