The patch below does not apply to the 4.4-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 02a3307aa9c20b4f6626255b028f07f6cfa16feb Mon Sep 17 00:00:00 2001

From: Liu Bo bo.liu@linux.alibaba.com Date: Wed, 16 May 2018 01:37:36 +0800 Subject: [PATCH] btrfs: fix reading stale metadata blocks after degraded raid1 mounts

If a btree block, aka. extent buffer, is not available in the extent buffer cache, it'll be read out from the disk instead, i.e.

btrfs_search_slot() read_block_for_search() # hold parent and its lock, go to read child btrfs_release_path() read_tree_block() # read child

Unfortunately, the parent lock got released before reading child, so commit 5bdd3536cbbe ("Btrfs: Fix block generation verification race") had used 0 as parent transid to read the child block. It forces read_tree_block() not to check if parent transid is different with the generation id of the child that it reads out from disk.

A simple PoC is included in btrfs/124,

0. A two-disk raid1 btrfs,

1. Right after mkfs.btrfs, block A is allocated to be device tree's root.

2. Mount this filesystem and put it in use, after a while, device tree's root got COW but block A hasn't been allocated/overwritten yet.

3. Umount it and reload the btrfs module to remove both disks from the global @fs_devices list.

4. mount -odegraded dev1 and write some data, so now block A is allocated to be a leaf in checksum tree. Note that only dev1 has the latest metadata of this filesystem.

5. Umount it and mount it again normally (with both disks), since raid1 can pick up one disk by the writer task's pid, if btrfs_search_slot() needs to read block A, dev2 which does NOT have the latest metadata might be read for block A, then we got a stale block A.

6. As parent transid is not checked, block A is marked as uptodate and put into the extent buffer cache, so the future search won't bother to read disk again, which means it'll make changes on this stale one and make it dirty and flush it onto disk.

To avoid the problem, parent transid needs to be passed to read_tree_block().

In order to get a valid parent transid, we need to hold the parent's lock until finishing reading child.

This patch needs to be slightly adapted for stable kernels, the &first_key parameter added to read_tree_block() is from 4.16+ (581c1760415c4). The fix is to replace 0 by 'gen'.

Fixes: 5bdd3536cbbe ("Btrfs: Fix block generation verification race") CC: stable@vger.kernel.org # 4.4+ Signed-off-by: Liu Bo bo.liu@linux.alibaba.com Reviewed-by: Filipe Manana fdmanana@suse.com Reviewed-by: Qu Wenruo wqu@suse.com [ update changelog ] Signed-off-by: David Sterba dsterba@suse.com

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c index 63488f0b850f..8c68961925b1 100644 --- a/fs/btrfs/ctree.c +++ b/fs/btrfs/ctree.c @@ -2436,10 +2436,8 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p, if (p->reada != READA_NONE) reada_for_search(fs_info, p, level, slot, key->objectid);

- btrfs_release_path(p); - ret = -EAGAIN; - tmp = read_tree_block(fs_info, blocknr, 0, parent_level - 1, + tmp = read_tree_block(fs_info, blocknr, gen, parent_level - 1, &first_key); if (!IS_ERR(tmp)) { /* @@ -2454,6 +2452,8 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p, } else { ret = PTR_ERR(tmp); } + + btrfs_release_path(p); return ret; }