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

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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.

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

Fixes: f9c589e142 ("xhci: TD-fragment, align the unsplittable case with a bounce buffer")
Cc: stable@vger.kernel.org
Suggested-by: Michal Pecio <michal.pecio@gmail.com>
Signed-off-by: Arthur Gautier <baloo@superbaloo.net>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://patch.msgid.link/20260831090448.95644-4-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Arthur Gautier 2026-08-31 12:04:48 +03:00 committed by Greg Kroah-Hartman
parent 05506a76f1
commit ff44dfb03a
1 changed files with 24 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

32
drivers/usb/host/xhci-ring.c
View File

@ -824,21 +824,18 @@ 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,
@ -854,10 +851,29 @@ 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;
int i = 0;
if (!td->bounce_seg || !ring || !td->urb)
return;
/* td->bounce_seg is the last one bounced, unmap them all */
for (seg = td->start_seg; i++ < ring->num_segs; 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)
{
@ -3685,7 +3701,7 @@ 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 */
/* TD bounced at least, and last on this seg */
td->bounce_seg = ring->enq_seg;
}
}