btrfs-progs: check: lowmem: fix crash when METADATA_ITEM has invalid level

[BUG]
When running lowmem mode with METADATA_ITEM which has invalid level, it
will crash with the following backtrace:

 (gdb) bt
 #0  0x0000555555616b0b in btrfs_header_bytenr (eb=0x4)
     at ./kernel-shared/ctree.h:2134
 #1  0x0000555555620c78 in check_tree_block_backref (root_id=5,
     bytenr=30457856, level=256) at check/mode-lowmem.c:3818
 #2  0x0000555555621f6c in check_extent_item (path=0x7fffffffd9c0)
     at check/mode-lowmem.c:4334
 #3  0x00005555556235a5 in check_leaf_items (root=0x555555688e10,
     path=0x7fffffffd9c0, nrefs=0x7fffffffda30, account_bytes=1)
     at check/mode-lowmem.c:4835
 #4  0x0000555555623c6d in walk_down_tree (root=0x555555688e10,
     path=0x7fffffffd9c0, level=0x7fffffffd984, nrefs=0x7fffffffda30,
     check_all=1) at check/mode-lowmem.c:4967
 #5  0x000055555562494f in check_btrfs_root (root=0x555555688e10, check_all=1)
     at check/mode-lowmem.c:5266
 #6  0x00005555556254ee in check_chunks_and_extents_lowmem ()
     at check/mode-lowmem.c:5556
 #7  0x00005555555f0b82 in do_check_chunks_and_extents () at check/main.c:9114
 #8  0x00005555555f50ea in cmd_check (cmd=0x55555567c640 <cmd_struct_check>,
     argc=3, argv=0x7fffffffdec0) at check/main.c:10892
 #9  0x000055555556b2b1 in cmd_execute (argv=0x7fffffffdec0, argc=3,
     cmd=0x55555567c640 <cmd_struct_check>) at cmds/commands.h:125

[CAUSE]
For function check_extent_item() it will go through inline extent items
and then check their backrefs.

But for METADATA_ITEM, it doesn't really validate key.offset, which is
u64 and can contain value way larger than BTRFS_MAX_LEVEL (mostly caused
by bit flip).

In that case, if we have a larger value like 256 in key.offset, then
later check_tree_block_backref() will use 256 as level, and overflow
path->nodes[level] and crash.

[FIX]
Just verify the level, no matter if it's from btrfs_tree_block_level()
(which is just u8), or it's from key.offset (which is u64).

To do the check properly and detect higher bits corruption, also change
the type of @Level from u8 to u64.

Now lowmem mode can detect the problem properly:

 ...
 [2/7] checking extents
 ERROR: tree block 30457856 has bad backref level, has 256 expect [0, 7]
 ERROR: extent[30457856 16384] level mismatch, wanted: 0, have: 256
 ERROR: errors found in extent allocation tree or chunk allocation
 [3/7] checking free space tree
 ...

Reviewed-by: Su Yue <l@damenly.su>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: adam900710

check/mode-lowmem.c

@@ -4242,7 +4242,8 @@ static int check_extent_item(struct btrfs_path *path)
 	u64 owner_offset;
 	u64 super_gen;
 	int metadata = 0;
-	int level;
+	/* To handle corrupted values in skinny backref */
+	u64 level;
 	struct btrfs_key key;
 	int ret;
 	int err = 0;
@@ -4303,6 +4304,16 @@ static int check_extent_item(struct btrfs_path *path)
 		/* New METADATA_ITEM */
 		level = key.offset;
 	}
+
+	if (metadata && level >= BTRFS_MAX_LEVEL) {
+		error(
+		"tree block %llu has bad backref level, has %llu expect [0, %u]",
+		      key.objectid, level, BTRFS_MAX_LEVEL - 1);
+		err |= BACKREF_MISMATCH;
+		/* This is a critical error, exit right now */
+		goto out;
+	}
+
 	ptr_offset = ptr - (unsigned long)ei;
 
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