linux-kernel - [PATCH 4.19 22/41] userfaultfd: shmem: allocate anonymous memory for MAP

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Message-Id: <20181206142951.633608581@linuxfoundation.org>
Date:   Thu,  6 Dec 2018 15:39:02 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Andrea Arcangeli <aarcange@...hat.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Hugh Dickins <hughd@...gle.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Jann Horn <jannh@...gle.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Peter Xu <peterx@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 4.19 22/41] userfaultfd: shmem: allocate anonymous memory for MAP_PRIVATE shmem

4.19-stable review patch.  If anyone has any objections, please let me know.

------------------

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

commit 5b51072e97d587186c2f5390c8c9c1fb7e179505 upstream.

Userfaultfd did not create private memory when UFFDIO_COPY was invoked
on a MAP_PRIVATE shmem mapping.  Instead it wrote to the shmem file,
even when that had not been opened for writing.  Though, fortunately,
that could only happen where there was a hole in the file.

Fix the shmem-backed implementation of UFFDIO_COPY to create private
memory for MAP_PRIVATE mappings.  The hugetlbfs-backed implementation
was already correct.

This change is visible to userland, if userfaultfd has been used in
unintended ways: so it introduces a small risk of incompatibility, but
is necessary in order to respect file permissions.

An app that uses UFFDIO_COPY for anything like postcopy live migration
won't notice the difference, and in fact it'll run faster because there
will be no copy-on-write and memory waste in the tmpfs pagecache
anymore.

Userfaults on MAP_PRIVATE shmem keep triggering only on file holes like
before.

The real zeropage can also be built on a MAP_PRIVATE shmem mapping
through UFFDIO_ZEROPAGE and that's safe because the zeropage pte is
never dirty, in turn even an mprotect upgrading the vma permission from
PROT_READ to PROT_READ|PROT_WRITE won't make the zeropage pte writable.

Link: http://lkml.kernel.org/r/20181126173452.26955-3-aarcange@redhat.com
Fixes: 4c27fe4c4c84 ("userfaultfd: shmem: add shmem_mcopy_atomic_pte for userfaultfd support")
Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
Reported-by: Mike Rapoport <rppt@...ux.ibm.com>
Reviewed-by: Hugh Dickins <hughd@...gle.com>
Cc: <stable@...r.kernel.org>
Cc: "Dr. David Alan Gilbert" <dgilbert@...hat.com>
Cc: Jann Horn <jannh@...gle.com>
Cc: Mike Kravetz <mike.kravetz@...cle.com>
Cc: Peter Xu <peterx@...hat.com>
Cc: stable@...r.kernel.org
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 mm/userfaultfd.c |   15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -381,7 +381,17 @@ static __always_inline ssize_t mfill_ato
 {
 	ssize_t err;

-	if (vma_is_anonymous(dst_vma)) {
+	/*
+	 * The normal page fault path for a shmem will invoke the
+	 * fault, fill the hole in the file and COW it right away. The
+	 * result generates plain anonymous memory. So when we are
+	 * asked to fill an hole in a MAP_PRIVATE shmem mapping, we'll
+	 * generate anonymous memory directly without actually filling
+	 * the hole. For the MAP_PRIVATE case the robustness check
+	 * only happens in the pagetable (to verify it's still none)
+	 * and not in the radix tree.
+	 */
+	if (!(dst_vma->vm_flags & VM_SHARED)) {
 		if (!zeropage)
 			err = mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma,
 					       dst_addr, src_addr, page);
@@ -486,7 +496,8 @@ retry:
 	 * dst_vma.
 	 */
 	err = -ENOMEM;
-	if (vma_is_anonymous(dst_vma) && unlikely(anon_vma_prepare(dst_vma)))
+	if (!(dst_vma->vm_flags & VM_SHARED) &&
+	    unlikely(anon_vma_prepare(dst_vma)))
 		goto out_unlock;

 	while (src_addr < src_start + len) {