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

Christophe Leroy
2021-03-03T15:46:13Z

Le 02/03/2021 à 10:53, Marco Elver a écrit :
> On Tue, 2 Mar 2021 at 10:27, Christophe Leroy
> <christophe.leroy@csgroup.eu> 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.
>
> 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.
>
> -- Add code to test_invalid_access(), to get the compiler to emit
> different code. E.g. add a bunch (unnecessary) function calls, or add
> barriers, etc.
>
> -- Play with compiler options. We already pass
> -fno-optimize-sibling-calls for kfence_test.o to avoid tail-call
> optimizations that'd hide stack trace entries. But perhaps there's
> something ppc-specific we missed?
>
> Well, the good thing is that KFENCE detects the bad access just fine.
> Since, according to the test, everything works from KFENCE's side, I'd
> be happy to give my Ack:
>
> Acked-by: Marco Elver <elver@google.com>
>

Thanks.

For you information, I've got a pile of warnings from mm/kfence/report.o . Is that expected ?

CC mm/kfence/report.o
In file included from ./include/linux/printk.h:7,
from ./include/linux/kernel.h:16,
from mm/kfence/report.c:10:
mm/kfence/report.c: In function 'kfence_report_error':
./include/linux/kern_levels.h:5:18: warning: format '%zd' expects argument of type 'signed size_t',
but argument 6 has type 'ptrdiff_t' {aka 'const long int'} [-Wformat=]
5 | #define KERN_SOH "\001" /* ASCII Start Of Header */
| ^~~~~~
./include/linux/kern_levels.h:11:18: note: in expansion of macro 'KERN_SOH'
11 | #define KERN_ERR KERN_SOH "3" /* error conditions */
| ^~~~~~~~
./include/linux/printk.h:343:9: note: in expansion of macro 'KERN_ERR'
343 | printk(KERN_ERR pr_fmt(fmt), ##__VA_ARGS__)
| ^~~~~~~~
mm/kfence/report.c:207:3: note: in expansion of macro 'pr_err'
207 | pr_err("Out-of-bounds %s at 0x%p (%luB %s of kfence-#%zd):\n",
| ^~~~~~
./include/linux/kern_levels.h:5:18: warning: format '%zd' expects argument of type 'signed size_t',
but argument 4 has type 'ptrdiff_t' {aka 'const long int'} [-Wformat=]
5 | #define KERN_SOH "\001" /* ASCII Start Of Header */
| ^~~~~~
./include/linux/kern_levels.h:11:18: note: in expansion of macro 'KERN_SOH'
11 | #define KERN_ERR KERN_SOH "3" /* error conditions */
| ^~~~~~~~
./include/linux/printk.h:343:9: note: in expansion of macro 'KERN_ERR'
343 | printk(KERN_ERR pr_fmt(fmt), ##__VA_ARGS__)
| ^~~~~~~~
mm/kfence/report.c:216:3: note: in expansion of macro 'pr_err'
216 | pr_err("Use-after-free %s at 0x%p (in kfence-#%zd):\n",
| ^~~~~~
./include/linux/kern_levels.h:5:18: warning: format '%zd' expects argument of type 'signed size_t',
but argument 2 has type 'ptrdiff_t' {aka 'const long int'} [-Wformat=]
5 | #define KERN_SOH "\001" /* ASCII Start Of Header */
| ^~~~~~
./include/linux/kern_levels.h:24:19: note: in expansion of macro 'KERN_SOH'
24 | #define KERN_CONT KERN_SOH "c"
| ^~~~~~~~
./include/linux/printk.h:385:9: note: in expansion of macro 'KERN_CONT'
385 | printk(KERN_CONT fmt, ##__VA_ARGS__)
| ^~~~~~~~~
mm/kfence/report.c:223:3: note: in expansion of macro 'pr_cont'
223 | pr_cont(" (in kfence-#%zd):\n", object_index);
| ^~~~~~~
./include/linux/kern_levels.h:5:18: warning: format '%zd' expects argument of type 'signed size_t',
but argument 3 has type 'ptrdiff_t' {aka 'const long int'} [-Wformat=]
5 | #define KERN_SOH "\001" /* ASCII Start Of Header */
| ^~~~~~
./include/linux/kern_levels.h:11:18: note: in expansion of macro 'KERN_SOH'
11 | #define KERN_ERR KERN_SOH "3" /* error conditions */
| ^~~~~~~~
./include/linux/printk.h:343:9: note: in expansion of macro 'KERN_ERR'
343 | printk(KERN_ERR pr_fmt(fmt), ##__VA_ARGS__)
| ^~~~~~~~
mm/kfence/report.c:233:3: note: in expansion of macro 'pr_err'
233 | pr_err("Invalid free of 0x%p (in kfence-#%zd):\n", (void *)address,
| ^~~~~~

Christophe