linux-kernel - [PATCH 4.17 23/31] userfaultfd: remove uffd flags from vma->vm_flags if UFFD_EVENT

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Message-Id: <20180804082633.996505438@linuxfoundation.org>
Date:   Sat,  4 Aug 2018 11:00:58 +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, Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        syzbot+121be635a7a35ddb7dcb@...kaller.appspotmail.com,
        Andrea Arcangeli <aarcange@...hat.com>,
        Eric Biggers <ebiggers3@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 4.17 23/31] userfaultfd: remove uffd flags from vma->vm_flags if UFFD_EVENT_FORK fails

4.17-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Mike Rapoport <rppt@...ux.vnet.ibm.com>

commit 31e810aa1033a7db50a2746cd34a2432237f6420 upstream.

The fix in commit 0cbb4b4f4c44 ("userfaultfd: clear the
vma->vm_userfaultfd_ctx if UFFD_EVENT_FORK fails") cleared the
vma->vm_userfaultfd_ctx but kept userfaultfd flags in vma->vm_flags
that were copied from the parent process VMA.

As the result, there is an inconsistency between the values of
vma->vm_userfaultfd_ctx.ctx and vma->vm_flags which triggers BUG_ON
in userfaultfd_release().

Clearing the uffd flags from vma->vm_flags in case of UFFD_EVENT_FORK
failure resolves the issue.

Link: http://lkml.kernel.org/r/1532931975-25473-1-git-send-email-rppt@linux.vnet.ibm.com
Fixes: 0cbb4b4f4c44 ("userfaultfd: clear the vma->vm_userfaultfd_ctx if UFFD_EVENT_FORK fails")
Signed-off-by: Mike Rapoport <rppt@...ux.vnet.ibm.com>
Reported-by: syzbot+121be635a7a35ddb7dcb@...kaller.appspotmail.com
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Eric Biggers <ebiggers3@...il.com>
Cc: <stable@...r.kernel.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 fs/userfaultfd.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -631,8 +631,10 @@ static void userfaultfd_event_wait_compl
 		/* the various vma->vm_userfaultfd_ctx still points to it */
 		down_write(&mm->mmap_sem);
 		for (vma = mm->mmap; vma; vma = vma->vm_next)
-			if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx)
+			if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx) {
 				vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
+				vma->vm_flags &= ~(VM_UFFD_WP | VM_UFFD_MISSING);
+			}
 		up_write(&mm->mmap_sem);
 
 		userfaultfd_ctx_put(release_new_ctx);