[PATCH v2] xhci: fix lost bounce buffers on TDs spanning several ring segments

[Arthur Gautier <baloo@superbaloo.net>]

When a TD reaches a link TRB with data that is not aligned to the
endpoint's wMaxPacketSize, xhci_align_td() stages the unalignable tail
through the bounce buffer of the ring segment holding that link TRB.
xhci_unmap_td_bounce_buffer() 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->bounce_seg,
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->bounce_seg, so every earlier bounce
buffer is neither copied back nor DMA unmapped.

The URB still completes with actual_length 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. verity_prefetch_io() 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 (bounce_buf, bounce_dma, bounce_len, bounce_offs)
already lives on the ring segment, so there is nothing extra to track.
Keep recording the last bounced segment in td->bounce_seg and, on
completion, walk the segments from td->start_seg up to it, unmapping
every segment that still has a pending bounce.

Stopping at td->bounce_seg rather than td->end_seg matters: a bounce
implies the TD continues past that segment's link TRB, so bounce_seg is
always strictly before end_seg, and a later TD may already have started
in end_seg 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 end_seg == start_seg.

Changes since v1:
- Walk td->start_seg -> td->bounce_seg instead of td->bounce_seg ->
  td->end_seg. end_seg can hold the start of a later TD which may
  already have been bounced there, so the v1 walk could copy a
  foreign bounce buffer into this URB and unmap it twice.
  (caught by Mathias and Michal)
- Keep recording the last bounced segment. Also handles a TD
  wrapping the whole ring.  (suggested by Michal)
- Test !td->bounce_seg first, it is the common case. (Michal)

v1: https://patchwork.kernel.org/project/linux-usb/patch/20260811033508.1050148-1-baloo@superbaloo.net/

Fixes: f9c589e142d0 ("xhci: TD-fragment, align the unsplittable case with a bounce buffer")
Suggested-by: Michal Pecio <michal.pecio@gmail.com>
Signed-off-by: Arthur Gautier <baloo@superbaloo.net>
---
 drivers/usb/host/xhci-ring.c | 31 +++++++++++++++++++++++--------
 1 file changed, 23 insertions(+), 8 deletions(-)

diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index 4f98d8269625..1772cdf9ffc5 100644
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -842,21 +842,18 @@ static void xhci_giveback_urb_in_irq(struct xhci_hcd *xhci,
 	usb_hcd_giveback_urb(hcd, urb, status);
 }
 
-static void xhci_unmap_td_bounce_buffer(struct xhci_hcd *xhci,
-		struct xhci_ring *ring, struct xhci_td *td)
+static void xhci_unmap_one_bounce_buffer(struct xhci_hcd *xhci,
+		struct xhci_ring *ring, struct xhci_td *td,
+		struct xhci_segment *seg)
 {
 	struct device *dev = xhci_to_hcd(xhci)->self.sysdev;
-	struct xhci_segment *seg = td->bounce_seg;
 	struct urb *urb = td->urb;
 	size_t len;
 
-	if (!ring || !seg || !urb)
-		return;
-
 	if (usb_urb_dir_out(urb)) {
 		dma_unmap_single(dev, seg->bounce_dma, ring->bounce_buf_len,
 				 DMA_TO_DEVICE);
-		return;
+		goto done;
 	}
 
 	dma_unmap_single(dev, seg->bounce_dma, ring->bounce_buf_len,
@@ -872,10 +869,28 @@ static void xhci_unmap_td_bounce_buffer(struct xhci_hcd *xhci,
 		memcpy(urb->transfer_buffer + seg->bounce_offs, seg->bounce_buf,
 		       seg->bounce_len);
 	}
+done:
 	seg->bounce_len = 0;
 	seg->bounce_offs = 0;
 }
 
+static void xhci_unmap_td_bounce_buffer(struct xhci_hcd *xhci,
+		struct xhci_ring *ring, struct xhci_td *td)
+{
+	struct xhci_segment *seg;
+
+	if (!td->bounce_seg || !ring || !td->urb)
+		return;
+
+	/* td->bounce_seg is the last one bounced, unmap them all */
+	for (seg = td->start_seg; ; seg = seg->next) {
+		if (seg->bounce_len)
+			xhci_unmap_one_bounce_buffer(xhci, ring, td, seg);
+		if (seg == td->bounce_seg)
+			break;
+	}
+}
+
 static void xhci_td_cleanup(struct xhci_hcd *xhci, struct xhci_td *td,
 			    struct xhci_ring *ep_ring, int status)
 {
@@ -3674,7 +3689,7 @@ int xhci_queue_bulk_tx(struct xhci_hcd *xhci, gfp_t mem_flags,
 						  &trb_buff_len,
 						  ring->enq_seg)) {
 					send_addr = ring->enq_seg->bounce_dma;
-					/* assuming TD won't span 2 segs */
+					/* a TD may be bounced in every seg it spans */
 					td->bounce_seg = ring->enq_seg;
 				}
 			}
-- 
2.55.0