From: hujianyang <hujianyang@huawei.com>
Date: Wed, 30 Apr 2014 06:06:06 +0000 (+0800)
Subject: UBIFS: fix an mmap and fsync race condition
X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=ac8df9ec7b4e25b87d5a71dfd9af4d8076d66bff;p=GitHub%2Fmt8127%2Fandroid_kernel_alcatel_ttab.git

UBIFS: fix an mmap and fsync race condition

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@huawei.com>
Tested-by: Laurence Withers <lwithers@guralp.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---

diff --git a/fs/ubifs/file.c b/fs/ubifs/file.c
index 14374530784c..881324c08430 100644
--- a/fs/ubifs/file.c
+++ b/fs/ubifs/file.c
@@ -1524,8 +1524,7 @@ static int ubifs_vm_page_mkwrite(struct vm_area_struct *vma,
 	}
 
 	wait_for_stable_page(page);
-	unlock_page(page);
-	return 0;
+	return VM_FAULT_LOCKED;
 
 out_unlock:
 	unlock_page(page);