Re: [v4.9-rt PATCH] ARM: mm: remove tasklist locking from update_sections_early()

[Kees Cook <keescook@chromium.org>]

On Wed, Apr 19, 2017 at 12:58 PM, Grygorii Strashko
<grygorii.strashko@ti.com> wrote:
>
>
> On 04/18/2017 07:15 PM, Kees Cook wrote:
>> On Tue, Apr 18, 2017 at 1:48 PM, Grygorii Strashko
>> <grygorii.strashko@ti.com> wrote:
>>> The below backtrace can be observed on -rt kernel with CONFIG_DEBUG_RODATA
>>> option enabled:
>>>
>>>  BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:993
>>>  in_atomic(): 1, irqs_disabled(): 128, pid: 14, name: migration/0
>>>  1 lock held by migration/0/14:
>>>   #0:  (tasklist_lock){+.+...}, at: [<c01183e8>] update_sections_early+0x24/0xdc
>>>  irq event stamp: 38
>>>  hardirqs last  enabled at (37): [<c08f6f7c>] _raw_spin_unlock_irq+0x24/0x68
>>>  hardirqs last disabled at (38): [<c01fdfe8>] multi_cpu_stop+0xd8/0x138
>>>  softirqs last  enabled at (0): [<c01303ec>] copy_process.part.5+0x238/0x1b64
>>>  softirqs last disabled at (0): [<  (null)>]   (null)
>>>  Preemption disabled at: [<c01fe244>] cpu_stopper_thread+0x80/0x10c
>>>  CPU: 0 PID: 14 Comm: migration/0 Not tainted 4.9.21-rt16-02220-g49e319c #15
>>>  Hardware name: Generic DRA74X (Flattened Device Tree)
>>>  [<c0112014>] (unwind_backtrace) from [<c010d370>] (show_stack+0x10/0x14)
>>>  [<c010d370>] (show_stack) from [<c049beb8>] (dump_stack+0xa8/0xd4)
>>>  [<c049beb8>] (dump_stack) from [<c01631a0>] (___might_sleep+0x1bc/0x2ac)
>>>  [<c01631a0>] (___might_sleep) from [<c08f7244>] (__rt_spin_lock+0x1c/0x30)
>>>  [<c08f7244>] (__rt_spin_lock) from [<c08f77a4>] (rt_read_lock+0x54/0x68)
>>>  [<c08f77a4>] (rt_read_lock) from [<c01183e8>] (update_sections_early+0x24/0xdc)
>>>  [<c01183e8>] (update_sections_early) from [<c01184b0>] (__fix_kernmem_perms+0x10/0x1c)
>>>  [<c01184b0>] (__fix_kernmem_perms) from [<c01fe010>] (multi_cpu_stop+0x100/0x138)
>>>  [<c01fe010>] (multi_cpu_stop) from [<c01fe24c>] (cpu_stopper_thread+0x88/0x10c)
>>>  [<c01fe24c>] (cpu_stopper_thread) from [<c015edc4>] (smpboot_thread_fn+0x174/0x31c)
>>>  [<c015edc4>] (smpboot_thread_fn) from [<c015a988>] (kthread+0xf0/0x108)
>>>  [<c015a988>] (kthread) from [<c0108818>] (ret_from_fork+0x14/0x3c)
>>>  Freeing unused kernel memory: 1024K (c0d00000 - c0e00000)
>>>
>>> The stop_machine() is called with cpus = NULL from fix_kernmem_perms() and
>>> mark_rodata_ro() which means only one CPU will execute
>>> update_sections_early() while all other CPUs will spin and wait. Hence,
>>> it's safe to remove tasklist locking from update_sections_early(). As part of
>>> this change also mark functions which are local to this module as static
>>> to simplify code analize in the future.
>>
>> Hm, yes, good point. It's only every called while other CPUs are stopped.
>>
>>>
>>> Cc: Kees Cook <keescook@chromium.org>
>>> Cc: Laura Abbott <labbott@redhat.com>
>>> Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
>>> ---
>>>  arch/arm/mm/init.c | 8 +++-----
>>>  1 file changed, 3 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
>>> index 370581a..a77953a 100644
>>> --- a/arch/arm/mm/init.c
>>> +++ b/arch/arm/mm/init.c
>
> /**
>  * update_sections_early intended to be called only through stop_machine
>  * framework and be executed by only one CPU while all other CPUs will spin and
>  * wait, so no locking is required in this function.
>  */
>>> @@ -693,30 +693,28 @@ static void update_sections_early(struct section_perm perms[], int n)
>>
>> Maybe this should be renamed update_sections_stopped()? Or at least
>> comments added to help see why it's safe.
>
> would it be ok if I add above comment before update_sections_early?

A comment is fine. It'll just help people looking at this in the future. :)

Thanks!

-Kees

-- 
Kees Cook
Pixel Security