Patch "mm, oom_reaper: gather each vma to prevent leaking TLB entry" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

mm, oom_reaper: gather each vma to prevent leaking TLB entry

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mm-oom_reaper-gather-each-vma-to-prevent-leaking-tlb-entry.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Wang Nan <[email protected]>
Date: Wed, 29 Nov 2017 16:09:58 -0800
Subject: mm, oom_reaper: gather each vma to prevent leaking TLB entry

From: Wang Nan <[email protected]>

commit 687cb0884a714ff484d038e9190edc874edcf146 upstream.

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.

---
 mm/oom_kill.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

@@ -524,7 +524,6 @@ static bool __oom_reap_task_mm(struct ta
 	 */
 	set_bit(MMF_UNSTABLE, &mm->flags);
 
-	tlb_gather_mmu(&tlb, mm, 0, -1);
 	for (vma = mm->mmap ; vma; vma = vma->vm_next) {
 		if (is_vm_hugetlb_page(vma))
 			continue;

@@ -546,11 +545,13 @@ static bool __oom_reap_task_mm(struct ta
 		 * 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))
+		if (vma_is_anonymous(vma) || !(vma->vm_flags & VM_SHARED)) {
+			tlb_gather_mmu(&tlb, mm, vma->vm_start, vma->vm_end);
 			unmap_page_range(&tlb, vma, vma->vm_start, vma->vm_end,
 					 &details);
+			tlb_finish_mmu(&tlb, vma->vm_start, vma->vm_end);
+		}
 	}
-	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)),

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/mm-oom_reaper-gather-each-vma-to-prevent-leaking-tlb-entry.patch