Re: [PATCH] xfs: fix slab-use-after-free in xrep_newbt_free()

[Zheng Shuhao]

Hi Darrick,

Thanks. Re: "What if some other thread initiates the shutdown?"

That is what I tested. The paused autoreap dfps are freed by the failed
xrep_defer_finish() in the repair thread, no matter which thread sets the
shutdown. xfs_defer_finish_noroll()'s out_shutdown path forces the
shutdown before xfs_defer_cancel_list(), which frees every
xfs_defer_pending on the scrub transaction, including the paused autoreap
EFIs. resv->autoreap.dfp is dangling by the time junkit runs.

If the shutdown is already set before repair starts, the dfps may still be
alive, but unpausing them is a no-op: deferred work cannot progress on a
dead log, and scrub teardown cancels the transaction and frees the dfps.
Any EFI that reached the log is replayed at mount, which is exactly the
intended autoreap behavior for a repair that never committed its new
btree.

The guard cannot miss the dangling case, because the cancel that frees the
dfps happens after the shutdown is forced.

Testing: helper threads issue XFS_IOC_GOINGDOWN (LOGFLUSH and NOLOGFLUSH)
after the xfs_defer_item_pause trace event -- 1 to 8 threads, 0-100 ms
delays -- while the main thread force-rebuilds a bmap btree. Unpatched:
KASAN UAF every time in xrep_newbt_free() -> xfs_defer_item_unpause().
Patched: no splats, scrub returns -EIO; after remount the log replays,
free block count is unchanged, data reads back intact, xfs_repair -n is
clean. Healthy forced rebuilds still do the normal pause/unpause.

Thanks,
Zheng Shuhao