Race in mm/ksm.c

Race in mm/ksm.c

[Abhishek Shah]

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Dear Kernel Maintainers,

We found a race in mm/ksm.c. During the execution of the function
*__ksm_run* which uses variable *ksm_run* to decide the list insertion
point, the variable *ksm_run* can be concurrently modified in the function
*run_store*, which we thought could be undesirable since “KSM pages in
newly forked mms can be missed” (See comment here:
https://elixir.bootlin.com/linux/v5.18-rc5/source/mm/ksm.c#L2498). We would
also like your thoughts on the security impact given it is a TOCTOU bug.

We provide more details below including the trace and reproducing
test cases.


*Trace*
BUG: KCSAN: data-race in __ksm_enter / run_store
write to 0xffffffff881edae0 of 8 bytes by task 6542 on cpu 0:
 run_store+0x19a/0x2d0 mm/ksm.c:2897
 kobj_attr_store+0x44/0x60 lib/kobject.c:824
 sysfs_kf_write+0x16f/0x1a0 fs/sysfs/file.c:136
 kernfs_fop_write_iter+0x2ae/0x370 fs/kernfs/file.c:291
 call_write_iter include/linux/fs.h:2050 [inline]
 new_sync_write fs/read_write.c:504 [inline]
 vfs_write+0x779/0x900 fs/read_write.c:591
 ksys_write+0xde/0x190 fs/read_write.c:644
 __do_sys_write fs/read_write.c:656 [inline]
 __se_sys_write fs/read_write.c:653 [inline]
 __x64_sys_write+0x43/0x50 fs/read_write.c:653
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffffffff881edae0 of 8 bytes by task 6541 on cpu 1:
 __ksm_enter+0x114/0x260 mm/ksm.c:2501
 ksm_madvise+0x291/0x350 mm/ksm.c:2451
 madvise_vma_behavior mm/madvise.c:1039 [inline]
 madvise_walk_vmas mm/madvise.c:1221 [inline]
 do_madvise+0x656/0xeb0 mm/madvise.c:1399
 __do_sys_madvise mm/madvise.c:1412 [inline]
 __se_sys_madvise mm/madvise.c:1410 [inline]
 __x64_sys_madvise+0x64/0x70 mm/madvise.c:1410
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 6541 Comm: syz-executor2-n Not tainted 5.18.0-rc5+ #107
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1
04/01/2014
---------------------
*Inputs *
Input CPU 0:
r0 = openat$sysctl(0xffffff9c,
&(0x7f0000000100)='/sys/kernel/mm/ksm/run\x00', 0x1, 0x0)
write$sysctl(r0, &(0x7f0000000000)='2\x00', 0x2)

Input CPU 1:
madvise(&(0x7f0000ffc000/0x4000)=nil, 0x4000, 0xc)
mlock2(&(0x7f0000ffe000/0x2000)=nil, 0x2000, 0x0)
madvise(&(0x7f0000ffd000/0x3000)=nil, 0x3000, 0x12)
clone(0x0, 0x0, 0x0, 0x0, 0x0)

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Re: Race in mm/ksm.c

[Kefeng Wang]

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On 2022/7/21 23:58, Abhishek Shah wrote:
> Dear Kernel Maintainers,
>
> We found a race in mm/ksm.c. During the execution of the function 
> /__ksm_run/ which uses variable /ksm_run/ to decide the list insertion 
> point, the variable /ksm_run/ can be concurrently modified in the 
> function /run_store/, which we thought could be undesirable since “KSM 
> pages in newly forked mms can be missed” (See comment here: 
> https://elixir.bootlin.com/linux/v5.18-rc5/source/mm/ksm.c#L2498 
> <https://elixir.bootlin.com/linux/v5.18-rc5/source/mm/ksm.c#L2498>). 
> We would also like your thoughts on the security impact given it is a 
> TOCTOU bug.
>
> We provide more details below including the trace and reproducing 
> test cases.


Hello， could the following changes to avoid the data-race issue？

diff --git a/mm/ksm.c b/mm/ksm.c
index 54f78c9eecae..f072753cbb3a 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -2497,6 +2497,7 @@ int __ksm_enter(struct mm_struct *mm)
  {
         struct mm_slot *mm_slot;
         int needs_wakeup;
+       bool ksm_run_merge;

         mm_slot = alloc_mm_slot();
         if (!mm_slot)
@@ -2505,6 +2506,10 @@ int __ksm_enter(struct mm_struct *mm)
         /* Check ksm_run too?  Would need tighter locking */
         needs_wakeup = list_empty(&ksm_mm_head.mm_list);

+       mutex_lock(&ksm_thread_mutex);
+       ksm_run_unmerge = !!(ksm_run & KSM_RUN_UNMERGE);
+       mutex_unlock(&ksm_thread_mutex);
+
         spin_lock(&ksm_mmlist_lock);
         insert_to_mm_slots_hash(mm, mm_slot);
         /*
@@ -2517,7 +2522,7 @@ int __ksm_enter(struct mm_struct *mm)
          * scanning cursor, otherwise KSM pages in newly forked mms will be
          * missed: then we might as well insert at the end of the list.
          */
-       if (ksm_run & KSM_RUN_UNMERGE)
+       if (ksm_run_unmerge)
                 list_add_tail(&mm_slot->mm_list, &ksm_mm_head.mm_list);
         else
                 list_add_tail(&mm_slot->mm_list, 
&ksm_scan.mm_slot->mm_list);


>
>
> *Trace*
> BUG: KCSAN: data-race in __ksm_enter / run_store
> write to 0xffffffff881edae0 of 8 bytes by task 6542 on cpu 0:
>  run_store+0x19a/0x2d0 mm/ksm.c:2897
>  kobj_attr_store+0x44/0x60 lib/kobject.c:824
>  sysfs_kf_write+0x16f/0x1a0 fs/sysfs/file.c:136
>  kernfs_fop_write_iter+0x2ae/0x370 fs/kernfs/file.c:291
>  call_write_iter include/linux/fs.h:2050 [inline]
>  new_sync_write fs/read_write.c:504 [inline]
>  vfs_write+0x779/0x900 fs/read_write.c:591
>  ksys_write+0xde/0x190 fs/read_write.c:644
>  __do_sys_write fs/read_write.c:656 [inline]
>  __se_sys_write fs/read_write.c:653 [inline]
>  __x64_sys_write+0x43/0x50 fs/read_write.c:653
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
>
> read to 0xffffffff881edae0 of 8 bytes by task 6541 on cpu 1:
>  __ksm_enter+0x114/0x260 mm/ksm.c:2501
>  ksm_madvise+0x291/0x350 mm/ksm.c:2451
>  madvise_vma_behavior mm/madvise.c:1039 [inline]
>  madvise_walk_vmas mm/madvise.c:1221 [inline]
>  do_madvise+0x656/0xeb0 mm/madvise.c:1399
>  __do_sys_madvise mm/madvise.c:1412 [inline]
>  __se_sys_madvise mm/madvise.c:1410 [inline]
>  __x64_sys_madvise+0x64/0x70 mm/madvise.c:1410
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
>
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 1 PID: 6541 Comm: syz-executor2-n Not tainted 5.18.0-rc5+ #107
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 
> 04/01/2014
> ---------------------
> *Inputs *
> Input CPU 0:
> r0 = openat$sysctl(0xffffff9c, 
> &(0x7f0000000100)='/sys/kernel/mm/ksm/run\x00', 0x1, 0x0)
> write$sysctl(r0, &(0x7f0000000000)='2\x00', 0x2)
>
> Input CPU 1:
> madvise(&(0x7f0000ffc000/0x4000)=nil, 0x4000, 0xc)
> mlock2(&(0x7f0000ffe000/0x2000)=nil, 0x2000, 0x0)
> madvise(&(0x7f0000ffd000/0x3000)=nil, 0x3000, 0x12)
> clone(0x0, 0x0, 0x0, 0x0, 0x0)

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Re: Race in mm/ksm.c

[Gabriel Ryan]

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Confirmed.

Thanks for the quick response and apologies for the delay!

Best,

Gabe

On Thu, Jul 21, 2022 at 9:57 PM Kefeng Wang <wangkefeng.wang@huawei.com>
wrote:

>
> On 2022/7/21 23:58, Abhishek Shah wrote:
>
> Dear Kernel Maintainers,
>
> We found a race in mm/ksm.c. During the execution of the function
> *__ksm_run* which uses variable *ksm_run* to decide the list insertion
> point, the variable *ksm_run* can be concurrently modified in the
> function *run_store*, which we thought could be undesirable since “KSM
> pages in newly forked mms can be missed” (See comment here:
> https://elixir.bootlin.com/linux/v5.18-rc5/source/mm/ksm.c#L2498). We
> would also like your thoughts on the security impact given it is a TOCTOU
> bug.
>
> We provide more details below including the trace and reproducing
> test cases.
>
>
> Hello， could the following changes to avoid the data-race issue？
>
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 54f78c9eecae..f072753cbb3a 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -2497,6 +2497,7 @@ int __ksm_enter(struct mm_struct *mm)
>  {
>         struct mm_slot *mm_slot;
>         int needs_wakeup;
> +       bool ksm_run_merge;
>
>         mm_slot = alloc_mm_slot();
>         if (!mm_slot)
> @@ -2505,6 +2506,10 @@ int __ksm_enter(struct mm_struct *mm)
>         /* Check ksm_run too?  Would need tighter locking */
>         needs_wakeup = list_empty(&ksm_mm_head.mm_list);
>
> +       mutex_lock(&ksm_thread_mutex);
> +       ksm_run_unmerge = !!(ksm_run & KSM_RUN_UNMERGE);
> +       mutex_unlock(&ksm_thread_mutex);
> +
>         spin_lock(&ksm_mmlist_lock);
>         insert_to_mm_slots_hash(mm, mm_slot);
>         /*
> @@ -2517,7 +2522,7 @@ int __ksm_enter(struct mm_struct *mm)
>          * scanning cursor, otherwise KSM pages in newly forked mms will be
>          * missed: then we might as well insert at the end of the list.
>          */
> -       if (ksm_run & KSM_RUN_UNMERGE)
> +       if (ksm_run_unmerge)
>                 list_add_tail(&mm_slot->mm_list, &ksm_mm_head.mm_list);
>         else
>                 list_add_tail(&mm_slot->mm_list,
> &ksm_scan.mm_slot->mm_list);
>
>
>
>
> *Trace*
> BUG: KCSAN: data-race in __ksm_enter / run_store
> write to 0xffffffff881edae0 of 8 bytes by task 6542 on cpu 0:
>  run_store+0x19a/0x2d0 mm/ksm.c:2897
>  kobj_attr_store+0x44/0x60 lib/kobject.c:824
>  sysfs_kf_write+0x16f/0x1a0 fs/sysfs/file.c:136
>  kernfs_fop_write_iter+0x2ae/0x370 fs/kernfs/file.c:291
>  call_write_iter include/linux/fs.h:2050 [inline]
>  new_sync_write fs/read_write.c:504 [inline]
>  vfs_write+0x779/0x900 fs/read_write.c:591
>  ksys_write+0xde/0x190 fs/read_write.c:644
>  __do_sys_write fs/read_write.c:656 [inline]
>  __se_sys_write fs/read_write.c:653 [inline]
>  __x64_sys_write+0x43/0x50 fs/read_write.c:653
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
>
> read to 0xffffffff881edae0 of 8 bytes by task 6541 on cpu 1:
>  __ksm_enter+0x114/0x260 mm/ksm.c:2501
>  ksm_madvise+0x291/0x350 mm/ksm.c:2451
>  madvise_vma_behavior mm/madvise.c:1039 [inline]
>  madvise_walk_vmas mm/madvise.c:1221 [inline]
>  do_madvise+0x656/0xeb0 mm/madvise.c:1399
>  __do_sys_madvise mm/madvise.c:1412 [inline]
>  __se_sys_madvise mm/madvise.c:1410 [inline]
>  __x64_sys_madvise+0x64/0x70 mm/madvise.c:1410
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
>
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 1 PID: 6541 Comm: syz-executor2-n Not tainted 5.18.0-rc5+ #107
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1
> 04/01/2014
> ---------------------
> *Inputs *
> Input CPU 0:
> r0 = openat$sysctl(0xffffff9c,
> &(0x7f0000000100)='/sys/kernel/mm/ksm/run\x00', 0x1, 0x0)
> write$sysctl(r0, &(0x7f0000000000)='2\x00', 0x2)
>
> Input CPU 1:
> madvise(&(0x7f0000ffc000/0x4000)=nil, 0x4000, 0xc)
> mlock2(&(0x7f0000ffe000/0x2000)=nil, 0x2000, 0x0)
> madvise(&(0x7f0000ffd000/0x3000)=nil, 0x3000, 0x12)
> clone(0x0, 0x0, 0x0, 0x0, 0x0)
>
>

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Re: Race in mm/ksm.c

[Kefeng Wang]

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On 2022/8/2 19:44, Gabriel Ryan wrote:
> Confirmed.

It means that the following changes could fix the issue, right?

If so, I will send a formal patch with your report and test, thanks.

>
> Thanks for the quick response and apologies for the delay!
>
> Best,
>
> Gabe
>
> On Thu, Jul 21, 2022 at 9:57 PM Kefeng Wang 
> <wangkefeng.wang@huawei.com> wrote:
>
>
>     On 2022/7/21 23:58, Abhishek Shah wrote:
>>     Dear Kernel Maintainers,
>>
>>     We found a race in mm/ksm.c. During the execution of the function
>>     /__ksm_run/ which uses variable /ksm_run/ to decide the list
>>     insertion point, the variable /ksm_run/ can be concurrently
>>     modified in the function /run_store/, which we thought could be
>>     undesirable since “KSM pages in newly forked mms can be missed”
>>     (See comment here:
>>     https://elixir.bootlin.com/linux/v5.18-rc5/source/mm/ksm.c#L2498
>>     <https://elixir.bootlin.com/linux/v5.18-rc5/source/mm/ksm.c#L2498>).
>>     We would also like your thoughts on the security impact given it
>>     is a TOCTOU bug.
>>
>>     We provide more details below including the trace and reproducing
>>     test cases.
>
>
>     Hello， could the following changes to avoid the data-race issue？
>
>     diff --git a/mm/ksm.c b/mm/ksm.c
>     index 54f78c9eecae..f072753cbb3a 100644
>     --- a/mm/ksm.c
>     +++ b/mm/ksm.c
>     @@ -2497,6 +2497,7 @@ int __ksm_enter(struct mm_struct *mm)
>      {
>             struct mm_slot *mm_slot;
>             int needs_wakeup;
>     +       bool ksm_run_merge;
>
>             mm_slot = alloc_mm_slot();
>             if (!mm_slot)
>     @@ -2505,6 +2506,10 @@ int __ksm_enter(struct mm_struct *mm)
>             /* Check ksm_run too?  Would need tighter locking */
>             needs_wakeup = list_empty(&ksm_mm_head.mm_list);
>
>     +       mutex_lock(&ksm_thread_mutex);
>     +       ksm_run_unmerge = !!(ksm_run & KSM_RUN_UNMERGE);
>     +       mutex_unlock(&ksm_thread_mutex);
>     +
>             spin_lock(&ksm_mmlist_lock);
>             insert_to_mm_slots_hash(mm, mm_slot);
>             /*
>     @@ -2517,7 +2522,7 @@ int __ksm_enter(struct mm_struct *mm)
>              * scanning cursor, otherwise KSM pages in newly forked
>     mms will be
>              * missed: then we might as well insert at the end of the
>     list.
>              */
>     -       if (ksm_run & KSM_RUN_UNMERGE)
>     +       if (ksm_run_unmerge)
>                     list_add_tail(&mm_slot->mm_list,
>     &ksm_mm_head.mm_list);
>             else
>                     list_add_tail(&mm_slot->mm_list,
>     &ksm_scan.mm_slot->mm_list);
>
>
>>
>>
>>     *Trace*
>>     BUG: KCSAN: data-race in __ksm_enter / run_store
>>     write to 0xffffffff881edae0 of 8 bytes by task 6542 on cpu 0:
>>      run_store+0x19a/0x2d0 mm/ksm.c:2897
>>      kobj_attr_store+0x44/0x60 lib/kobject.c:824
>>      sysfs_kf_write+0x16f/0x1a0 fs/sysfs/file.c:136
>>      kernfs_fop_write_iter+0x2ae/0x370 fs/kernfs/file.c:291
>>      call_write_iter include/linux/fs.h:2050 [inline]
>>      new_sync_write fs/read_write.c:504 [inline]
>>      vfs_write+0x779/0x900 fs/read_write.c:591
>>      ksys_write+0xde/0x190 fs/read_write.c:644
>>      __do_sys_write fs/read_write.c:656 [inline]
>>      __se_sys_write fs/read_write.c:653 [inline]
>>      __x64_sys_write+0x43/0x50 fs/read_write.c:653
>>      do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>>      do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
>>      entry_SYSCALL_64_after_hwframe+0x44/0xae
>>
>>     read to 0xffffffff881edae0 of 8 bytes by task 6541 on cpu 1:
>>      __ksm_enter+0x114/0x260 mm/ksm.c:2501
>>      ksm_madvise+0x291/0x350 mm/ksm.c:2451
>>      madvise_vma_behavior mm/madvise.c:1039 [inline]
>>      madvise_walk_vmas mm/madvise.c:1221 [inline]
>>      do_madvise+0x656/0xeb0 mm/madvise.c:1399
>>      __do_sys_madvise mm/madvise.c:1412 [inline]
>>      __se_sys_madvise mm/madvise.c:1410 [inline]
>>      __x64_sys_madvise+0x64/0x70 mm/madvise.c:1410
>>      do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>>      do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
>>      entry_SYSCALL_64_after_hwframe+0x44/0xae
>>
>>     Reported by Kernel Concurrency Sanitizer on:
>>     CPU: 1 PID: 6541 Comm: syz-executor2-n Not tainted 5.18.0-rc5+ #107
>>     Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
>>     1.15.0-1 04/01/2014
>>     ---------------------
>>     *Inputs *
>>     Input CPU 0:
>>     r0 = openat$sysctl(0xffffff9c,
>>     &(0x7f0000000100)='/sys/kernel/mm/ksm/run\x00', 0x1, 0x0)
>>     write$sysctl(r0, &(0x7f0000000000)='2\x00', 0x2)
>>
>>     Input CPU 1:
>>     madvise(&(0x7f0000ffc000/0x4000)=nil, 0x4000, 0xc)
>>     mlock2(&(0x7f0000ffe000/0x2000)=nil, 0x2000, 0x0)
>>     madvise(&(0x7f0000ffd000/0x3000)=nil, 0x3000, 0x12)
>>     clone(0x0, 0x0, 0x0, 0x0, 0x0)
>

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Re: Race in mm/ksm.c

[Gabriel Ryan]

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Yes, we are unable to trigger the reported race after applying your patch,
and by inspection the patch appears to have fixed the issue by extending
ksm_thread_mutex to cover the ksm_run flag check in __ksm_enter.

We have also contacted the kernel security team since the race may have
security implications.

Thanks,

Gabe

On Tue, Aug 2, 2022 at 8:18 AM Kefeng Wang <wangkefeng.wang@huawei.com>
wrote:

>
> On 2022/8/2 19:44, Gabriel Ryan wrote:
>
> Confirmed.
>
> It means that the following changes could fix the issue, right?
>
> If so, I will send a formal patch with your report and test, thanks.
>
>
> Thanks for the quick response and apologies for the delay!
>
> Best,
>
> Gabe
>
> On Thu, Jul 21, 2022 at 9:57 PM Kefeng Wang <wangkefeng.wang@huawei.com>
> wrote:
>
>>
>> On 2022/7/21 23:58, Abhishek Shah wrote:
>>
>> Dear Kernel Maintainers,
>>
>> We found a race in mm/ksm.c. During the execution of the function
>> *__ksm_run* which uses variable *ksm_run* to decide the list insertion
>> point, the variable *ksm_run* can be concurrently modified in the
>> function *run_store*, which we thought could be undesirable since “KSM
>> pages in newly forked mms can be missed” (See comment here:
>> https://elixir.bootlin.com/linux/v5.18-rc5/source/mm/ksm.c#L2498). We
>> would also like your thoughts on the security impact given it is a TOCTOU
>> bug.
>>
>> We provide more details below including the trace and reproducing
>> test cases.
>>
>>
>> Hello， could the following changes to avoid the data-race issue？
>>
>> diff --git a/mm/ksm.c b/mm/ksm.c
>> index 54f78c9eecae..f072753cbb3a 100644
>> --- a/mm/ksm.c
>> +++ b/mm/ksm.c
>> @@ -2497,6 +2497,7 @@ int __ksm_enter(struct mm_struct *mm)
>>  {
>>         struct mm_slot *mm_slot;
>>         int needs_wakeup;
>> +       bool ksm_run_merge;
>>
>>         mm_slot = alloc_mm_slot();
>>         if (!mm_slot)
>> @@ -2505,6 +2506,10 @@ int __ksm_enter(struct mm_struct *mm)
>>         /* Check ksm_run too?  Would need tighter locking */
>>         needs_wakeup = list_empty(&ksm_mm_head.mm_list);
>>
>> +       mutex_lock(&ksm_thread_mutex);
>> +       ksm_run_unmerge = !!(ksm_run & KSM_RUN_UNMERGE);
>> +       mutex_unlock(&ksm_thread_mutex);
>> +
>>         spin_lock(&ksm_mmlist_lock);
>>         insert_to_mm_slots_hash(mm, mm_slot);
>>         /*
>> @@ -2517,7 +2522,7 @@ int __ksm_enter(struct mm_struct *mm)
>>          * scanning cursor, otherwise KSM pages in newly forked mms will
>> be
>>          * missed: then we might as well insert at the end of the list.
>>          */
>> -       if (ksm_run & KSM_RUN_UNMERGE)
>> +       if (ksm_run_unmerge)
>>                 list_add_tail(&mm_slot->mm_list, &ksm_mm_head.mm_list);
>>         else
>>                 list_add_tail(&mm_slot->mm_list,
>> &ksm_scan.mm_slot->mm_list);
>>
>>
>>
>>
>> *Trace*
>> BUG: KCSAN: data-race in __ksm_enter / run_store
>> write to 0xffffffff881edae0 of 8 bytes by task 6542 on cpu 0:
>>  run_store+0x19a/0x2d0 mm/ksm.c:2897
>>  kobj_attr_store+0x44/0x60 lib/kobject.c:824
>>  sysfs_kf_write+0x16f/0x1a0 fs/sysfs/file.c:136
>>  kernfs_fop_write_iter+0x2ae/0x370 fs/kernfs/file.c:291
>>  call_write_iter include/linux/fs.h:2050 [inline]
>>  new_sync_write fs/read_write.c:504 [inline]
>>  vfs_write+0x779/0x900 fs/read_write.c:591
>>  ksys_write+0xde/0x190 fs/read_write.c:644
>>  __do_sys_write fs/read_write.c:656 [inline]
>>  __se_sys_write fs/read_write.c:653 [inline]
>>  __x64_sys_write+0x43/0x50 fs/read_write.c:653
>>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>>  do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
>>  entry_SYSCALL_64_after_hwframe+0x44/0xae
>>
>> read to 0xffffffff881edae0 of 8 bytes by task 6541 on cpu 1:
>>  __ksm_enter+0x114/0x260 mm/ksm.c:2501
>>  ksm_madvise+0x291/0x350 mm/ksm.c:2451
>>  madvise_vma_behavior mm/madvise.c:1039 [inline]
>>  madvise_walk_vmas mm/madvise.c:1221 [inline]
>>  do_madvise+0x656/0xeb0 mm/madvise.c:1399
>>  __do_sys_madvise mm/madvise.c:1412 [inline]
>>  __se_sys_madvise mm/madvise.c:1410 [inline]
>>  __x64_sys_madvise+0x64/0x70 mm/madvise.c:1410
>>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>>  do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
>>  entry_SYSCALL_64_after_hwframe+0x44/0xae
>>
>> Reported by Kernel Concurrency Sanitizer on:
>> CPU: 1 PID: 6541 Comm: syz-executor2-n Not tainted 5.18.0-rc5+ #107
>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1
>> 04/01/2014
>> ---------------------
>> *Inputs *
>> Input CPU 0:
>> r0 = openat$sysctl(0xffffff9c,
>> &(0x7f0000000100)='/sys/kernel/mm/ksm/run\x00', 0x1, 0x0)
>> write$sysctl(r0, &(0x7f0000000000)='2\x00', 0x2)
>>
>> Input CPU 1:
>> madvise(&(0x7f0000ffc000/0x4000)=nil, 0x4000, 0xc)
>> mlock2(&(0x7f0000ffe000/0x2000)=nil, 0x2000, 0x0)
>> madvise(&(0x7f0000ffd000/0x3000)=nil, 0x3000, 0x12)
>> clone(0x0, 0x0, 0x0, 0x0, 0x0)
>>
>>

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