ext4: WARN if a full dir leaf block has only one dentry

The maximum length of a filename is 255 and the minimum block size is 1024,
so it is always guaranteed that the number of entries is greater than or
equal to 2 when do_split() is called. So unless ext4_dx_add_entry() and
make_indexed_dir() or some other functions are buggy, 'split == 0' will
not occur.

Setting 'continued' to 0 in this case masks the problem that the file
system has become corrupted, even though it prevents possible out-of-bounds
access. Hence WARN_ON_ONCE() is used to check if 'split' is 0, and if it is
then warns and returns an error to abort split.

Suggested-by: Theodore Ts'o <tytso@mit.edu>
Link: https://lore.kernel.org/r/20240823160518.GA424729@mit.edu
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20241008121152.3771906-1-libaokun@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 790db7eac6c2ad5e1790e363e4ac273162e35013..08d15cd2b5946fbadb92962891026067a9e269e3 100644 (file)
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -2000,8 +2000,17 @@ static struct ext4_dir_entry_2 *do_split(handle_t *handle, struct inode *dir,
        else
                split = count/2;
 
+       if (WARN_ON_ONCE(split == 0)) {
+               /* Should never happen, but avoid out-of-bounds access below */
+               ext4_error_inode_block(dir, (*bh)->b_blocknr, 0,
+                       "bad indexed directory? hash=%08x:%08x count=%d move=%u",
+                       hinfo->hash, hinfo->minor_hash, count, move);
+               err = -EFSCORRUPTED;
+               goto out;
+       }
+
        hash2 = map[split].hash;
-       continued = split > 0 ? hash2 == map[split - 1].hash : 0;
+       continued = hash2 == map[split - 1].hash;
        dxtrace(printk(KERN_INFO "Split block %lu at %x, %i/%i\n",
                        (unsigned long)dx_get_block(frame->at),
                                        hash2, split, count-split));
@@ -2043,10 +2052,11 @@ static struct ext4_dir_entry_2 *do_split(handle_t *handle, struct inode *dir,
        return de;
 
 journal_error:
+       ext4_std_error(dir->i_sb, err);
+out:
        brelse(*bh);
        brelse(bh2);
        *bh = NULL;
-       ext4_std_error(dir->i_sb, err);
        return ERR_PTR(err);
 }