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

Christophe Leroy
2021-03-03T16:33:02Z

Le 03/03/2021 à 11:39, Marco Elver a écrit :
> On Wed, 3 Mar 2021 at 11:32, Christophe Leroy
> <christophe.leroy@csgroup.eu> wrote:
>>
>>
>>
>> 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
>
> No this is not expected. Is 'signed size_t' != 'long int' on ppc32?
>

No, it is an 'int' not a 'long int', see arch/powerpc/include/uapi/asm/posix_types.h

#ifdef __powerpc64__
typedef unsigned long __kernel_old_dev_t;
#define __kernel_old_dev_t __kernel_old_dev_t
#else
typedef unsigned int __kernel_size_t;
typedef int __kernel_ssize_t;
typedef long __kernel_ptrdiff_t;
#define __kernel_size_t __kernel_size_t

What is probably specific to powerpc is that ptrdiff_t is not same as ssize_t unlike in
include/uapi/asm-generic/posix_types.h :

/*
* Most 32 bit architectures use "unsigned int" size_t,
* and all 64 bit architectures use "unsigned long" size_t.
*/
#ifndef __kernel_size_t
#if __BITS_PER_LONG != 64
typedef unsigned int __kernel_size_t;
typedef int __kernel_ssize_t;
typedef int __kernel_ptrdiff_t;
#else
typedef __kernel_ulong_t __kernel_size_t;
typedef __kernel_long_t __kernel_ssize_t;
typedef __kernel_long_t __kernel_ptrdiff_t;
#endif
#endif

I have no warning on ppc64.

Christophe