[PATCH] btrfs: add a sanity check for csum root before fill the data csum

[Edward Adam Davis <eadavis@qq.com>]

Syzbot reported a null-ptr-deref in btrfs_lookup_csums_bitmap.
The btrfs info contains IGNOREDATACSUMS, which prevents the csum root from
being loaded.
Before filling in the csum data, check the flag BTRFS_FS_STATE_NO_DATA_CSUMS
to confirm that the csum root has been loaded.

Reported-and-tested-by: syzbot+5d2b33d7835870519b5f@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=5d2b33d7835870519b5f
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
---
 fs/btrfs/scrub.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
index 3a3427428074..1ba4d8ba902b 100644
--- a/fs/btrfs/scrub.c
+++ b/fs/btrfs/scrub.c
@@ -1602,7 +1602,8 @@ static int scrub_find_fill_first_stripe(struct btrfs_block_group *bg,
 	}
 
 	/* Now fill the data csum. */
-	if (bg->flags & BTRFS_BLOCK_GROUP_DATA) {
+	if (!test_bit(BTRFS_FS_STATE_NO_DATA_CSUMS, &fs_info->fs_state) &&
+	    bg->flags & BTRFS_BLOCK_GROUP_DATA) {
 		int sector_nr;
 		unsigned long csum_bitmap = 0;
 
-- 
2.43.0