[PATCH 3.2 004/125] UBIFS: fix an mmap and fsync race condition

[Ben Hutchings]

3.2.61-rc1 review patch.  If anyone has any objections, please let me know.

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

From: hujianyang <hujianyang@xxxxxxxxxx>

commit 691a7c6f28ac90cccd0dbcf81348ea90b211bdd0 upstream.

There is a race condition in UBIFS:

Thread A (mmap)                        Thread B (fsync)

->__do_fault                           ->write_cache_pages
   -> ubifs_vm_page_mkwrite
       -> budget_space
       -> lock_page
       -> release/convert_page_budget
       -> SetPagePrivate
       -> TestSetPageDirty
       -> unlock_page
                                       -> lock_page
                                           -> TestClearPageDirty
                                           -> ubifs_writepage
                                               -> do_writepage
                                                   -> release_budget
                                                   -> ClearPagePrivate
                                                   -> unlock_page
   -> !(ret & VM_FAULT_LOCKED)
   -> lock_page
   -> set_page_dirty
       -> ubifs_set_page_dirty
           -> TestSetPageDirty (set page dirty without budgeting)
   -> unlock_page

This leads to situation where we have a diry page but no budget allocated for
this page, so further write-back may fail with -ENOSPC.

In this fix we return from page_mkwrite without performing unlock_page. We
return VM_FAULT_LOCKED instead. After doing this, the race above will not
happen.

Signed-off-by: hujianyang <hujianyang@xxxxxxxxxx>
Tested-by: Laurence Withers <lwithers@xxxxxxxxxx>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@xxxxxxxxxxxxxxx>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 fs/ubifs/file.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/fs/ubifs/file.c
+++ b/fs/ubifs/file.c
@@ -1522,8 +1522,7 @@ static int ubifs_vm_page_mkwrite(struct
 			ubifs_release_dirty_inode_budget(c, ui);
 	}
 
-	unlock_page(page);
-	return 0;
+	return VM_FAULT_LOCKED;
 
 out_unlock:
 	unlock_page(page);

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