Patch "dax: fix PMD data corruption when fault races with write" has been added to the 4.11-stable tree

This is a note to let you know that I've just added the patch titled

dax: fix PMD data corruption when fault races with write

to the 4.11-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
dax-fix-pmd-data-corruption-when-fault-races-with-write.patch
and it can be found in the queue-4.11 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Ross Zwisler <[email protected]>
Date: Fri, 12 May 2017 15:47:00 -0700
Subject: dax: fix PMD data corruption when fault races with write

From: Ross Zwisler <[email protected]>

commit 876f29460cbd4086b43475890c1bf2488fa11d40 upstream.

This is based on a patch from Jan Kara that fixed the equivalent race in the DAX PTE fault path.

Currently DAX PMD read fault can race with write(2) in the following way:

CPU1 - write(2) CPU2 - read fault

                                dax_iomap_pmd_fault()
                                  ->iomap_begin() - sees hole

dax_iomap_rw()
iomap_apply()
->iomap_begin - allocates blocks
dax_iomap_actor()
invalidate_inode_pages2_range()
- there's nothing to invalidate

grab_mapping_entry()
- we add huge zero page to the radix tree and map it to page tables

The result is that hole page is mapped into page tables (and thus zeros are seen in mmap) while file has data written in that place.

Fix the problem by locking exception entry before mapping blocks for the fault. That way we are sure invalidate_inode_pages2_range() call for racing write will either block on entry lock waiting for the fault to finish (and unmap stale page tables after that) or read fault will see already allocated blocks by write(2).

---
 fs/dax.c |   28 ++++++++++++++--------------
 1 file changed, 14 insertions(+), 14 deletions(-)

@@ -1353,6 +1353,16 @@ static int dax_iomap_pmd_fault(struct vm
 		goto fallback;
 
 	/*
+	 * grab_mapping_entry() will make sure we get a 2M empty entry, a DAX
+	 * PMD or a HZP entry.  If it can't (because a 4k page is already in
+	 * the tree, for instance), it will return -EEXIST and we just fall
+	 * back to 4k entries.
+	 */
+	entry = grab_mapping_entry(mapping, pgoff, RADIX_DAX_PMD);
+	if (IS_ERR(entry))
+		goto fallback;
+
+	/*
 	 * Note that we don't use iomap_apply here.  We aren't doing I/O, only
 	 * setting up a mapping, so really we're using iomap_begin() as a way
 	 * to look up our filesystem block.

@@ -1360,21 +1370,11 @@ static int dax_iomap_pmd_fault(struct vm
 	pos = (loff_t)pgoff << PAGE_SHIFT;
 	error = ops->iomap_begin(inode, pos, PMD_SIZE, iomap_flags, &iomap);
 	if (error)
-		goto fallback;
+		goto unlock_entry;
 
 	if (iomap.offset + iomap.length < pos + PMD_SIZE)
 		goto finish_iomap;
 
-	/*
-	 * grab_mapping_entry() will make sure we get a 2M empty entry, a DAX
-	 * PMD or a HZP entry.  If it can't (because a 4k page is already in
-	 * the tree, for instance), it will return -EEXIST and we just fall
-	 * back to 4k entries.
-	 */
-	entry = grab_mapping_entry(mapping, pgoff, RADIX_DAX_PMD);
-	if (IS_ERR(entry))
-		goto finish_iomap;
-
 	switch (iomap.type) {
 	case IOMAP_MAPPED:
 		result = dax_pmd_insert_mapping(vmf, &iomap, pos, &entry);

@@ -1382,7 +1382,7 @@ static int dax_iomap_pmd_fault(struct vm
 	case IOMAP_UNWRITTEN:
 	case IOMAP_HOLE:
 		if (WARN_ON_ONCE(write))
-			goto unlock_entry;
+			break;
 		result = dax_pmd_load_hole(vmf, &iomap, &entry);
 		break;
 	default:

@@ -1390,8 +1390,6 @@ static int dax_iomap_pmd_fault(struct vm
 		break;
 	}
 
- unlock_entry:
-	put_locked_mapping_entry(mapping, pgoff, entry);
  finish_iomap:
 	if (ops->iomap_end) {
 		int copied = PMD_SIZE;

@@ -1407,6 +1405,8 @@ static int dax_iomap_pmd_fault(struct vm
 		ops->iomap_end(inode, pos, PMD_SIZE, copied, iomap_flags,
 				&iomap);
 	}
+ unlock_entry:
+	put_locked_mapping_entry(mapping, pgoff, entry);
  fallback:
 	if (result == VM_FAULT_FALLBACK) {
 		split_huge_pmd(vma, vmf->pmd, vmf->address);

Patches currently in stable-queue which might be from [email protected] are

queue-4.11/fs-block_dev-always-invalidate-cleancache-in-invalidate_bdev.patch queue-4.11/ext4-return-to-starting-transaction-in-ext4_dax_huge_fault.patch queue-4.11/dax-prevent-invalidation-of-mapped-dax-entries.patch queue-4.11/dax-fix-pmd-data-corruption-when-fault-races-with-write.patch queue-4.11/fs-fix-data-invalidation-in-the-cleancache-during-direct-io.patch queue-4.11/mm-fix-data-corruption-due-to-stale-mmap-reads.patch