eCryptfs: Write bug for non-ecryptfs file

The following code segment in ecryptfs_write_begin(mmap.c) is problematic,

1 if (!PageUptodate(page)) {
2 if (!(crypt_stat->flags & ECRYPTFS_ENCRYPTED)) {
3 ...
4 } else if(crypt_stat->flags & ECRYPTFS_VIEW_AS_ENCRYPTED) {
5 ...
6 } else {
7 if (prev_page_end_size >= i_size_read(page->mapping->host)) {
8 zero_user(page, 0, PAGE_CACHE_SIZE);
9 }
10 ...
11 }
12 ...
13 /* Writing to a new page, and creating a small hole from start
14 * of page? Zero it out. */
15 if ((i_size_read(mapping->host)==prev_page_end_size) && (pos!=0))
16 zero_user(page, 0, PAGE_CACHE_SIZE);

1 The check on 'pos!=0' for the IF statement in line 15 implies that while
pos==0, the page needs not be zeroed. Unfortunately, that is not true, suppose an empty non-ecryptfs file has been created beforehand, currently the writter want to write at pos==0 with a length of m bytes, m<PAGE_CACHE_SIZE, now the page will not be zeroed, and set to Uptodate! Then the data within (m,PAGE_CACHE_SIZE) read by any reader will be undefined.
2 Even remove 'pos!=0', there is further more a case which does not be taken
into account,
if a file satisfies (i_size_read(mapping->host) > prev_page_end_size), and is non-ecryptfs file, then the page is not zeroed, but the data within (i_size_read(mapping->host), (prev_page_end_size+1)<<PAGE_CACHE_SHIFT) need be zeroed as well.
3 For ecryptfs file, if satisfies (prev_page_end_size == i_size_read(page->
mapping->host) && pos!=0, this page will be double zeroed.

This patch solves the above problem.

---
 fs/ecryptfs/mmap.c |    7 ++-----
 1 files changed, 2 insertions(+), 5 deletions(-)

index 10ec695..27c0da8 100644
@@ -308,6 +308,8 @@ static int ecryptfs_write_begin(struct file *file,
 			&ecryptfs_inode_to_private(mapping->host)->crypt_stat;
 
 		if (!(crypt_stat->flags & ECRYPTFS_ENCRYPTED)) {
+			if ((i_size_read(mapping->host) >> PAGE_CACHE_SHIFT) <= index)
+				zero_user(page, 0, PAGE_CACHE_SIZE);
 			rc = ecryptfs_read_lower_page_segment(
 				page, index, 0, PAGE_CACHE_SIZE, mapping->host);
 			if (rc) {

@@ -379,11 +381,6 @@ static int ecryptfs_write_begin(struct file *file,
 			}
 		}
 	}
-	/* Writing to a new page, and creating a small hole from start
-	 * of page?  Zero it out. */
-	if ((i_size_read(mapping->host) == prev_page_end_size)
-	    && (pos != 0))
-		zero_user(page, 0, PAGE_CACHE_SIZE);
 out:
 	if (unlikely(rc)) {
 		unlock_page(page);

1.7.6.5