xhci_cmd_address_device() sized the Output Device Context for Slot + EP0
(64 bytes); xhci_cmd_configure_endpoint() then allocated a bigger copy and
repointed DCBAA[slot_id]. The controller keeps the address it was given at
Address Device (QEMU caches it), so every bulk Endpoint Context was written
past the 64-byte block into the next kmalloc header -- on every USB
mass-storage attach, every ISA. After O-C's smoke campaign reshuffled the
heap, a kfree at thumbdrive unplug walked into it: #GP in coalesce_neighbors().
Found with temporary instrumentation (load base, code bytes at RIP, event
crumbs, a heap-list checker): the corrupt word was an EP Context (Bulk IN,
Running, MPS 1024) exactly DCI 3 x 32 bytes into the 64-byte context.
Now: XHCI_DEVICE_CTX_BYTES (32 contexts x 32 bytes) allocated once per
slot at Address Device, zeroed whole, never reallocated; Configure
Endpoint just fills it. Also ends the per-attach leak of the old context.
Verified: heap checker clean from boot; campaign -> O-A -> MINT -> unplug
-> re-attach passes with 0 exceptions; 3-ISA acceptance passes, parity and
dict_hash set unchanged.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
