Code: static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
{
	struct mmu_gather tlb;
	struct vm_area_struct *vma;
	bool ret = true;

	/*
	 * We have to make sure to not race with the victim exit path
	 * and cause premature new oom victim selection:
	 * __oom_reap_task_mm		exit_mm
	 *   mmget_not_zero
	 *				  mmput
	 *				    atomic_dec_and_test
	 *				  exit_oom_victim
	 *				[...]
	 *				out_of_memory
	 *				  select_bad_process
	 *				    # no TIF_MEMDIE task selects new victim
	 *  unmap_page_range # frees some memory
	 */
	mutex_lock(&oom_lock);

	if (!down_read_trylock(&mm->mmap_sem)) {
		ret = false;
		trace_skip_task_reaping(tsk->pid);
		goto unlock_oom;
	}

	/*
	 * If the mm has notifiers then we would need to invalidate them around
	 * unmap_page_range and that is risky because notifiers can sleep and
	 * what they do is basically undeterministic.  So let's have a short
	 * sleep to give the oom victim some more time.
	 * TODO: we really want to get rid of this ugly hack and make sure that
	 * notifiers cannot block for unbounded amount of time and add
	 * mmu_notifier_invalidate_range_{start,end} around unmap_page_range
	 */
	if (mm_has_notifiers(mm)) {
		up_read(&mm->mmap_sem);
		schedule_timeout_idle(HZ);
		goto unlock_oom;
	}

	/*
	 * MMF_OOM_SKIP is set by exit_mmap when the OOM reaper can't
	 * work on the mm anymore. The check for MMF_OOM_SKIP must run
	 * under mmap_sem for reading because it serializes against the
	 * down_write();up_write() cycle in exit_mmap().
	 */
	if (test_bit(MMF_OOM_SKIP, &mm->flags)) {
		up_read(&mm->mmap_sem);
		trace_skip_task_reaping(tsk->pid);
		goto unlock_oom;
	}

	trace_start_task_reaping(tsk->pid);

	/*
	 * Tell all users of get_user/copy_from_user etc... that the content
	 * is no longer stable. No barriers really needed because unmapping
	 * should imply barriers already and the reader would hit a page fault
	 * if it stumbled over a reaped memory.
 	 */
 	set_bit(MMF_UNSTABLE, &mm->flags);
 
	tlb_gather_mmu(&tlb, mm, 0, -1);
 	for (vma = mm->mmap ; vma; vma = vma->vm_next) {
 		if (!can_madv_dontneed_vma(vma))
 			continue;

		/*
		 * Only anonymous pages have a good chance to be dropped
		 * without additional steps which we cannot afford as we
		 * are OOM already.
		 *
		 * We do not even care about fs backed pages because all
		 * which are reclaimable have already been reclaimed and
 		 * we do not want to block exit_mmap by keeping mm ref
 		 * count elevated without a good reason.
 		 */
		if (vma_is_anonymous(vma) || !(vma->vm_flags & VM_SHARED))
 			unmap_page_range(&tlb, vma, vma->vm_start, vma->vm_end,
 					 NULL);
 	}
	tlb_finish_mmu(&tlb, 0, -1);
 	pr_info("oom_reaper: reaped process %d (%s), now anon-rss:%lukB, file-rss:%lukB, shmem-rss:%lukB\n",
 			task_pid_nr(tsk), tsk->comm,
 			K(get_mm_counter(mm, MM_ANONPAGES)),
			K(get_mm_counter(mm, MM_FILEPAGES)),
			K(get_mm_counter(mm, MM_SHMEMPAGES)));
	up_read(&mm->mmap_sem);

	trace_finish_task_reaping(tsk->pid);
unlock_oom:
	mutex_unlock(&oom_lock);
	return ret;
}

Vulnerability Type: DoS 
CWE ID: CWE-416
Summary: The __oom_reap_task_mm function in mm/oom_kill.c in the Linux kernel before 4.14.4 mishandles gather operations, which allows attackers to cause a denial of service (TLB entry leak or use-after-free) or possibly have unspecified other impact by triggering a copy_to_user call within a certain time window.
Commit Message: mm, oom_reaper: gather each vma to prevent leaking TLB entry

tlb_gather_mmu(&tlb, mm, 0, -1) means gathering the whole virtual memory
space.  In this case, tlb->fullmm is true.  Some archs like arm64
doesn't flush TLB when tlb->fullmm is true:

  commit 5a7862e83000 ("arm64: tlbflush: avoid flushing when fullmm == 1").

Which causes leaking of tlb entries.

Will clarifies his patch:
 "Basically, we tag each address space with an ASID (PCID on x86) which
  is resident in the TLB. This means we can elide TLB invalidation when
  pulling down a full mm because we won't ever assign that ASID to
  another mm without doing TLB invalidation elsewhere (which actually
  just nukes the whole TLB).

  I think that means that we could potentially not fault on a kernel
  uaccess, because we could hit in the TLB"

There could be a window between complete_signal() sending IPI to other
cores and all threads sharing this mm are really kicked off from cores.
In this window, the oom reaper may calls tlb_flush_mmu_tlbonly() to
flush TLB then frees pages.  However, due to the above problem, the TLB
entries are not really flushed on arm64.  Other threads are possible to
access these pages through TLB entries.  Moreover, a copy_to_user() can
also write to these pages without generating page fault, causes
use-after-free bugs.

This patch gathers each vma instead of gathering full vm space.  In this
case tlb->fullmm is not true.  The behavior of oom reaper become similar
to munmapping before do_exit, which should be safe for all archs.

Link: http://lkml.kernel.org/r/20171107095453.179940-1-wangnan0@huawei.com
Fixes: aac453635549 ("mm, oom: introduce oom reaper")
Signed-off-by: Wang Nan <wangnan0@huawei.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Bob Liu <liubo95@huawei.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Roman Gushchin <guro@fb.com>
Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>