From 31e810aa1033a7db50a2746cd34a2432237f6420 Mon Sep 17 00:00:00 2001
From: Mike Rapoport <rppt@linux.vnet.ibm.com>
Date: Thu, 2 Aug 2018 15:36:09 -0700
Subject: [PATCH] userfaultfd: remove uffd flags from vma->vm_flags if
 UFFD_EVENT_FORK fails
Git-commit: 31e810aa1033a7db50a2746cd34a2432237f6420
Patch-mainline: v4.18-rc8
References: bsc#1118809

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@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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Jan Kara <jack@suse.cz>

---
 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);
-- 
2.16.4