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Date:   Wed, 26 Aug 2020 10:47:08 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, ak@...ux.intel.com,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [External] Re: [PATCH] mm/hugetlb: Fix a race between hugetlb
 sysctl handlers

On Wed, Aug 26, 2020 at 8:03 AM Mike Kravetz <mike.kravetz@...cle.com> wrote:
>
> On 8/24/20 8:01 PM, Muchun Song wrote:
> > On Tue, Aug 25, 2020 at 5:21 AM Mike Kravetz <mike.kravetz@...cle.com> wrote:
> >>
> >> I too am looking at this now and do not completely understand the race.
> >> It could be that:
> >>
> >> hugetlb_sysctl_handler_common
> >> ...
> >>         table->data = &tmp;
> >>
> >> and, do_proc_doulongvec_minmax()
> >> ...
> >>         return __do_proc_doulongvec_minmax(table->data, table, write, ...
> >> with __do_proc_doulongvec_minmax(void *data, struct ctl_table *table, ...
> >> ...
> >>         i = (unsigned long *) data;
> >>         ...
> >>                 *i = val;
> >>
> >> So, __do_proc_doulongvec_minmax can be dereferencing and writing to the pointer
> >> in one thread when hugetlb_sysctl_handler_common is setting it in another?
> >
> > Yes, you are right.
> >
> >>
> >> Another confusing part of the message is the stack trace which includes
> >> ...
> >>      ? set_max_huge_pages+0x3da/0x4f0
> >>      ? alloc_pool_huge_page+0x150/0x150
> >>
> >> which are 'downstream' from these routines.  I don't understand why these
> >> are in the trace.
> >
> > I am also confused. But this issue can be reproduced easily by letting more
> > than one thread write to `/proc/sys/vm/nr_hugepages`. With this patch applied,
> > the issue can not be reproduced and disappears.
>
> There certainly is an issue here as one thread can modify data in another.
> However, I am having a hard time seeing what causes the 'kernel NULL pointer
> dereference'.

If you write 0 to '/proc/sys/vm/nr_hugepages', you will get the
    kernel NULL pointer dereference, address: 0000000000000000

If you write 1024 to '/proc/sys/vm/nr_hugepages', you will get the
    kernel NULL pointer dereference, address: 0000000000000400

The address of dereference is the value which you write to the
'/proc/sys/vm/nr_hugepages'.

>
> I tried to reproduce the issue myself but was unsuccessful.  I have 16 threads
> writing to /proc/sys/vm/nr_hugepages in an infinite loop.  After several hours
> running, I did not hit the issue.  Just curious, what architecture is the
> system?  any special config or compiler options?
>
> If you can easily reproduce, can you post the detailed oops message?
>
> The 'NULL pointer' seems strange because after the first assignment to
> table->data the value should never be NULL.  Certainly it can be modified
> by another thread, but I can not see how it can be NULL.  At the beginning
> of __do_proc_doulongvec_minmax, there is a check for NULL pointer with:

CPU0:                                     CPU1:
                                          proc_sys_write
hugetlb_sysctl_handler                      proc_sys_call_handler
hugetlb_sysctl_handler_common                 hugetlb_sysctl_handler
  table->data = &tmp;                           hugetlb_sysctl_handler_common
                                                  table->data = &tmp;
    proc_doulongvec_minmax
      do_proc_doulongvec_minmax             sysctl_head_finish
        __do_proc_doulongvec_minmax
          i = table->data;
          *i = val;     // corrupt CPU1 stack

If the val is 0, you will see the NULL.

>
>         if (!data || !table->maxlen || !*lenp || (*ppos && !write)) {
>                 *lenp = 0;
>                 return 0;
>         }
>
> I looked at the code my compiler produced for __do_proc_doulongvec_minmax.
> It appears to use the same value/register for the pointer throughout the
> routine.  IOW, I do not see how the pointer can be NULL for the assignment
> when the routine does:
>
>                         *i = val;
>
> Again, your analysis/patch points out a real issue.  I just want to get
> a better understanding to make sure there is not another issue causing
> the NULL pointer dereference.

Below is my test script. There are 8 threads to execute the following script.
In my qemu, it is easy to panic. Thanks.

#!/bin/sh

while :
do
        echo 128 > /proc/sys/vm/nr_hugepages
        echo 0 > /proc/sys/vm/nr_hugepages
done

> --
> Mike Kravetz



-- 
Yours,
Muchun

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