userfaultfd: use ENOENT instead of EFAULT if the atomic copy user fails

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@redhat.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Hugh Dickins <hughd@google.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Jann Horn <jannh@google.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: <stable@vger.kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 5 insertions, 5 deletions

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 7f2a28ab46d537..705a3e9cc910e1 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4080,7 +4080,7 @@ 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 42b70978e81489..6c54a6874e4143 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2238,7 +2238,7 @@ 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 5029f241908f48..46c8949e5f8f83 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 @@ retry:
 
 		cond_resched();
 
-		if (unlikely(err == -EFAULT)) {
+		if (unlikely(err == -ENOENT)) {
 			up_read(&dst_mm->mmap_sem);
 			BUG_ON(!page);
 
@@ -530,7 +530,7 @@ retry:
 				       src_addr, &page, zeropage);
 		cond_resched();
 
-		if (unlikely(err == -EFAULT)) {
+		if (unlikely(err == -ENOENT)) {
 			void *page_kaddr;
 
 			up_read(&dst_mm->mmap_sem);