Subject: Re: [RFC PATCH v1] powerpc: Enable KFENCE for PPC32

From: Michael Ellerman <[email-masked]>
Date: Tue, 02 Mar 2021 22:40:03 +1100

Christophe Leroy <[email-masked]> writes:
> Le 02/03/2021 à 10:53, Marco Elver a écrit :
>> On Tue, 2 Mar 2021 at 10:27, Christophe Leroy
>> <[email-masked]> wrote:
>>> Le 02/03/2021 à 10:21, Alexander Potapenko a écrit :
>>>>> [   14.998426] BUG: KFENCE: invalid read in finish_task_switch.isra.0+0x54/0x23c
>>>>> [   14.998426]
>>>>> [   15.007061] Invalid read at 0x(ptrval):
>>>>> [   15.010906]  finish_task_switch.isra.0+0x54/0x23c
>>>>> [   15.015633]  kunit_try_run_case+0x5c/0xd0
>>>>> [   15.019682]  kunit_generic_run_threadfn_adapter+0x24/0x30
>>>>> [   15.025099]  kthread+0x15c/0x174
>>>>> [   15.028359]  ret_from_kernel_thread+0x14/0x1c
>>>>> [   15.032747]
>>>>> [   15.034251] CPU: 0 PID: 111 Comm: kunit_try_catch Tainted: G    B
>>>>> 5.12.0-rc1-s3k-dev-01534-g4f14ae75edf0-dirty #4674
>>>>> [   15.045811] ==================================================================
>>>>> [   15.053324]     # test_invalid_access: EXPECTATION FAILED at mm/kfence/kfence_test.c:636
>>>>> [   15.053324]     Expected report_matches(&expect) to be true, but is false
>>>>> [   15.068359]     not ok 21 - test_invalid_access
>>>>
>>>> The test expects the function name to be test_invalid_access, i. e.
>>>> the first line should be "BUG: KFENCE: invalid read in
>>>> test_invalid_access".
>>>> The error reporting function unwinds the stack, skips a couple of
>>>> "uninteresting" frames
>>>> (https://elixir.bootlin.com/linux/v5.12-rc1/source/mm/kfence/report.c#L43)
>>>> and uses the first "interesting" one frame to print the report header
>>>> (https://elixir.bootlin.com/linux/v5.12-rc1/source/mm/kfence/report.c#L226).
>>>>
>>>> It's strange that test_invalid_access is missing altogether from the
>>>> stack trace - is that expected?
>>>> Can you try printing the whole stacktrace without skipping any frames
>>>> to see if that function is there?
>>>>
>>>
>>> Booting with 'no_hash_pointers" I get the following. Does it helps ?
>>>
>>> [   16.837198] ==================================================================
>>> [   16.848521] BUG: KFENCE: invalid read in finish_task_switch.isra.0+0x54/0x23c
>>> [   16.848521]
>>> [   16.857158] Invalid read at 0xdf98800a:
>>> [   16.861004]  finish_task_switch.isra.0+0x54/0x23c
>>> [   16.865731]  kunit_try_run_case+0x5c/0xd0
>>> [   16.869780]  kunit_generic_run_threadfn_adapter+0x24/0x30
>>> [   16.875199]  kthread+0x15c/0x174
>>> [   16.878460]  ret_from_kernel_thread+0x14/0x1c
>>> [   16.882847]
>>> [   16.884351] CPU: 0 PID: 111 Comm: kunit_try_catch Tainted: G    B
>>> 5.12.0-rc1-s3k-dev-01534-g4f14ae75edf0-dirty #4674
>>> [   16.895908] NIP:  c016eb8c LR: c02f50dc CTR: c016eb38
>>> [   16.900963] REGS: e2449d90 TRAP: 0301   Tainted: G    B
>>> (5.12.0-rc1-s3k-dev-01534-g4f14ae75edf0-dirty)
>>> [   16.911386] MSR:  00009032 <EE,ME,IR,DR,RI>  CR: 22000004  XER: 00000000
>>> [   16.918153] DAR: df98800a DSISR: 20000000
>>> [   16.918153] GPR00: c02f50dc e2449e50 c1140d00 e100dd24 c084b13c 00000008 c084b32b c016eb38
>>> [   16.918153] GPR08: c0850000 df988000 c0d10000 e2449eb0 22000288
>>> [   16.936695] NIP [c016eb8c] test_invalid_access+0x54/0x108
>>> [   16.942125] LR [c02f50dc] kunit_try_run_case+0x5c/0xd0
>>> [   16.947292] Call Trace:
>>> [   16.949746] [e2449e50] [c005a5ec] finish_task_switch.isra.0+0x54/0x23c (unreliable)
>> 
>> The "(unreliable)" might be a clue that it's related to ppc32 stack
>> unwinding. Any ppc expert know what this is about?
>> 
>>> [   16.957443] [e2449eb0] [c02f50dc] kunit_try_run_case+0x5c/0xd0
>>> [   16.963319] [e2449ed0] [c02f63ec] kunit_generic_run_threadfn_adapter+0x24/0x30
>>> [   16.970574] [e2449ef0] [c004e710] kthread+0x15c/0x174
>>> [   16.975670] [e2449f30] [c001317c] ret_from_kernel_thread+0x14/0x1c
>>> [   16.981896] Instruction dump:
>>> [   16.984879] 8129d608 38e7eb38 81020280 911f004c 39000000 995f0024 907f0028 90ff001c
>>> [   16.992710] 3949000a 915f0020 3d40c0d1 3d00c085 <8929000a> 3908adb0 812a4b98 3d40c02f
>>> [   17.000711] ==================================================================
>>> [   17.008223]     # test_invalid_access: EXPECTATION FAILED at mm/kfence/kfence_test.c:636
>>> [   17.008223]     Expected report_matches(&expect) to be true, but is false
>>> [   17.023243]     not ok 21 - test_invalid_access
>> 
>> On a fault in test_invalid_access, KFENCE prints the stack trace based
>> on the information in pt_regs. So we do not think there's anything we
>> can do to improve stack printing pe-se.
>
> stack printing, probably not. Would be good anyway to mark the last level [unreliable] as the ppc does.
>
> IIUC, on ppc the address in the stack frame of the caller is written by the caller. In most tests, 
> there is some function call being done before the fault, for instance 
> test_kmalloc_aligned_oob_read() does a call to kunit_do_assertion which populates the address of the 
> call in the stack. However this is fragile.
>
> This works for function calls because in order to call a subfunction, a function has to set up a 
> stack frame in order to same the value in the Link Register, which contains the address of the 
> function's parent and that will be clobbered by the sub-function call.
>
> However, it cannot be done by exceptions, because exceptions can happen in a function that has no 
> stack frame (because that function has no need to call a subfunction and doesn't need to same 
> anything on the stack). If the exception handler was writting the caller's address in the stack 
> frame, it would in fact write it in the parent's frame, leading to a mess.
>
> But in fact the information is in pt_regs, it is in regs->nip so KFENCE should be able to use that 
> instead of the stack.
>
>> 
>> What's confusing is that it's only this test, and none of the others.
>> Given that, it might be code-gen related, which results in some subtle
>> issue with stack unwinding. There are a few things to try, if you feel
>> like it:
>> 
>> -- Change the unwinder, if it's possible for ppc32.
>
> I don't think it is possible.

I think this actually is the solution.

It seems the good architectures have all added support for
arch_stack_walk(), and we have not.

Looking at some of the implementations of arch_stack_walk() it seems
it's expected that the first entry emitted includes the PC (or NIP on
ppc).

For us stack_trace_save() calls save_stack_trace() which only emits
entries from the stack, which doesn't necessarily include the function
NIP is pointing to.

So I think it's probably on us to update to that new API. Or at least
update our save_stack_trace() to fabricate an entry using the NIP, as it
seems that's what callers expect.

cheers