linux-kernel - [PATCH AUTOSEL for 4.9 005/156] userfaultfd: shmem: __do_fault requires VM_FAULT

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Date:   Thu, 7 Dec 2017 15:48:55 +0000
From:   alexander.levin@...izon.com
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        alexander.levin@...izon.com
Subject: [PATCH AUTOSEL for 4.9 005/156] userfaultfd: shmem: __do_fault
 requires VM_FAULT_NOPAGE

From: Andrea Arcangeli <aarcange@...hat.com>

[ Upstream commit 6bbc4a4144b1a69743022ac68dfaf6e7d993abb9 ]

__do_fault assumes vmf->page has been initialized and is valid if
VM_FAULT_NOPAGE is not returned by vma->vm_ops->fault(vma, vmf).

handle_userfault() in turn should return VM_FAULT_NOPAGE if it doesn't
return VM_FAULT_SIGBUS or VM_FAULT_RETRY (the other two possibilities).

This VM_FAULT_NOPAGE case is only invoked when signal are pending and it
didn't matter for anonymous memory before.  It only started to matter
since shmem was introduced.  hugetlbfs also takes a different path and
doesn't exercise __do_fault.

Link: http://lkml.kernel.org/r/20170228154201.GH5816@redhat.com
Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
Cc: "Kirill A. Shutemov" <kirill@...temov.name>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Sasha Levin <alexander.levin@...izon.com>
---
 fs/userfaultfd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index b86054cc41db..784d667475ae 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -419,7 +419,7 @@ int handle_userfault(struct fault_env *fe, unsigned long reason)
 			 * in such case.
 			 */
 			down_read(&mm->mmap_sem);
-			ret = 0;
+			ret = VM_FAULT_NOPAGE;
 		}
 	}
 
-- 
2.11.0