[PATCH 1/5] userfaultfd: use ENOENT instead of EFAULT if the atomic copy user fails

[Tyler Hicks]

From: Andrea Arcangeli <aarcange at redhat.com>

Patch series "userfaultfd shmem updates".

Jann found two bugs in the userfaultfd shmem MAP_SHARED backend: the
lack of the VM_MAYWRITE check and the lack of i_size checks.

Then looking into the above we also fixed the MAP_PRIVATE case.

Hugh by source review also found a data loss source if UFFDIO_COPY is
used on shmem MAP_SHARED PROT_READ mappings (the production usages
incidentally run with PROT_READ|PROT_WRITE, so the data loss couldn't
happen in those production usages like with QEMU).

The whole patchset is marked for stable.

We verified QEMU postcopy live migration with guest running on shmem
MAP_PRIVATE run as well as before after the fix of shmem MAP_PRIVATE.
Regardless if it's shmem or hugetlbfs or MAP_PRIVATE or MAP_SHARED, QEMU
unconditionally invokes a punch hole if the guest mapping is filebacked
and a MADV_DONTNEED too (needed to get rid of the MAP_PRIVATE COWs and
for the anon backend).

This patch (of 5):

We internally used EFAULT to communicate with the caller, switch to
ENOENT, so EFAULT can be used as a non internal retval.

Link: http://lkml.kernel.org/r/20181126173452.26955-2-aarcange@redhat.com
Fixes: 4c27fe4c4c84 ("userfaultfd: shmem: add shmem_mcopy_atomic_pte for userfaultfd support")
Signed-off-by: Andrea Arcangeli <aarcange at redhat.com>
Reviewed-by: Mike Rapoport <rppt at linux.ibm.com>
Reviewed-by: Hugh Dickins <hughd at google.com>
Cc: Mike Kravetz <mike.kravetz at oracle.com>
Cc: Jann Horn <jannh at google.com>
Cc: Peter Xu <peterx at redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert at redhat.com>
Cc: <stable at vger.kernel.org>
Cc: stable at vger.kernel.org
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>

CVE-2018-18397

(cherry picked from commit 9e368259ad988356c4c95150fafd1a06af095d98)
Signed-off-by: Tyler Hicks <tyhicks at canonical.com>
---
 mm/hugetlb.c     | 2 +-
 mm/shmem.c       | 2 +-
 mm/userfaultfd.c | 6 +++---
 3 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index b5bcf50a7d9f..f7a5539f8ddc 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4070,7 +4070,7 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
 
 		/* fallback to copy_from_user outside mmap_sem */
 		if (unlikely(ret)) {
-			ret = -EFAULT;
+			ret = -ENOENT;
 			*pagep = page;
 			/* don't free the page */
 			goto out;
diff --git a/mm/shmem.c b/mm/shmem.c
index 0c6023603b25..a8853fe0c8b6 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2307,7 +2307,7 @@ static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
 				*pagep = page;
 				shmem_inode_unacct_blocks(inode, 1);
 				/* don't free the page */
-				return -EFAULT;
+				return -ENOENT;
 			}
 		} else {		/* mfill_zeropage_atomic */
 			clear_highpage(page);
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 5029f241908f..46c8949e5f8f 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -48,7 +48,7 @@ static int mcopy_atomic_pte(struct mm_struct *dst_mm,
 
 		/* fallback to copy_from_user outside mmap_sem */
 		if (unlikely(ret)) {
-			ret = -EFAULT;
+			ret = -ENOENT;
 			*pagep = page;
 			/* don't free the page */
 			goto out;
@@ -274,7 +274,7 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
 
 		cond_resched();
 
-		if (unlikely(err == -EFAULT)) {
+		if (unlikely(err == -ENOENT)) {
 			up_read(&dst_mm->mmap_sem);
 			BUG_ON(!page);
 
@@ -530,7 +530,7 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
 				       src_addr, &page, zeropage);
 		cond_resched();
 
-		if (unlikely(err == -EFAULT)) {
+		if (unlikely(err == -ENOENT)) {
 			void *page_kaddr;
 
 			up_read(&dst_mm->mmap_sem);
-- 
2.7.4