linux-kernel - [PATCH RESEND] userfaultfd: remove uffd flags from vma->vm_flags if UFFD_EVENT

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Date:   Mon, 30 Jul 2018 09:26:15 +0300
From:   Mike Rapoport <rppt@...ux.vnet.ibm.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Eric Biggers <ebiggers3@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        syzbot <syzbot+121be635a7a35ddb7dcb@...kaller.appspotmail.com>,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>, stable@...r.kernel.org
Subject: [PATCH RESEND] 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@...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
---
 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