Re: [v4.9-rt PATCH v2] ARM: mm: remove tasklist locking from update_sections_early()

[Kees Cook <keescook@chromium.org>]

On Wed, Apr 19, 2017 at 5:36 PM, Laura Abbott <labbott@redhat.com> wrote:
> On 04/19/2017 01:10 PM, Grygorii Strashko 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
>
> Acked-by: Laura Abbott <labbott@redhat.com>

Acked-by: Kees Cook <keescook@chromium.org>

Please throw this at the ARM patch tracker (with our Acks).
http://www.arm.linux.org.uk/developer/patches/info.php

Thanks!

-Kees

>
>
>>
>> Cc: Kees Cook <keescook@chromium.org>
>> Cc: Laura Abbott <labbott@redhat.com>
>> Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
>> ---
>> As I've checked it also can be applied to LKML as is.
>>
>> Changes in v2:
>>   - added comment to update_sections_early()
>>
>> v1: https://patchwork.kernel.org/patch/9686289/
>>
>>   arch/arm/mm/init.c | 13 ++++++++-----
>>   1 file changed, 8 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
>> index 370581a..838f6b35 100644
>> --- a/arch/arm/mm/init.c
>> +++ b/arch/arm/mm/init.c
>> @@ -689,34 +689,37 @@ void set_section_perms(struct section_perm *perms,
>> int n, bool set,
>>     }
>>   +/**
>> + * update_sections_early intended to be called only through stop_machine
>> + * framework and executed by only one CPU while all other CPUs will spin
>> and
>> + * wait, so no locking is required in this function.
>> + */
>>   static void update_sections_early(struct section_perm perms[], int n)
>>   {
>>         struct task_struct *t, *s;
>>   -     read_lock(&tasklist_lock);
>>         for_each_process(t) {
>>                 if (t->flags & PF_KTHREAD)
>>                         continue;
>>                 for_each_thread(t, s)
>>                         set_section_perms(perms, n, true, s->mm);
>>         }
>> -       read_unlock(&tasklist_lock);
>>         set_section_perms(perms, n, true, current->active_mm);
>>         set_section_perms(perms, n, true, &init_mm);
>>   }
>>   -int __fix_kernmem_perms(void *unused)
>> +static int __fix_kernmem_perms(void *unused)
>>   {
>>         update_sections_early(nx_perms, ARRAY_SIZE(nx_perms));
>>         return 0;
>>   }
>>   -void fix_kernmem_perms(void)
>> +static void fix_kernmem_perms(void)
>>   {
>>         stop_machine(__fix_kernmem_perms, NULL, NULL);
>>   }
>>   -int __mark_rodata_ro(void *unused)
>> +static int __mark_rodata_ro(void *unused)
>>   {
>>         update_sections_early(ro_perms, ARRAY_SIZE(ro_perms));
>>         return 0;
>>
>



-- 
Kees Cook
Pixel Security