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

From: Andrea Arcangeli <[email protected]>
Subject: 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.

---

 mm/hugetlb.c     |    2 +-
 mm/shmem.c       |    2 +-
 mm/userfaultfd.c |    6 +++---
 3 files changed, 5 insertions(+), 5 deletions(-)

@@ -4080,7 +4080,7 @@ int hugetlb_mcopy_atomic_pte(struct mm_s
 
 		/* fallback to copy_from_user outside mmap_sem */
 		if (unlikely(ret)) {
-			ret = -EFAULT;
+			ret = -ENOENT;
 			*pagep = page;
 			/* don't free the page */
 			goto out;

@@ -2238,7 +2238,7 @@ static int shmem_mfill_atomic_pte(struct
 				*pagep = page;
 				shmem_inode_unacct_blocks(inode, 1);
 				/* don't free the page */
-				return -EFAULT;
+				return -ENOENT;
 			}
 		} else {		/* mfill_zeropage_atomic */
 			clear_highpage(page);

@@ -48,7 +48,7 @@ static int mcopy_atomic_pte(struct mm_st
 
 		/* 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);

_