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)
>