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Date:   Mon,  4 Jul 2022 16:27:21 +0200
From:   Lukas Czerner <lczerner@...hat.com>
To:     linux-ext4@...r.kernel.org
Cc:     tytso@....edu
Subject: [PATCH 2/2] ext4: make sure ext4_append() always allocates new block

ext4_append() must always allocate a new block, otherwise we run the
risk of overwriting existing directory block corrupting the directory
tree in the process resulting in all manner of problems later on.

Add a sanity check to see if the logical block is already allocated and
error out if it is.

Signed-off-by: Lukas Czerner <lczerner@...hat.com>
---
 fs/ext4/namei.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index cf460aa4f81d..4af441494e09 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -54,6 +54,7 @@ static struct buffer_head *ext4_append(handle_t *handle,
 					struct inode *inode,
 					ext4_lblk_t *block)
 {
+	struct ext4_map_blocks map;
 	struct buffer_head *bh;
 	int err;
 
@@ -63,6 +64,21 @@ static struct buffer_head *ext4_append(handle_t *handle,
 		return ERR_PTR(-ENOSPC);
 
 	*block = inode->i_size >> inode->i_sb->s_blocksize_bits;
+	map.m_lblk = *block;
+	map.m_len = 1;
+
+	/*
+	 * We're appending new directory block. Make sure the block is not
+	 * allocated yet, otherwise we will end up corrupting the
+	 * directory.
+	 */
+	err = ext4_map_blocks(NULL, inode, &map, 0);
+	if (err < 0)
+		return ERR_PTR(err);
+	if (err) {
+		EXT4_ERROR_INODE(inode, "Logical block already allocated");
+		return ERR_PTR(-EFSCORRUPTED);
+	}
 
 	bh = ext4_bread(handle, inode, *block, EXT4_GET_BLOCKS_CREATE);
 	if (IS_ERR(bh))
-- 
2.35.3

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