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usb: a configuration block is as long as the device says it is

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The driver read the first 512 bytes of a configuration block into a fixed
buffer and parsed those. The block's length is the device's own choice
(wTotalLength, a u16), so anything larger was silently cut: interfaces past
the cut did not exist as far as the host was concerned, while the
SET_CONFIGURATION that follows still configured the device for all of them.
A headset is 500-900 bytes, a UVC webcam 1-3 KB, a multifunction printer
600+.

Now allocated at the declared length, so the ceiling is the field's u16 —
the specification's number rather than one of ours. A block shorter than its
own 9-byte header is refused rather than trusted.

The bring-up line reports the declared length and the bytes actually read,
so a truncation can never again be invisible, and usb-large-descriptor
asserts they match with a backreference.

That case has an honest limit, recorded in its comment: QEMU cannot produce
a block over 512 bytes. The boot keyboard, mouse and stick are 34-44, and
the largest device available is usb-audio in multi-channel mode at 211 —
which is exactly why the suite never caught this, and why it cannot now
reproduce the original trigger. What it does catch is the class: any clamp
below the attached device's block fails it, verified by pinning the buffer
to 128 and watching "config block 211 bytes, read 128" turn the case red.

Suite 115 -> 116.
This commit is contained in:
Daniel Samson
2026-08-08 11:53:01 +01:00
parent 69fbef40c0
commit 6328823ef1
4 changed files with 42 additions and 8 deletions
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docs/bounds-track-plan.md
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@@ -15,7 +15,7 @@ next one starts.*
| L1 | Reclamation: a dead task's registrations die with its claims | **stopped — the step was wrong; see open question 4** |
| L2 | Bounds build check + allowlist; declare what we have already touched | **done** — `zig build bounds`, 273 allowlisted, 5 declared |
| L3 | xHCI: slot count from `HCSPARAMS1.MaxSlots`, not 8 | **done** — QEMU reports 64; the driver tracked 8 |
| L4 | USB: configuration descriptor sized by `wTotalLength`, not 512 | not started |
| L4 | USB: configuration descriptor sized by `wTotalLength`, not 512 | **done** — QEMU tops out at 211 bytes, so the case catches the class, not the original trigger |
| L5 | USB: interfaces from the descriptor, and the misattributed-endpoint bug | not started |
| L6 | xHCI: a failed `allocateDevice` stops leaking an enabled slot | not started |