Re: kernel BUG at fs/userfaultfd.c:LINE! (2)

[Mike Rapoport <rppt@linux.vnet.ibm.com>]

On Tue, Jul 17, 2018 at 12:28:06PM -0700, Eric Biggers wrote:
> [+Cc userfaultfd developers and linux-mm]
> 
> The reproducer hits the BUG_ON() in userfaultfd_release():
> 
> 	BUG_ON(!!vma->vm_userfaultfd_ctx.ctx ^
> 	       !!(vma->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));

Thanks for the CC.

The fix is below.

--
Sincerely yours,
Mike.

>From 3711b6a72abe3796b2b97fdba8eadd995b29f195 Mon Sep 17 00:00:00 2001
From: Mike Rapoport <rppt@linux.vnet.ibm.com>
Date: Wed, 18 Jul 2018 09:45:54 +0300
Subject: [PATCH] userfaultfd: remove uffd flags from vma->vm_flags if
 UFFD_EVENT_FORK fails

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.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Reported-by: syzbot+121be635a7a35ddb7dcb@syzkaller.appspotmail.com
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Eric Biggers <ebiggers3@gmail.com>
Cc: stable@vger.kernel.org
---
 fs/userfaultfd.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 594d192b2331..bad9cea37f12 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -633,8 +633,10 @@ static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
 		/* 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);
-- 
2.7.4