ext4: check magic even the extent block bh is verified

hulk inclusion
category: bugfix
bugzilla: 109246
CVE: NA
---------------------------

Our stress testing with IO error can trigger follow OOB with a very low
probability.

[59898.282466] BUG: KASAN: slab-out-of-bounds in ext4_find_extent+0x2e4/0x480
...
[59898.287162] Call Trace:
[59898.287575]  dump_stack+0x8b/0xb9
[59898.288070]  print_address_description+0x73/0x280
[59898.289903]  ext4_find_extent+0x2e4/0x480
[59898.290553]  ext4_ext_map_blocks+0x125/0x1470
[59898.295481]  ext4_map_blocks+0x5ee/0x940
[59898.315984]  ext4_mpage_readpages+0x63c/0xdb0
[59898.320231]  read_pages+0xe6/0x370
[59898.321589]  __do_page_cache_readahead+0x233/0x2a0
[59898.321594]  ondemand_readahead+0x157/0x450
[59898.321598]  generic_file_read_iter+0xcb2/0x1550
[59898.328828]  __vfs_read+0x233/0x360
[59898.328840]  vfs_read+0xa5/0x190
[59898.330126]  ksys_read+0xa5/0x150
[59898.331405]  do_syscall_64+0x6d/0x1f0
[59898.331418]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

Digging deep and we found it's actually a xattr block which can happened
with follow steps:

1. extent update for file1 and will remove a leaf extent block(block A)
2. we need update the idx extent block too
3. block A has been allocated as a xattr block and will set verified
3. io error happened for this idx block and will the buffer has been
   released late
4. extent find for file1 will read the idx block and see block A again
5. since the buffer of block A is already verified, we will use it
   directly, which can lead the upper OOB

Same as __ext4_xattr_check_block, we can check magic even the buffer is
verified to fix the problem.
Signed-off-by: Nyangerkun <yangerkun@huawei.com>
Reviewed-by: NZhang Yi <yi.zhang@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

fs/ext4/extents.c

@@ -590,6 +590,14 @@ __read_extent_tree_block(const char *function, unsigned int line,
 			goto errout;
 	}
 	if (buffer_verified(bh)) {
+		if (unlikely(ext_block_hdr(bh)->eh_magic != EXT4_EXT_MAGIC)) {
+			err = -EFSCORRUPTED;
+			ext4_error_inode(inode, function, line, 0,
+				"invalid magic for verified extent block %llu",
+				(unsigned long long)bh->b_blocknr);
+			goto errout;
+		}
+
 		if (!(flags & EXT4_EX_FORCE_CACHE))
 			return bh;
 	} else {