[PATCH 3.19.y-ckt 01/92] ext4: allocate entire range in zero range

[Kamal Mostafa]

3.19.8-ckt1 -stable review patch.  If anyone has any objections, please let me know.

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

From: Lukas Czerner <lczerner at redhat.com>

commit 0f2af21aae11972fa924374ddcf52e88347cf5a8 upstream.

Currently there is a bug in zero range code which causes zero range
calls to only allocate block aligned portion of the range, while
ignoring the rest in some cases.

In some cases, namely if the end of the range is past i_size, we do
attempt to preallocate the last nonaligned block. However this might
cause kernel to BUG() in some carefully designed zero range requests
on setups where page size > block size.

Fix this problem by first preallocating the entire range, including
the nonaligned edges and converting the written extents to unwritten
in the next step. This approach will also give us the advantage of
having the range to be as linearly contiguous as possible.

Signed-off-by: Lukas Czerner <lczerner at redhat.com>
Signed-off-by: Theodore Ts'o <tytso at mit.edu>
Reference: CVE-2015-0275
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 fs/ext4/extents.c | 31 +++++++++++++++++++------------
 1 file changed, 19 insertions(+), 12 deletions(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index bed4308..aa52242 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -4803,12 +4803,6 @@ static long ext4_zero_range(struct file *file, loff_t offset,
 	else
 		max_blocks -= lblk;
 
-	flags = EXT4_GET_BLOCKS_CREATE_UNWRIT_EXT |
-		EXT4_GET_BLOCKS_CONVERT_UNWRITTEN |
-		EXT4_EX_NOCACHE;
-	if (mode & FALLOC_FL_KEEP_SIZE)
-		flags |= EXT4_GET_BLOCKS_KEEP_SIZE;
-
 	mutex_lock(&inode->i_mutex);
 
 	/*
@@ -4825,15 +4819,28 @@ static long ext4_zero_range(struct file *file, loff_t offset,
 		ret = inode_newsize_ok(inode, new_size);
 		if (ret)
 			goto out_mutex;
-		/*
-		 * If we have a partial block after EOF we have to allocate
-		 * the entire block.
-		 */
-		if (partial_end)
-			max_blocks += 1;
 	}
 
+	flags = EXT4_GET_BLOCKS_CREATE_UNWRIT_EXT;
+	if (mode & FALLOC_FL_KEEP_SIZE)
+		flags |= EXT4_GET_BLOCKS_KEEP_SIZE;
+
+	/* Preallocate the range including the unaligned edges */
+	if (partial_begin || partial_end) {
+		ret = ext4_alloc_file_blocks(file,
+				round_down(offset, 1 << blkbits) >> blkbits,
+				(round_up((offset + len), 1 << blkbits) -
+				 round_down(offset, 1 << blkbits)) >> blkbits,
+				new_size, flags, mode);
+		if (ret)
+			goto out_mutex;
+
+	}
+
+	/* Zero range excluding the unaligned edges */
 	if (max_blocks > 0) {
+		flags |= (EXT4_GET_BLOCKS_CONVERT_UNWRITTEN |
+			  EXT4_EX_NOCACHE);
 
 		/* Now release the pages and zero block aligned part of pages*/
 		truncate_pagecache_range(inode, start, end - 1);
-- 
1.9.1