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Date:   Fri,  1 May 2020 15:23:29 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        syzbot+e27980339d305f2dbfd9@...kaller.appspotmail.com,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 5.4 50/83] mm: shmem: disable interrupt when acquiring info->lock in userfaultfd_copy path

From: Yang Shi <yang.shi@...ux.alibaba.com>

commit 94b7cc01da5a3cc4f3da5e0ff492ef008bb555d6 upstream.

Syzbot reported the below lockdep splat:

    WARNING: possible irq lock inversion dependency detected
    5.6.0-rc7-syzkaller #0 Not tainted
    --------------------------------------------------------
    syz-executor.0/10317 just changed the state of lock:
    ffff888021d16568 (&(&info->lock)->rlock){+.+.}, at: spin_lock include/linux/spinlock.h:338 [inline]
    ffff888021d16568 (&(&info->lock)->rlock){+.+.}, at: shmem_mfill_atomic_pte+0x1012/0x21c0 mm/shmem.c:2407
    but this lock was taken by another, SOFTIRQ-safe lock in the past:
     (&(&xa->xa_lock)->rlock#5){..-.}

    and interrupts could create inverse lock ordering between them.

    other info that might help us debug this:
     Possible interrupt unsafe locking scenario:

           CPU0                    CPU1
           ----                    ----
      lock(&(&info->lock)->rlock);
                                   local_irq_disable();
                                   lock(&(&xa->xa_lock)->rlock#5);
                                   lock(&(&info->lock)->rlock);
      <Interrupt>
        lock(&(&xa->xa_lock)->rlock#5);

     *** DEADLOCK ***

The full report is quite lengthy, please see:

  https://lore.kernel.org/linux-mm/alpine.LSU.2.11.2004152007370.13597@eggly.anvils/T/#m813b412c5f78e25ca8c6c7734886ed4de43f241d

It is because CPU 0 held info->lock with IRQ enabled in userfaultfd_copy
path, then CPU 1 is splitting a THP which held xa_lock and info->lock in
IRQ disabled context at the same time.  If softirq comes in to acquire
xa_lock, the deadlock would be triggered.

The fix is to acquire/release info->lock with *_irq version instead of
plain spin_{lock,unlock} to make it softirq safe.

Fixes: 4c27fe4c4c84 ("userfaultfd: shmem: add shmem_mcopy_atomic_pte for userfaultfd support")
Reported-by: syzbot+e27980339d305f2dbfd9@...kaller.appspotmail.com
Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Tested-by: syzbot+e27980339d305f2dbfd9@...kaller.appspotmail.com
Acked-by: Hugh Dickins <hughd@...gle.com>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Link: http://lkml.kernel.org/r/1587061357-122619-1-git-send-email-yang.shi@linux.alibaba.com
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 mm/shmem.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2403,11 +2403,11 @@ static int shmem_mfill_atomic_pte(struct
 
 	lru_cache_add_anon(page);
 
-	spin_lock(&info->lock);
+	spin_lock_irq(&info->lock);
 	info->alloced++;
 	inode->i_blocks += BLOCKS_PER_PAGE;
 	shmem_recalc_inode(inode);
-	spin_unlock(&info->lock);
+	spin_unlock_irq(&info->lock);
 
 	inc_mm_counter(dst_mm, mm_counter_file(page));
 	page_add_file_rmap(page, false);


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