[3.19.y-ckt stable] Patch "mm: avoid setting up anonymous pages into file mapping" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    mm: avoid setting up anonymous pages into file mapping

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.19.y-queue

This patch is scheduled to be released in version 3.19.8-ckt5.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.19.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From c10363ac467b6f08bb8d7a9f50a366d3b21737dc Mon Sep 17 00:00:00 2001
From: "Kirill A. Shutemov" <kirill.shutemov at linux.intel.com>
Date: Mon, 6 Jul 2015 23:18:37 +0300
Subject: mm: avoid setting up anonymous pages into file mapping

commit 6b7339f4c31ad69c8e9c0b2859276e22cf72176d upstream.

Reading page fault handler code I've noticed that under right
circumstances kernel would map anonymous pages into file mappings: if
the VMA doesn't have vm_ops->fault() and the VMA wasn't fully populated
on ->mmap(), kernel would handle page fault to not populated pte with
do_anonymous_page().

Let's change page fault handler to use do_anonymous_page() only on
anonymous VMA (->vm_ops == NULL) and make sure that the VMA is not
shared.

For file mappings without vm_ops->fault() or shred VMA without vm_ops,
page fault on pte_none() entry would lead to SIGBUS.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov at linux.intel.com>
Acked-by: Oleg Nesterov <oleg at redhat.com>
Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Willy Tarreau <w at 1wt.eu>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
[ kamal: backport to 3.19-stable: s/do_fault/do_linear_fault/ ]
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 mm/memory.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 6aa7822..f5d0e3d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2630,6 +2630,10 @@ static int do_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,

 	pte_unmap(page_table);

+	/* File mapping without ->vm_ops ? */
+	if (vma->vm_flags & VM_SHARED)
+		return VM_FAULT_SIGBUS;
+
 	/* Check if we need to add a guard page to the stack */
 	if (check_stack_guard_page(vma, address) < 0)
 		return VM_FAULT_SIGSEGV;
@@ -3040,6 +3044,9 @@ static int do_linear_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 			- vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;

 	pte_unmap(page_table);
+	/* The VMA was not fully populated on mmap() or missing VM_DONTEXPAND */
+	if (!vma->vm_ops->fault)
+		return VM_FAULT_SIGBUS;
 	if (!(flags & FAULT_FLAG_WRITE))
 		return do_read_fault(mm, vma, address, pmd, pgoff, flags,
 				orig_pte);
@@ -3214,13 +3221,12 @@ static int handle_pte_fault(struct mm_struct *mm,
 	barrier();
 	if (!pte_present(entry)) {
 		if (pte_none(entry)) {
-			if (vma->vm_ops) {
-				if (likely(vma->vm_ops->fault))
-					return do_linear_fault(mm, vma, address,
-						pte, pmd, flags, entry);
-			}
-			return do_anonymous_page(mm, vma, address,
-						 pte, pmd, flags);
+			if (vma->vm_ops)
+				return do_linear_fault(mm, vma, address, pte,
+						pmd, flags, entry);
+
+			return do_anonymous_page(mm, vma, address, pte, pmd,
+					flags);
 		}
 		if (pte_file(entry))
 			return do_nonlinear_fault(mm, vma, address,
--
1.9.1