btrfs: harden block_group::bg_list against list_del() races

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: btrfs: harden block_group::bg_list against list_del() races As far as I can tell, these calls of list_del_init() on bg_list cannot run concurrently with btrfs_mark_bg_unused() or btrfs_mark_bg_to_reclaim(), as they are in transaction error paths and situations where the block group is readonly. However, if there is any chance at all of racing with mark_bg_unused(), or a different future user of bg_list, better to be safe than sorry. Otherwise we risk the following interleaving (bg_list refcount in parens) T1 (some random op) T2 (btrfs_mark_bg_unused) !list_empty(&bg->bg_list); (1) list_del_init(&bg->bg_list); (1) list_move_tail (1) btrfs_put_block_group (0) btrfs_delete_unused_bgs bg = list_first_entry list_del_init(&bg->bg_list); btrfs_put_block_group(bg); (-1) Ultimately, this results in a broken ref count that hits zero one deref early and the real final deref underflows the refcount, resulting in a WARNING.

References

• https://git.kernel.org/stable/c/bf089c4d1141b27332c092b1dcca5022c415a3b6
• https://git.kernel.org/stable/c/909e60fb469d4101c6b08cf6e622efb062bb24a1
• https://git.kernel.org/stable/c/185fd73e5ac06027c4be9a129e59193f6a3ef202
• https://git.kernel.org/stable/c/7511e29cf1355b2c47d0effb39e463119913e2f6