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Date:   Tue, 22 Dec 2020 14:31:52 -0500
From:   Andrea Arcangeli <aarcange@redhat.com>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Xu <peterx@redhat.com>,
        Nadav Amit <nadav.amit@gmail.com>, Yu Zhao <yuzhao@google.com>,
        linux-mm <linux-mm@kvack.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Pavel Emelyanov <xemul@openvz.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        stable <stable@vger.kernel.org>,
        Minchan Kim <minchan@kernel.org>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect
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On Mon, Dec 21, 2020 at 07:19:35PM -0800, Andy Lutomirski wrote:
> instance of this (with mmap_sem held for write) in x86:
> mark_screen_rdonly().  Dare I ask how broken this is?  We could likely

That one is buggy despite it takes the mmap_write_lock... inverting
the last two lines would fix it though.

-	mmap_write_unlock(mm);
	flush_tlb_mm_range(mm, 0xA0000, 0xA0000 + 32*PAGE_SIZE, PAGE_SHIFT, false);
+	mmap_write_unlock(mm);

The issue here is that rightfully wp_page_copy copies outside the PT
lock (good thing) and it correctly assumes during the copy nobody can
modify the page if the fault happened in a write access and the pte
had no _PAGE_WRITE bit set.

The bug is clearly in userfaultfd_writeprotect that doesn't enforce
the above invariant.

If the userfaultfd_wrprotect(mode_wp = false) run after the deferred
TLB flush this could never happen because the uffd_wp bit was still
set and the page fault would hit the handle_userfault dead end and
it'd have to be restarted from scratch.

Leaving stale tlb entries after dropping the PT lock and with only the
mmap_lock_read is only ok if the page fault has a "check" that catches
that specific case, so that specific case is fully serialized by the
PT lock alone and the "catch" path is fully aware about the stale TLB
entries that were left around.

So looking at the two cases that predates uffd-wp that already did a
RW->RO transition with the mmap_lock_read, they both comply with the
wp_page_copy invariant but with two different techniques:

1) change_protection is called by the scheduler with the
  mmap_read_lock and with a deferred TLB flush, and things don't fall
  apart completely there simply because we catch that in do_numa_page:

	if (pte_protnone(vmf->orig_pte) && vma_is_accessible(vmf->vma))
+               /* catch it: no risk to end up in wp_page_copy even if the change_protection
+	           is running concurrently and the tlb flush is still pending */
		return do_numa_page(vmf);

  do_numa_page doesn't issue any further tlb flush, but basically
  restores the pte to the previous value, so then the pending flush
  becomes a noop, since there was no modification to the pte in the
  first place after do_numa_page runs.

2) ksm write_protect_page is also called with mmap_read_lock, and it
   will simply not defer the flush. If the tlb flush is done inside
   the PT lock it is ok to take mmap_read_lock since the page fault
   code will not make any assumption about the TLB before reading the
   pagetable content under PT lock.

   In fact if mm_tlb_flush_pending(mm) is true, write_protect_page in
   KSM has to issue an extra synchronous flush before dropping the PT
   lock, even if it finds the _PAGE_WRITE bit is already clear,
   precisely because there can be another deferred flush that cleared
   the pte but didn't flush the TLB yet.

userfaultfd_writeprotect() is already using the technique in 1) to be
safe, except the un-wrprotect can break it if run while the tlb flush
is still pending...

The good thing is this guarantee must be provided not more granular
even than a vma, not per-mm and a vma cannot be registered on two
different uffd ctx at the same time, so the locking can happen all
internal to the uffd ctx.

My previous suggestion to use a mutex to serialize
userfaultfd_writeprotect with a mutex will still work, but we can run
as many wrprotect and un-wrprotect as we want in parallel, as long as
they're not simultaneous, we can do much better than a mutex.

Ideally we would need a new two_group_semaphore, where each group can
run as many parallel instances as it wants, but no instance of one
group can run in parallel with any instance of the other group. AFIK
such a kind of lock doesn't exist right now.

wrprotect if left alone never had an issue since we had a working
"catch" check for it well before wp_page_copy could run. unprotect
also if left alone was ok since it's a permission upgrade.

Alternatively, we can solve this with the same technique as in 2),
because all it matters is that the 4k pte modification doesn't take
the mmap_lock_write. A modification to a single 4k pte or a single 2m
hugepmd is very likely indication that there's going to be a flood of
those in parallel from all CPUs and likely there's also a flood of
page faults from all CPUs in parallel. In addition for a 4k wrprotect
or un-wrprotect there's not a significant benefit in deferring the TLB
flush.

I don't think we can take the mmap_write_lock unconditionally because
one of the main reasons to deal with the extra complexity of resolving
page faults in userland is to bypass mprotect entirely.

Example, JITs can use unprivileged uffd to eliminate the software-set
dirty bit every time it modifies memory, that would then require
frequent wrprotect/un-wrprotect during page faults and the less likely
we have to block for I/O during those CPU-only events, the
better. This is one of the potential use cases, but there's plenty
more.

So the alternative would be to take mmap_read_lock for "len <=
HPAGE_PMD_SIZE" and the mmap_write_lock otherwise, and to add a
parameter in change_protection to tell it to flush before dropping the
PT lock and not defer the flush. So this is going to be more intrusive
in the VM and it's overall unnecessary.

The below is mostly untested... but it'd be good to hear some feedback
before doing more work in this direction.

From 4ace4d1b53f5cb3b22a5c2dc33facc4150b112d6 Mon Sep 17 00:00:00 2001
From: Andrea Arcangeli <aarcange@redhat.com>
Date: Tue, 22 Dec 2020 14:30:16 -0500
Subject: [PATCH 1/1] mm: userfaultfd: avoid leaving stale TLB after
 userfaultfd_writeprotect()

change_protection() is called by userfaultfd_writeprotect() with the
mmap_lock_read like in change_prot_numa().

The page fault code in wp_copy_page() rightfully assumes if the CPU
issued a write fault and the write bit in the pagetable is not set, no
CPU can write to the page. That's wrong assumption after
userfaultfd_writeprotect(). That's also wrong assumption after
change_prot_numa() where the regular page fault code also would assume
that if the present bit is not set and the page fault is running,
there should be no stale TLB entry, but there is still.

So to stay safe, the page fault code must be prevented to run as long
as long as the TLB flush remains pending. That is already achieved by
the do_numa_page() path for change_prot_numa() and by the
userfaultfd_pte_wp() path for userfaultfd_writeprotect().

The problem that needs fixing is that an un-wrprotect
(i.e. userfaultfd_writeprotect() with UFFDIO_WRITEPROTECT_MODE_WP not
set) could run in between the original wrprotect
(i.e. userfaultfd_writeprotect() with UFFDIO_WRITEPROTECT_MODE_WP set)
and wp_copy_page, while the TLB flush remains pending.

In such case the userfaultfd_pte_wp() check will stop being effective
to prevent the regular page fault code to run.

CPU0			CPU 1		CPU 2
------			--------	-------
userfaultfd_wrprotect(mode_wp = true)
PT lock
atomic set _PAGE_UFFD_WP and clear _PAGE_WRITE
PT unlock

			do_page_fault FAULT_FLAG_WRITE
					userfaultfd_wrprotect(mode_wp = false)
					PT lock
					ATOMIC clear _PAGE_UFFD_WP <- problem
					/* _PAGE_WRITE not set */
					PT unlock
					XXXXXXXXXXXXXX BUG RACE window open here

			PT lock
			FAULT_FLAG_WRITE is set by CPU
			_PAGE_WRITE is still clear in pte
			PT unlock

			wp_page_copy
			copy_user_page runs with stale TLB

deferred tlb flush <- too late
XXXXXXXXXXXXXX BUG RACE window close here
================================================================================

If the userfaultfd_wrprotect(mode_wp = false) can only run after the
deferred TLB flush the above cannot happen either, because the uffd_wp
bit will remain set until after the TLB flush and the page fault would
reliably hit the handle_userfault() dead end as long as the TLB is
stale.

So to fix this we need to prevent any un-wrprotect to start until the
last outstanding wrprotect completed and to prevent any further
wrprotect until the last outstanding un-wrprotect completed. Then
wp_page_copy can still run in parallel but only with the un-wrprotect,
and that's fine since it's a permission promotion.

We would need a new two_group_semaphore, where each group can run as
many parallel instances as it wants, but no instance of one group can
run in parallel with any instance of the other group. The below rwsem
with two atomics approximates that kind lock.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
 fs/userfaultfd.c | 39 +++++++++++++++++++++++++++++++++++++++
 mm/memory.c      | 20 ++++++++++++++++++++
 2 files changed, 59 insertions(+)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 894cc28142e7..3729ca99dae5 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -76,6 +76,20 @@ struct userfaultfd_ctx {
 	bool mmap_changing;
 	/* mm with one ore more vmas attached to this userfaultfd_ctx */
 	struct mm_struct *mm;
+	/*
+	 * We cannot let userfaultd_writeprotect(mode_wp = false)
+	 * clear _PAGE_UFFD_WP from the pgtable, until the last
+	 * outstanding userfaultd_writeprotect(mode_wp = true)
+	 * completes, because the TLB flush is deferred. The page
+	 * fault must be forced to enter handle_userfault() while the
+	 * TLB flush is deferred, and that's achieved with the
+	 * _PAGE_UFFD_WP bit. The _PAGE_UFFD_WP can only be cleared
+	 * after there are no stale TLB entries left, only then it'll
+	 * be safe again for the page fault to continue and not hit
+	 * the handle_userfault() dead end anymore.
+	 */
+	struct rw_semaphore wrprotect_rwsem;
+	atomic64_t wrprotect_count[2];
 };
 
 struct userfaultfd_fork_ctx {
@@ -1783,6 +1797,7 @@ static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx,
 	struct uffdio_writeprotect __user *user_uffdio_wp;
 	struct userfaultfd_wake_range range;
 	bool mode_wp, mode_dontwake;
+	bool contention;
 
 	if (READ_ONCE(ctx->mmap_changing))
 		return -EAGAIN;
@@ -1808,9 +1823,30 @@ static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx,
 	if (mode_wp && mode_dontwake)
 		return -EINVAL;
 
+	VM_WARN_ON(atomic64_read(&ctx->wrprotect_count[0]) < 0);
+	VM_WARN_ON(atomic64_read(&ctx->wrprotect_count[1]) < 0);
+	atomic64_inc(&ctx->wrprotect_count[mode_wp ? 0 : 1]);
+	smp_mb__after_atomic();
+	contention = atomic64_read(&ctx->wrprotect_count[mode_wp ? 1 : 0]) > 0;
+	if (!contention)
+		down_read(&ctx->wrprotect_rwsem);
+	else {
+		down_write(&ctx->wrprotect_rwsem);
+		if (!atomic64_read(&ctx->wrprotect_count[mode_wp ? 1 : 0])) {
+			contention = false;
+			downgrade_write(&ctx->wrprotect_rwsem);
+		}
+
+	}
 	ret = mwriteprotect_range(ctx->mm, uffdio_wp.range.start,
 				  uffdio_wp.range.len, mode_wp,
 				  &ctx->mmap_changing);
+	if (!contention)
+		up_read(&ctx->wrprotect_rwsem);
+	else
+		up_write(&ctx->wrprotect_rwsem);
+	smp_mb__before_atomic();
+	atomic64_dec(&ctx->wrprotect_count[mode_wp ? 0 : 1]);
 	if (ret)
 		return ret;
 
@@ -1958,6 +1994,9 @@ static void init_once_userfaultfd_ctx(void *mem)
 	init_waitqueue_head(&ctx->fault_wqh);
 	init_waitqueue_head(&ctx->event_wqh);
 	init_waitqueue_head(&ctx->fd_wqh);
+	init_rwsem(&ctx->wrprotect_rwsem);
+	atomic64_set(&ctx->wrprotect_count[0], 0);
+	atomic64_set(&ctx->wrprotect_count[1], 0);
 	seqcount_spinlock_init(&ctx->refile_seq, &ctx->fault_pending_wqh.lock);
 }
 
diff --git a/mm/memory.c b/mm/memory.c
index 7d608765932b..4ff9f21d5a7b 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3085,6 +3085,12 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
 {
 	struct vm_area_struct *vma = vmf->vma;
 
+	/*
+	 * STALE_TLB_WARNING: while the uffd_wp bit is set, the TLB
+	 * can be stale. We cannot allow do_wp_page to proceed or
+	 * it'll wrongly assume that nobody can still be writing to
+	 * the page if !pte_write.
+	 */
 	if (userfaultfd_pte_wp(vma, *vmf->pte)) {
 		pte_unmap_unlock(vmf->pte, vmf->ptl);
 		return handle_userfault(vmf, VM_UFFD_WP);
@@ -4274,6 +4280,12 @@ static inline vm_fault_t create_huge_pmd(struct vm_fault *vmf)
 static inline vm_fault_t wp_huge_pmd(struct vm_fault *vmf, pmd_t orig_pmd)
 {
 	if (vma_is_anonymous(vmf->vma)) {
+		/*
+		 * STALE_TLB_WARNING: while the uffd_wp bit is set,
+		 * the TLB can be stale. We cannot allow wp_huge_pmd()
+		 * to proceed or it'll wrongly assume that nobody can
+		 * still be writing to the page if !pmd_write.
+		 */
 		if (userfaultfd_huge_pmd_wp(vmf->vma, orig_pmd))
 			return handle_userfault(vmf, VM_UFFD_WP);
 		return do_huge_pmd_wp_page(vmf, orig_pmd);
@@ -4388,6 +4400,10 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
 	if (!pte_present(vmf->orig_pte))
 		return do_swap_page(vmf);
 
+	/*
+	 * STALE_TLB_WARNING: if the pte is NUMA protnone the TLB can
+	 * be stale.
+	 */
 	if (pte_protnone(vmf->orig_pte) && vma_is_accessible(vmf->vma))
 		return do_numa_page(vmf);
 
@@ -4503,6 +4519,10 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
 			return 0;
 		}
 		if (pmd_trans_huge(orig_pmd) || pmd_devmap(orig_pmd)) {
+			/*
+			 * STALE_TLB_WARNING: if the pmd is NUMA
+			 * protnone the TLB can be stale.
+			 */
 			if (pmd_protnone(orig_pmd) && vma_is_accessible(vma))
 				return do_huge_pmd_numa_page(&vmf, orig_pmd);