ext4: zero out the unused memory region in the extent tree block

This commit zeroes out the unused memory region in the buffer_head
corresponding to the extent metablock after writing the extent header
and the corresponding extent node entries.

This is done to prevent random uninitialized data from getting into
the filesystem when the extent block is synced.

This fixes CVE-2019-11833.
Signed-off-by: Sriram Rajagopalan <sriramr@arista.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org

fs/ext4/extents.c

@@ -1035,6 +1035,7 @@ static int ext4_ext_split(handle_t *handle, struct inode *inode,
 	__le32 border;
 	ext4_fsblk_t *ablocks = NULL; /* array of allocated blocks */
 	int err = 0;
+	size_t ext_size = 0;
 
 	/* make decision: where to split? */
 	/* FIXME: now decision is simplest: at current extent */
@@ -1126,6 +1127,10 @@ static int ext4_ext_split(handle_t *handle, struct inode *inode,
 		le16_add_cpu(&neh->eh_entries, m);
 	}
 
+	/* zero out unused area in the extent block */
+	ext_size = sizeof(struct ext4_extent_header) +
+		sizeof(struct ext4_extent) * le16_to_cpu(neh->eh_entries);
+	memset(bh->b_data + ext_size, 0, inode->i_sb->s_blocksize - ext_size);
 	ext4_extent_block_csum_set(inode, neh);
 	set_buffer_uptodate(bh);
 	unlock_buffer(bh);
@@ -1205,6 +1210,11 @@ static int ext4_ext_split(handle_t *handle, struct inode *inode,
 				sizeof(struct ext4_extent_idx) * m);
 			le16_add_cpu(&neh->eh_entries, m);
 		}
+		/* zero out unused area in the extent block */
+		ext_size = sizeof(struct ext4_extent_header) +
+		   (sizeof(struct ext4_extent) * le16_to_cpu(neh->eh_entries));
+		memset(bh->b_data + ext_size, 0,
+			inode->i_sb->s_blocksize - ext_size);
 		ext4_extent_block_csum_set(inode, neh);
 		set_buffer_uptodate(bh);
 		unlock_buffer(bh);
@@ -1270,6 +1280,7 @@ static int ext4_ext_grow_indepth(handle_t *handle, struct inode *inode,
 	ext4_fsblk_t newblock, goal = 0;
 	struct ext4_super_block *es = EXT4_SB(inode->i_sb)->s_es;
 	int err = 0;
+	size_t ext_size = 0;
 
 	/* Try to prepend new index to old one */
 	if (ext_depth(inode))
@@ -1295,9 +1306,11 @@ static int ext4_ext_grow_indepth(handle_t *handle, struct inode *inode,
 		goto out;
 	}
 
+	ext_size = sizeof(EXT4_I(inode)->i_data);
 	/* move top-level index/leaf into new block */
-	memmove(bh->b_data, EXT4_I(inode)->i_data,
-		sizeof(EXT4_I(inode)->i_data));
+	memmove(bh->b_data, EXT4_I(inode)->i_data, ext_size);
+	/* zero out unused area in the extent block */
+	memset(bh->b_data + ext_size, 0, inode->i_sb->s_blocksize - ext_size);
 
 	/* set size of new block */
 	neh = ext_block_hdr(bh);