Re: [PATCH v2] tracepoint: Do not fail unregistering a probe due to memory allocation

Alexey Kardashevskiy
2021-01-27T07:15:13Z

On 18/11/2020 23:46, Steven Rostedt wrote:
> On Tue, 17 Nov 2020 20:54:24 -0800
> Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
>
>>> extern int
>>> @@ -310,7 +312,12 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>>> do { \
>>> it_func = (it_func_ptr)->func; \
>>> __data = (it_func_ptr)->data; \
>>> - ((void(*)(void *, proto))(it_func))(__data, args); \
>>> + /* \
>>> + * Removed functions that couldn't be allocated \
>>> + * are replaced with TRACEPOINT_STUB. \
>>> + */ \
>>> + if (likely(it_func != TRACEPOINT_STUB)) \
>>> + ((void(*)(void *, proto))(it_func))(__data, args); \
>>
>> I think you're overreacting to the problem.
>
> I will disagree with that.
>
>> Adding run-time check to extremely unlikely problem seems wasteful.
>
> Show me a real benchmark that you can notice a problem here. I bet that
> check is even within the noise of calling an indirect function. Especially
> on a machine with retpolines.
>
>> 99.9% of the time allocate_probes() will do kmalloc from slab of small
>> objects.
>> If that slab is out of memory it means it cannot allocate a single page.
>> In such case so many things will be failing to alloc that system
>> is unlikely operational. oom should have triggered long ago.
>> Imo Matt's approach to add __GFP_NOFAIL to allocate_probes()
>
> Looking at the GFP_NOFAIL comment:
>
> * %__GFP_NOFAIL: The VM implementation _must_ retry infinitely: the caller
> * cannot handle allocation failures. The allocation could block
> * indefinitely but will never return with failure. Testing for
> * failure is pointless.
> * New users should be evaluated carefully (and the flag should be
> * used only when there is no reasonable failure policy) but it is
> * definitely preferable to use the flag rather than opencode endless
> * loop around allocator.
>
> I realized I made a mistake in my patch for using it, as my patch is a
> failure policy. It looks like something we want to avoid in general.
>
> Thanks, I'll send a v3 that removes it.
>
>> when it's called from func_remove() is much better.
>> The error was reported by syzbot that was using
>> memory fault injections. ENOMEM in allocate_probes() was
>> never seen in real life and highly unlikely will ever be seen.
>
> And the biggest thing you are missing here, is that if you are running on a
> machine that has static calls, this code is never hit unless you have more
> than one callback on a single tracepoint. That's because when there's only
> one callback, it gets called directly, and this loop is not involved.

I am running syzkaller and the kernel keeps crashing in
__traceiter_##_name. This patch makes these crashes happen lot less
often (and so did the v1) but the kernel still crashes (examples below
but the common thing is that they crash in tracepoints). Disasm points
to __DO_TRACE_CALL(name) and this fixes it:

========================
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -313,6 +313,7 @@ static inline struct tracepoint
*tracepoint_ptr_deref(tracepoint_ptr_t *p)
\
it_func_ptr = \

rcu_dereference_raw((&__tracepoint_##_name)->funcs); \
+ if (it_func_ptr) \
do { \
it_func = (it_func_ptr)->func; \
========================

I am running on a powerpc box which does not have CONFIG_HAVE_STATIC_CALL.

I wonder if it is still the same problem which mentioned v3 might fix or
it is something different? Thanks,

[ 285.072538] Kernel attempted to read user page (0) - exploit attempt?
(uid: 0)
[ 285.073657] BUG: Kernel NULL pointer dereference on read at 0x00000000
[ 285.075129] Faulting instruction address: 0xc0000000002edf48
cpu 0xd: Vector: 300 (Data Access) at [c0000000115db530]
pc: c0000000002edf48: lock_acquire+0x2e8/0x5d0
lr: c0000000006ee450: step_into+0x940/0xc20
sp: c0000000115db7d0
msr: 8000000000009033
dar: 0
dsisr: 40000000
current = 0xc0000000115af280
paca = 0xc00000005ff9fe00 irqmask: 0x03 irq_happened: 0x01
pid = 182, comm = systemd-journal
Linux version 5.11.0-rc5-le_syzkaller_a+fstn1 (aik@fstn1-p1) (gcc
(Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0, GNU ld (GNU Binutils for Ubuntu)
2.30) #65 SMP Wed Jan 27 16:46:46 AEDT 2021
enter ? for help
[c0000000115db8c0] c0000000006ee450 step_into+0x940/0xc20
[c0000000115db950] c0000000006efddc walk_component+0xbc/0x340
[c0000000115db9d0] c0000000006f0418 link_path_walk.part.29+0x3b8/0x5b0
[c0000000115dbaa0] c0000000006f0b1c path_openat+0x11c/0x1190
[c0000000115dbb40] c0000000006f4334 do_filp_open+0xb4/0x180
[c0000000115dbc80] c0000000006c83cc do_sys_openat2+0x48c/0x610
[c0000000115dbd20] c0000000006caf9c do_sys_open+0xcc/0x140
[c0000000115dbdb0] c00000000004ba48 system_call_exception+0x178/0x2b0
[c0000000115dbe10] c00000000000e060 system_call_common+0xf0/0x27c
--- Exception: c00 (System Call) at 00007fff7fb3e758
SP (7ffff28e2900) is in userspace

[ 92.747130] FAT-fs (loop7): bogus number of reserved sectors
[ 92.747193] Kernel attempted to read user page (0) - exploit attempt?
(uid: 0)
[ 92.748393] FAT-fs (loop7): Can't find a valid FAT filesystem
[ 92.750579] BUG: Kernel NULL pointer dereference on read at 0x00000000
[ 92.751855] Faulting instruction address: 0xc0000000002ed928
cpu 0xd: Vector: 300 (Data Access) at [c00000001138f5c0]
pc: c0000000002ed928: lock_release+0x138/0x470
lr: c0000000002e0084: up_write+0x34/0x1e0
sp: c00000001138f860
msr: 8000000000009033
dar: 0
dsisr: 40000000
current = 0xc00000004fe7b480
paca = 0xc00000005ff9fe00 irqmask: 0x03 irq_happened: 0x01
pid = 10670, comm = syz-executor.3
Linux version 5.11.0-rc5-le_syzkaller_a+fstn1 (aik@fstn1-p1) (gcc
(Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0, GNU ld (GNU Binutils for Ubuntu)
2.30) #65 SMP Wed Jan 27 16:46:46 AEDT 2021
enter ? for help
[c00000001138f910] c0000000002e0084 up_write+0x34/0x1e0
[c00000001138f980] c0000000006151fc anon_vma_clone+0x1ec/0x370
[c00000001138f9f0] c00000000060180c __split_vma+0x11c/0x340
[c00000001138fa40] c000000000601cb0 __do_munmap+0x1c0/0x920
[c00000001138fad0] c000000000604d20 mmap_region+0x3b0/0xae0
[c00000001138fbd0] c0000000006059d4 do_mmap+0x584/0x830
[c00000001138fc60] c0000000005b0f90 vm_mmap_pgoff+0x170/0x260
[c00000001138fcf0] c000000000600818 ksys_mmap_pgoff+0x198/0x3a0
[c00000001138fd60] c0000000000155ec sys_mmap+0xcc/0x150
[c00000001138fdb0] c00000000004ba48 system_call_exception+0x178/0x2b0
[c00000001138fe10] c00000000000e060 system_call_common+0xf0/0x27c
--- Exception: c00 (System Call) at 0000000010058ad0
SP (7fffae45e1c0) is in userspace

--
Alexey