From: Josef Bacik <josef@toxicpanda.com>
Date: Fri, 13 Mar 2020 11:44:46 -0400
Git-commit: f28de8d8fd376ba57aaed53cdc1a0ba2238e8ebf
Patch-mainline: 5.7-rc1
References: bsc#1176019
Subject: [PATCH] btrfs: clear DEAD_RELOC_TREE before dropping the reloc root

The DEAD_RELOC_TREE flag is in place in order to avoid a use after free
in init_reloc_root, tracking the presence of reloc_root.  However adding
the explicit tree references in previous patches makes the use after
free impossible because at this point we no longer have a reloc_control
set on the fs_info and thus cannot enter the function.

So move this to be coupled with clearing the root->reloc_root so we're
consistent with all other operations of the reloc root.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
---
 fs/btrfs/relocation.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index a0c8f7949a19..54724ae45380 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -2275,18 +2275,18 @@ static int clean_dirty_subvols(struct reloc_control *rc)
 
 			list_del_init(&root->reloc_dirty_list);
 			root->reloc_root = NULL;
-			if (reloc_root) {
-
-				ret2 = btrfs_drop_snapshot(reloc_root, 0, 1);
-				if (ret2 < 0 && !ret)
-					ret = ret2;
-			}
 			/*
 			 * Need barrier to ensure clear_bit() only happens after
 			 * root->reloc_root = NULL. Pairs with have_reloc_root.
 			 */
 			smp_wmb();
 			clear_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state);
+
+			if (reloc_root) {
+				ret2 = btrfs_drop_snapshot(reloc_root, 0, 1);
+				if (ret2 < 0 && !ret)
+					ret = ret2;
+			}
 			btrfs_put_root(root);
 		} else {
 			/* Orphan reloc tree, just clean it up */
-- 
2.26.2