linux-kernel - [PATCH 1/1] userfaultfd: check VM_MAYWRITE was set after verifying the uffd is registered

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Message-Id: <20181206212028.18726-2-aarcange@redhat.com>
Date:   Thu,  6 Dec 2018 16:20:28 -0500
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Hugh Dickins <hughd@...gle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Jann Horn <jannh@...gle.com>, Peter Xu <peterx@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>
Subject: [PATCH 1/1] userfaultfd: check VM_MAYWRITE was set after verifying the uffd is registered

Calling UFFDIO_UNREGISTER on virtual ranges not yet registered in uffd
could trigger an harmless false positive WARN_ON. Check the vma is
already registered before checking VM_MAYWRITE to shut off the
false positive warning.

Cc: <stable@...r.kernel.org>
Fixes: 29ec90660d68 ("userfaultfd: shmem/hugetlbfs: only allow to register VM_MAYWRITE vmas")
Reported-by: syzbot+06c7092e7d71218a2c16@...kaller.appspotmail.com
Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
---
 fs/userfaultfd.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index cd58939dc977..7a85e609fc27 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1566,7 +1566,6 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
 		cond_resched();
 
 		BUG_ON(!vma_can_userfault(vma));
-		WARN_ON(!(vma->vm_flags & VM_MAYWRITE));
 
 		/*
 		 * Nothing to do: this vma is already registered into this
@@ -1575,6 +1574,8 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
 		if (!vma->vm_userfaultfd_ctx.ctx)
 			goto skip;
 
+		WARN_ON(!(vma->vm_flags & VM_MAYWRITE));
+
 		if (vma->vm_start > start)
 			start = vma->vm_start;
 		vma_end = min(end, vma->vm_end);