Re: Race in mm/ksm.c

[Gabriel Ryan <gabe@cs.columbia.edu>]

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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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