On 04/18/2017 07:15 PM, Kees Cook wrote:
> On Tue, Apr 18, 2017 at 1:48 PM, Grygorii Strashko
> <grygorii.strashko@xxxxxx> 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@xxxxxxxxxxxx>
>> Cc: Laura Abbott <labbott@xxxxxxxxxx>
>> Signed-off-by: Grygorii Strashko <grygorii.strashko@xxxxxx>
>> ---
>> 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?
Also I can rename it to update_sections_stopped() if you want - not sure about the name.
>
>> {
>> 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;
>
> Yeah, the static marks are all correct, thanks for fixing these!
>
> -Kees
>
--
regards,
-grygorii
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