CVE-2026-90015: xhci: fix lost bounce buffers on TDs spanning several ring segments

In the Linux kernel, the following vulnerability has been resolved:

xhci: fix lost bounce buffers on TDs spanning several ring segments

When a TD reaches a link TRB with data that is not aligned to the endpoint's wMaxPacketSize, xhcialigntd() stages the unalignable tail through the bounce buffer of the ring segment holding that link TRB. xhciunmaptdbouncebuffer() later unmaps it and, for IN transfers, copies the data back into the URB's buffer.

The enqueue path records the segment that was bounced in td->bounceseg, under the assumption that a TD never spans more than two ring segments. That assumption does not hold: a TD large enough to span three or more segments crosses several link TRBs and can be bounced at each of them. Only the last one survives in td->bounceseg, so every earlier bounce buffer is neither copied back nor DMA unmapped.

The URB still completes with actuallength equal to the requested length and no error, so the transfer looks successful while a wMaxPacketSize sized hole in the destination buffer silently keeps its previous contents. It also leaks a DMA mapping per dropped bounce.

Any sufficiently large and fragmented bulk transfer can hit this. It was found with a USB mass storage device behind xHCI backing a dm-verity target with 512 byte hash blocks, where the stale data is detected rather than silently consumed. The device enumerates as SuperSpeed, so wMaxPacketSize is 1024, while dm-bufio issues one 512 byte bio per hash block. verityprefetchio() makes the block layer merge hundreds of them into a single request of up to 512 scatterlist entries of 512 bytes each. At 256 TRBs per ring segment such a TD spans three segments, and every segment boundary falls on an odd multiple of 512, i.e. unaligned to wMaxPacketSize. dm-bufio then caches a hash block holding stale data and dm-verity declares the metadata block corrupted:

device-mapper: verity: 8:2: metadata block 10850 is corrupted

A reproducer running this under qemu is available at https://github.com/baloo/xhci-verity

The bounce state (bouncebuf, bouncedma, bouncelen, bounceoffs) already lives on the ring segment, so there is nothing extra to track. Keep recording the last bounced segment in td->bounceseg and, on completion, walk the segments from td->startseg up to it, unmapping every segment that still has a pending bounce.

Stopping at td->bounceseg rather than td->endseg matters: a bounce implies the TD continues past that segment's link TRB, so bounceseg is always strictly before endseg, and a later TD may already have started in endseg and been bounced there. Walking that far would copy a foreign bounce buffer into this URB and unmap it twice. It also keeps the walk correct if a TD ever wraps the whole ring so that endseg == startseg.

[mn: Add ring->numsegs check to prevent unlikely infinite for loop.]