Re: [External] Re: [PATCH v2] stackleak: Fix a race between stack erasing sysctl handlers

Re: [External] Re: [PATCH v2] stackleak: Fix a race between stack erasing sysctl handlers

[Alexander Popov]

On 07.09.2020 16:53, Muchun Song wrote:
> On Mon, Sep 7, 2020 at 7:24 PM Alexander Popov <alex.popov@linux.com> wrote:
>>
>> On 07.09.2020 05:54, Muchun Song wrote:
>>> Hi all,
>>>
>>> Any comments or suggestions? Thanks.
>>>
>>> On Fri, Aug 28, 2020 at 11:19 AM Muchun Song <songmuchun@bytedance.com> wrote:
>>>>
>>>> There is a race between the assignment of `table->data` and write value
>>>> to the pointer of `table->data` in the __do_proc_doulongvec_minmax() on
>>>> the other thread.
>>>>
>>>>     CPU0:                                 CPU1:
>>>>                                           proc_sys_write
>>>>     stack_erasing_sysctl                    proc_sys_call_handler
>>>>       table->data = &state;                   stack_erasing_sysctl
>>>>                                                 table->data = &state;
>>>>       proc_doulongvec_minmax
>>>>         do_proc_doulongvec_minmax             sysctl_head_finish
>>>>           __do_proc_doulongvec_minmax           unuse_table
>>>>             i = table->data;
>>>>             *i = val;  // corrupt CPU1's stack
>>
>> Hello everyone!
>>
>> As I remember, I implemented stack_erasing_sysctl() very similar to other sysctl
>> handlers. Is that issue relevant for other handlers as well?
> 
> Yeah, it's very similar. But the difference is that others use a
> global variable as the
> `table->data`, but here we use a local variable as the `table->data`.
> The local variable
> is allocated from the stack. So other thread could corrupt the stack
> like the diagram
> above.

Hi Muchun,

I don't think that the proposed copying of struct ctl_table to local variable is
a good fix of that issue. There might be other bugs caused by concurrent
execution of stack_erasing_sysctl().

I would recommend using some locking instead.

But you say there are other similar issues. Should it be fixed on higher level
in kernel/sysctl.c?

[Adding more knowing people to CC]

Thanks!

>> Muchun, could you elaborate how CPU1's stack is corrupted and how you detected
>> that? Thanks!
> 
> Why did I find this problem? Because I solve another problem which is
> very similar to
> this issue. You can reference the following fix patch. Thanks.
> 
>   https://lkml.org/lkml/2020/8/22/105
>>
>>>> Fix this by duplicating the `table`, and only update the duplicate of
>>>> it.
>>>>
>>>> Fixes: 964c9dff0091 ("stackleak: Allow runtime disabling of kernel stack erasing")
>>>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>>>> ---
>>>> changelogs in v2:
>>>>  1. Add more details about how the race happened to the commit message.
>>>>
>>>>  kernel/stackleak.c | 11 ++++++++---
>>>>  1 file changed, 8 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/kernel/stackleak.c b/kernel/stackleak.c
>>>> index a8fc9ae1d03d..fd95b87478ff 100644
>>>> --- a/kernel/stackleak.c
>>>> +++ b/kernel/stackleak.c
>>>> @@ -25,10 +25,15 @@ int stack_erasing_sysctl(struct ctl_table *table, int write,
>>>>         int ret = 0;
>>>>         int state = !static_branch_unlikely(&stack_erasing_bypass);
>>>>         int prev_state = state;
>>>> +       struct ctl_table dup_table = *table;
>>>>
>>>> -       table->data = &state;
>>>> -       table->maxlen = sizeof(int);
>>>> -       ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
>>>> +       /*
>>>> +        * In order to avoid races with __do_proc_doulongvec_minmax(), we
>>>> +        * can duplicate the @table and alter the duplicate of it.
>>>> +        */
>>>> +       dup_table.data = &state;
>>>> +       dup_table.maxlen = sizeof(int);
>>>> +       ret = proc_dointvec_minmax(&dup_table, write, buffer, lenp, ppos);
>>>>         state = !!state;
>>>>         if (ret || !write || state == prev_state)
>>>>                 return ret;
>>>> --
>>>> 2.11.0

Re: [External] Re: [PATCH v2] stackleak: Fix a race between stack erasing sysctl handlers

[Muchun Song]

On Mon, Sep 14, 2020 at 9:56 PM Alexander Popov <alex.popov@linux.com> wrote:
>
> On 07.09.2020 16:53, Muchun Song wrote:
> > On Mon, Sep 7, 2020 at 7:24 PM Alexander Popov <alex.popov@linux.com> wrote:
> >>
> >> On 07.09.2020 05:54, Muchun Song wrote:
> >>> Hi all,
> >>>
> >>> Any comments or suggestions? Thanks.
> >>>
> >>> On Fri, Aug 28, 2020 at 11:19 AM Muchun Song <songmuchun@bytedance.com> wrote:
> >>>>
> >>>> There is a race between the assignment of `table->data` and write value
> >>>> to the pointer of `table->data` in the __do_proc_doulongvec_minmax() on
> >>>> the other thread.
> >>>>
> >>>>     CPU0:                                 CPU1:
> >>>>                                           proc_sys_write
> >>>>     stack_erasing_sysctl                    proc_sys_call_handler
> >>>>       table->data = &state;                   stack_erasing_sysctl
> >>>>                                                 table->data = &state;
> >>>>       proc_doulongvec_minmax
> >>>>         do_proc_doulongvec_minmax             sysctl_head_finish
> >>>>           __do_proc_doulongvec_minmax           unuse_table
> >>>>             i = table->data;
> >>>>             *i = val;  // corrupt CPU1's stack
> >>
> >> Hello everyone!
> >>
> >> As I remember, I implemented stack_erasing_sysctl() very similar to other sysctl
> >> handlers. Is that issue relevant for other handlers as well?
> >
> > Yeah, it's very similar. But the difference is that others use a
> > global variable as the
> > `table->data`, but here we use a local variable as the `table->data`.
> > The local variable
> > is allocated from the stack. So other thread could corrupt the stack
> > like the diagram
> > above.
>
> Hi Muchun,
>
> I don't think that the proposed copying of struct ctl_table to local variable is
> a good fix of that issue. There might be other bugs caused by concurrent
> execution of stack_erasing_sysctl().

I can not figure out how the bug happened when there is concurrent
execution of stack_erasing_sysctl().

>
> I would recommend using some locking instead.
>
> But you say there are other similar issues. Should it be fixed on higher level
> in kernel/sysctl.c?

Yeah, we can see the same issue here.

    https://lkml.org/lkml/2020/8/22/105.

I agree with you. Maybe a fix on the higher level is a good choice in
kernel/sysctl.c. If someone also agrees with this solution, I can do
this work.

>
> [Adding more knowing people to CC]
>
> Thanks!
>
> >> Muchun, could you elaborate how CPU1's stack is corrupted and how you detected
> >> that? Thanks!
> >
> > Why did I find this problem? Because I solve another problem which is
> > very similar to
> > this issue. You can reference the following fix patch. Thanks.
> >
> >   https://lkml.org/lkml/2020/8/22/105
> >>
> >>>> Fix this by duplicating the `table`, and only update the duplicate of
> >>>> it.
> >>>>
> >>>> Fixes: 964c9dff0091 ("stackleak: Allow runtime disabling of kernel stack erasing")
> >>>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> >>>> ---
> >>>> changelogs in v2:
> >>>>  1. Add more details about how the race happened to the commit message.
> >>>>
> >>>>  kernel/stackleak.c | 11 ++++++++---
> >>>>  1 file changed, 8 insertions(+), 3 deletions(-)
> >>>>
> >>>> diff --git a/kernel/stackleak.c b/kernel/stackleak.c
> >>>> index a8fc9ae1d03d..fd95b87478ff 100644
> >>>> --- a/kernel/stackleak.c
> >>>> +++ b/kernel/stackleak.c
> >>>> @@ -25,10 +25,15 @@ int stack_erasing_sysctl(struct ctl_table *table, int write,
> >>>>         int ret = 0;
> >>>>         int state = !static_branch_unlikely(&stack_erasing_bypass);
> >>>>         int prev_state = state;
> >>>> +       struct ctl_table dup_table = *table;
> >>>>
> >>>> -       table->data = &state;
> >>>> -       table->maxlen = sizeof(int);
> >>>> -       ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
> >>>> +       /*
> >>>> +        * In order to avoid races with __do_proc_doulongvec_minmax(), we
> >>>> +        * can duplicate the @table and alter the duplicate of it.
> >>>> +        */
> >>>> +       dup_table.data = &state;
> >>>> +       dup_table.maxlen = sizeof(int);
> >>>> +       ret = proc_dointvec_minmax(&dup_table, write, buffer, lenp, ppos);
> >>>>         state = !!state;
> >>>>         if (ret || !write || state == prev_state)
> >>>>                 return ret;
> >>>> --
> >>>> 2.11.0



-- 
Yours,
Muchun

Re: [External] Re: [PATCH v2] stackleak: Fix a race between stack erasing sysctl handlers

[Muchun Song]

On Mon, Sep 14, 2020 at 9:56 PM Alexander Popov <alex.popov@linux.com> wrote:
>
> On 07.09.2020 16:53, Muchun Song wrote:
> > On Mon, Sep 7, 2020 at 7:24 PM Alexander Popov <alex.popov@linux.com> wrote:
> >>
> >> On 07.09.2020 05:54, Muchun Song wrote:
> >>> Hi all,
> >>>
> >>> Any comments or suggestions? Thanks.
> >>>
> >>> On Fri, Aug 28, 2020 at 11:19 AM Muchun Song <songmuchun@bytedance.com> wrote:
> >>>>
> >>>> There is a race between the assignment of `table->data` and write value
> >>>> to the pointer of `table->data` in the __do_proc_doulongvec_minmax() on
> >>>> the other thread.
> >>>>
> >>>>     CPU0:                                 CPU1:
> >>>>                                           proc_sys_write
> >>>>     stack_erasing_sysctl                    proc_sys_call_handler
> >>>>       table->data = &state;                   stack_erasing_sysctl
> >>>>                                                 table->data = &state;
> >>>>       proc_doulongvec_minmax
> >>>>         do_proc_doulongvec_minmax             sysctl_head_finish
> >>>>           __do_proc_doulongvec_minmax           unuse_table
> >>>>             i = table->data;
> >>>>             *i = val;  // corrupt CPU1's stack
> >>
> >> Hello everyone!
> >>
> >> As I remember, I implemented stack_erasing_sysctl() very similar to other sysctl
> >> handlers. Is that issue relevant for other handlers as well?
> >
> > Yeah, it's very similar. But the difference is that others use a
> > global variable as the
> > `table->data`, but here we use a local variable as the `table->data`.
> > The local variable
> > is allocated from the stack. So other thread could corrupt the stack
> > like the diagram
> > above.
>
> Hi Muchun,
>
> I don't think that the proposed copying of struct ctl_table to local variable is
> a good fix of that issue. There might be other bugs caused by concurrent
> execution of stack_erasing_sysctl().

Hi Alexander,

Yeah, we can fix this issue on a higher level in kernel/sysctl.c. But
we will rework some kernel/sysctl.c base code. Because the commit:

    964c9dff0091 ("stackleak: Allow runtime disabling of kernel stack erasing")

is introduced from linux-4.20. So we should backport this fix patch to the other
stable tree. Be the safe side, we can apply this patch to only fix the
stack_erasing_sysctl. In this case, the impact of backport is minimal.

In the feature, we can fix the issue(another patch) like this on a higher
level in kernel/sysctl.c and only apply it in the later kernel version. Is
this OK?

>
> I would recommend using some locking instead.
>
> But you say there are other similar issues. Should it be fixed on higher level
> in kernel/sysctl.c?
>
> [Adding more knowing people to CC]
>
> Thanks!
>
> >> Muchun, could you elaborate how CPU1's stack is corrupted and how you detected
> >> that? Thanks!
> >
> > Why did I find this problem? Because I solve another problem which is
> > very similar to
> > this issue. You can reference the following fix patch. Thanks.
> >
> >   https://lkml.org/lkml/2020/8/22/105
> >>
> >>>> Fix this by duplicating the `table`, and only update the duplicate of
> >>>> it.
> >>>>
> >>>> Fixes: 964c9dff0091 ("stackleak: Allow runtime disabling of kernel stack erasing")
> >>>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> >>>> ---
> >>>> changelogs in v2:
> >>>>  1. Add more details about how the race happened to the commit message.
> >>>>
> >>>>  kernel/stackleak.c | 11 ++++++++---
> >>>>  1 file changed, 8 insertions(+), 3 deletions(-)
> >>>>
> >>>> diff --git a/kernel/stackleak.c b/kernel/stackleak.c
> >>>> index a8fc9ae1d03d..fd95b87478ff 100644
> >>>> --- a/kernel/stackleak.c
> >>>> +++ b/kernel/stackleak.c
> >>>> @@ -25,10 +25,15 @@ int stack_erasing_sysctl(struct ctl_table *table, int write,
> >>>>         int ret = 0;
> >>>>         int state = !static_branch_unlikely(&stack_erasing_bypass);
> >>>>         int prev_state = state;
> >>>> +       struct ctl_table dup_table = *table;
> >>>>
> >>>> -       table->data = &state;
> >>>> -       table->maxlen = sizeof(int);
> >>>> -       ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
> >>>> +       /*
> >>>> +        * In order to avoid races with __do_proc_doulongvec_minmax(), we
> >>>> +        * can duplicate the @table and alter the duplicate of it.
> >>>> +        */
> >>>> +       dup_table.data = &state;
> >>>> +       dup_table.maxlen = sizeof(int);
> >>>> +       ret = proc_dointvec_minmax(&dup_table, write, buffer, lenp, ppos);
> >>>>         state = !!state;
> >>>>         if (ret || !write || state == prev_state)
> >>>>                 return ret;
> >>>> --
> >>>> 2.11.0



-- 
Yours,
Muchun

Re: [External] Re: [PATCH v2] stackleak: Fix a race between stack erasing sysctl handlers

[Alexander Popov]

On 22.09.2020 08:59, Muchun Song wrote:
> On Mon, Sep 14, 2020 at 9:56 PM Alexander Popov <alex.popov@linux.com> wrote:
>>
>> On 07.09.2020 16:53, Muchun Song wrote:
>>> On Mon, Sep 7, 2020 at 7:24 PM Alexander Popov <alex.popov@linux.com> wrote:
>>>>
>>>> On 07.09.2020 05:54, Muchun Song wrote:
>>>>> Hi all,
>>>>>
>>>>> Any comments or suggestions? Thanks.
>>>>>
>>>>> On Fri, Aug 28, 2020 at 11:19 AM Muchun Song <songmuchun@bytedance.com> wrote:
>>>>>>
>>>>>> There is a race between the assignment of `table->data` and write value
>>>>>> to the pointer of `table->data` in the __do_proc_doulongvec_minmax() on
>>>>>> the other thread.
>>>>>>
>>>>>>     CPU0:                                 CPU1:
>>>>>>                                           proc_sys_write
>>>>>>     stack_erasing_sysctl                    proc_sys_call_handler
>>>>>>       table->data = &state;                   stack_erasing_sysctl
>>>>>>                                                 table->data = &state;
>>>>>>       proc_doulongvec_minmax
>>>>>>         do_proc_doulongvec_minmax             sysctl_head_finish
>>>>>>           __do_proc_doulongvec_minmax           unuse_table
>>>>>>             i = table->data;
>>>>>>             *i = val;  // corrupt CPU1's stack
>>>>
>>>> Hello everyone!
>>>>
>>>> As I remember, I implemented stack_erasing_sysctl() very similar to other sysctl
>>>> handlers. Is that issue relevant for other handlers as well?
>>>
>>> Yeah, it's very similar. But the difference is that others use a
>>> global variable as the
>>> `table->data`, but here we use a local variable as the `table->data`.
>>> The local variable
>>> is allocated from the stack. So other thread could corrupt the stack
>>> like the diagram
>>> above.
>>
>> Hi Muchun,
>>
>> I don't think that the proposed copying of struct ctl_table to local variable is
>> a good fix of that issue. There might be other bugs caused by concurrent
>> execution of stack_erasing_sysctl().
> 
> Hi Alexander,
> 
> Yeah, we can fix this issue on a higher level in kernel/sysctl.c. But
> we will rework some kernel/sysctl.c base code. Because the commit:
> 
>     964c9dff0091 ("stackleak: Allow runtime disabling of kernel stack erasing")
> 
> is introduced from linux-4.20. So we should backport this fix patch to the other
> stable tree. Be the safe side, we can apply this patch to only fix the
> stack_erasing_sysctl. In this case, the impact of backport is minimal.
> 
> In the feature, we can fix the issue(another patch) like this on a higher
> level in kernel/sysctl.c and only apply it in the later kernel version. Is
> this OK?

Muchun, I would recommend:
  1) fixing the reason of the issue in kernel/sysctl.c
or
  2) use some locking in stack_erasing_sysctl() to fix the issue locally.

Honestly, I don't like this "dup_table" approach in the patch below. It doesn't
remove the data race.

Thank you!
Alexander

>> I would recommend using some locking instead.
>>
>> But you say there are other similar issues. Should it be fixed on higher level
>> in kernel/sysctl.c?
>>
>> [Adding more knowing people to CC]
>>
>> Thanks!
>>
>>>> Muchun, could you elaborate how CPU1's stack is corrupted and how you detected
>>>> that? Thanks!
>>>
>>> Why did I find this problem? Because I solve another problem which is
>>> very similar to
>>> this issue. You can reference the following fix patch. Thanks.
>>>
>>>   https://lkml.org/lkml/2020/8/22/105
>>>>
>>>>>> Fix this by duplicating the `table`, and only update the duplicate of
>>>>>> it.
>>>>>>
>>>>>> Fixes: 964c9dff0091 ("stackleak: Allow runtime disabling of kernel stack erasing")
>>>>>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>>>>>> ---
>>>>>> changelogs in v2:
>>>>>>  1. Add more details about how the race happened to the commit message.
>>>>>>
>>>>>>  kernel/stackleak.c | 11 ++++++++---
>>>>>>  1 file changed, 8 insertions(+), 3 deletions(-)
>>>>>>
>>>>>> diff --git a/kernel/stackleak.c b/kernel/stackleak.c
>>>>>> index a8fc9ae1d03d..fd95b87478ff 100644
>>>>>> --- a/kernel/stackleak.c
>>>>>> +++ b/kernel/stackleak.c
>>>>>> @@ -25,10 +25,15 @@ int stack_erasing_sysctl(struct ctl_table *table, int write,
>>>>>>         int ret = 0;
>>>>>>         int state = !static_branch_unlikely(&stack_erasing_bypass);
>>>>>>         int prev_state = state;
>>>>>> +       struct ctl_table dup_table = *table;
>>>>>>
>>>>>> -       table->data = &state;
>>>>>> -       table->maxlen = sizeof(int);
>>>>>> -       ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
>>>>>> +       /*
>>>>>> +        * In order to avoid races with __do_proc_doulongvec_minmax(), we
>>>>>> +        * can duplicate the @table and alter the duplicate of it.
>>>>>> +        */
>>>>>> +       dup_table.data = &state;
>>>>>> +       dup_table.maxlen = sizeof(int);
>>>>>> +       ret = proc_dointvec_minmax(&dup_table, write, buffer, lenp, ppos);
>>>>>>         state = !!state;
>>>>>>         if (ret || !write || state == prev_state)
>>>>>>                 return ret;
>>>>>> --
>>>>>> 2.11.0
> 
> 
> 

Re: [External] Re: [PATCH v2] stackleak: Fix a race between stack erasing sysctl handlers

[Muchun Song]

On Mon, Sep 28, 2020 at 2:32 PM Alexander Popov <alex.popov@linux.com> wrote:
>
> On 22.09.2020 08:59, Muchun Song wrote:
> > On Mon, Sep 14, 2020 at 9:56 PM Alexander Popov <alex.popov@linux.com> wrote:
> >>
> >> On 07.09.2020 16:53, Muchun Song wrote:
> >>> On Mon, Sep 7, 2020 at 7:24 PM Alexander Popov <alex.popov@linux.com> wrote:
> >>>>
> >>>> On 07.09.2020 05:54, Muchun Song wrote:
> >>>>> Hi all,
> >>>>>
> >>>>> Any comments or suggestions? Thanks.
> >>>>>
> >>>>> On Fri, Aug 28, 2020 at 11:19 AM Muchun Song <songmuchun@bytedance.com> wrote:
> >>>>>>
> >>>>>> There is a race between the assignment of `table->data` and write value
> >>>>>> to the pointer of `table->data` in the __do_proc_doulongvec_minmax() on
> >>>>>> the other thread.
> >>>>>>
> >>>>>>     CPU0:                                 CPU1:
> >>>>>>                                           proc_sys_write
> >>>>>>     stack_erasing_sysctl                    proc_sys_call_handler
> >>>>>>       table->data = &state;                   stack_erasing_sysctl
> >>>>>>                                                 table->data = &state;
> >>>>>>       proc_doulongvec_minmax
> >>>>>>         do_proc_doulongvec_minmax             sysctl_head_finish
> >>>>>>           __do_proc_doulongvec_minmax           unuse_table
> >>>>>>             i = table->data;
> >>>>>>             *i = val;  // corrupt CPU1's stack
> >>>>
> >>>> Hello everyone!
> >>>>
> >>>> As I remember, I implemented stack_erasing_sysctl() very similar to other sysctl
> >>>> handlers. Is that issue relevant for other handlers as well?
> >>>
> >>> Yeah, it's very similar. But the difference is that others use a
> >>> global variable as the
> >>> `table->data`, but here we use a local variable as the `table->data`.
> >>> The local variable
> >>> is allocated from the stack. So other thread could corrupt the stack
> >>> like the diagram
> >>> above.
> >>
> >> Hi Muchun,
> >>
> >> I don't think that the proposed copying of struct ctl_table to local variable is
> >> a good fix of that issue. There might be other bugs caused by concurrent
> >> execution of stack_erasing_sysctl().
> >
> > Hi Alexander,
> >
> > Yeah, we can fix this issue on a higher level in kernel/sysctl.c. But
> > we will rework some kernel/sysctl.c base code. Because the commit:
> >
> >     964c9dff0091 ("stackleak: Allow runtime disabling of kernel stack erasing")
> >
> > is introduced from linux-4.20. So we should backport this fix patch to the other
> > stable tree. Be the safe side, we can apply this patch to only fix the
> > stack_erasing_sysctl. In this case, the impact of backport is minimal.
> >
> > In the feature, we can fix the issue(another patch) like this on a higher
> > level in kernel/sysctl.c and only apply it in the later kernel version. Is
> > this OK?
>
> Muchun, I would recommend:
>   1) fixing the reason of the issue in kernel/sysctl.c
> or
>   2) use some locking in stack_erasing_sysctl() to fix the issue locally.

Yeah, this is work.

>
> Honestly, I don't like this "dup_table" approach in the patch below. It doesn't
> remove the data race.

Alexander, I don't understand where the race is? I think that the duplicate is
enough. But If you prefer using the lock to protect the data. I also
can do that.

>
> Thank you!
> Alexander
>
> >> I would recommend using some locking instead.
> >>
> >> But you say there are other similar issues. Should it be fixed on higher level
> >> in kernel/sysctl.c?
> >>
> >> [Adding more knowing people to CC]
> >>
> >> Thanks!
> >>
> >>>> Muchun, could you elaborate how CPU1's stack is corrupted and how you detected
> >>>> that? Thanks!
> >>>
> >>> Why did I find this problem? Because I solve another problem which is
> >>> very similar to
> >>> this issue. You can reference the following fix patch. Thanks.
> >>>
> >>>   https://lkml.org/lkml/2020/8/22/105
> >>>>
> >>>>>> Fix this by duplicating the `table`, and only update the duplicate of
> >>>>>> it.
> >>>>>>
> >>>>>> Fixes: 964c9dff0091 ("stackleak: Allow runtime disabling of kernel stack erasing")
> >>>>>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> >>>>>> ---
> >>>>>> changelogs in v2:
> >>>>>>  1. Add more details about how the race happened to the commit message.
> >>>>>>
> >>>>>>  kernel/stackleak.c | 11 ++++++++---
> >>>>>>  1 file changed, 8 insertions(+), 3 deletions(-)
> >>>>>>
> >>>>>> diff --git a/kernel/stackleak.c b/kernel/stackleak.c
> >>>>>> index a8fc9ae1d03d..fd95b87478ff 100644
> >>>>>> --- a/kernel/stackleak.c
> >>>>>> +++ b/kernel/stackleak.c
> >>>>>> @@ -25,10 +25,15 @@ int stack_erasing_sysctl(struct ctl_table *table, int write,
> >>>>>>         int ret = 0;
> >>>>>>         int state = !static_branch_unlikely(&stack_erasing_bypass);
> >>>>>>         int prev_state = state;
> >>>>>> +       struct ctl_table dup_table = *table;
> >>>>>>
> >>>>>> -       table->data = &state;
> >>>>>> -       table->maxlen = sizeof(int);
> >>>>>> -       ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
> >>>>>> +       /*
> >>>>>> +        * In order to avoid races with __do_proc_doulongvec_minmax(), we
> >>>>>> +        * can duplicate the @table and alter the duplicate of it.
> >>>>>> +        */
> >>>>>> +       dup_table.data = &state;
> >>>>>> +       dup_table.maxlen = sizeof(int);
> >>>>>> +       ret = proc_dointvec_minmax(&dup_table, write, buffer, lenp, ppos);
> >>>>>>         state = !!state;
> >>>>>>         if (ret || !write || state == prev_state)
> >>>>>>                 return ret;
> >>>>>> --
> >>>>>> 2.11.0
> >
> >
> >
>


-- 
Yours,
Muchun