[PATCH] btrfs: verify the tranisd of the to-be-written dirty extent buffer

[Qu Wenruo <wqu@suse.com>]

[BUG]
There is a bug report that a bitflip in the transid part of an extent
buffer makes btrfs to reject certain tree blocks:

  BTRFS error (device dm-0): parent transid verify failed on 1382301696 wanted 262166 found 22

[CAUSE]
Note the failed transid check, hex(262166) = 0x40016, while
hex(22) = 0x16.

It's an obvious bitflip.

Furthermore, the reporter also confirmed the bitflip is from the
hardware, so it's a real hardware caused bitflip, and such problem can
not be detected by the existing tree-checker framework.

As tree-checker can only verify the content inside one tree block, while
generation of a tree block can only be verified against its parent.

So such problem remain undetected.

[FIX]
Although tree-checker can not verify it at write-time, we still have a
quick (but not the most accurate) way to catch such obvious corruption.

Function csum_one_extent_buffer() is called before we submit metadata
write.

Thus it means, all the extent buffer passed in should be dirty tree
blocks, and should be newer than last committed transaction.

Using that we can catch the above bitflip.

Although it's not a perfect solution, as if the corrupted generation is
higher than the correct value, we have no way to catch it at all.

Reported-by: Christoph Anton Mitterer <calestyo@scientia.org>
Link: https://lore.kernel.org/linux-btrfs/2dfcbc130c55cc6fd067b93752e90bd2b079baca.camel@scientia.org/
Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/disk-io.c | 26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index b6a81c39d5f4..a89aa523413b 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -441,17 +441,31 @@ static int csum_one_extent_buffer(struct extent_buffer *eb)
 	else
 		ret = btrfs_check_leaf_full(eb);
 
-	if (ret < 0) {
-		btrfs_print_tree(eb, 0);
+	if (ret < 0)
+		goto error;
+
+	/*
+	 * Also check the generation, the eb reached here must be newer than
+	 * last committed. Or something seriously wrong happened.
+	 */
+	if (btrfs_header_generation(eb) <= fs_info->last_trans_committed) {
+		ret = -EUCLEAN;
 		btrfs_err(fs_info,
-			"block=%llu write time tree block corruption detected",
-			eb->start);
-		WARN_ON(IS_ENABLED(CONFIG_BTRFS_DEBUG));
-		return ret;
+			"block=%llu bad generation, have %llu expect > %llu",
+			  eb->start, btrfs_header_generation(eb),
+			  fs_info->last_trans_committed);
+		goto error;
 	}
 	write_extent_buffer(eb, result, 0, fs_info->csum_size);
 
 	return 0;
+error:
+	btrfs_print_tree(eb, 0);
+	btrfs_err(fs_info,
+		"block=%llu write time tree block corruption detected",
+		eb->start);
+	WARN_ON(IS_ENABLED(CONFIG_BTRFS_DEBUG));
+	return ret;
 }
 
 /* Checksum all dirty extent buffers in one bio_vec */
-- 
2.35.1