Re: [PATCH] usb: xhci: Fix invalid pointer dereference in Etron workaround

["Michał Pecio" <michal.pecio@gmail.com>]

Hi Mathias,

Gentle reminder about a pretty annoying bug, it would seem like a good
idea to have it fixed in 6.15-rc2.

Basically, if you use SS control transfers on Etron, this happens:

[88483.258966] xhci_hcd 0000:06:00.0: last TRB on seg we're gonna dieee!!!
[88483.260825] BUG: unable to handle page fault for address: ffffc9000177a00c
[88483.262441] #PF: supervisor read access in kernel mode
[88483.263889] #PF: error_code(0x0000) - not-present page
[88483.265306] PGD 100000067 P4D 100000067 PUD 100234067 PMD 103dd4067 PTE 0
[88483.266648] Oops: Oops: 0000 [#1] SMP
[88483.267980] CPU: 2 UID: 1000 PID: 20753 Comm: v4l2-ctl Not tainted 6.15.0-rc1 #7 PREEMPT 
[88483.269327] Hardware name: HP HP EliteDesk 705 G3 MT/8265, BIOS P06 Ver. 02.45 07/16/2024
[88483.270689] RIP: 0010:xhci_queue_ctrl_tx+0xaf/0x410 [xhci_hcd]


On Mon, 17 Mar 2025 22:29:27 +0100, Michal Pecio wrote:
> This check is performed before prepare_transfer() and prepare_ring(),
> so enqueue can already point at the final link TRB of a segment. And
> indeed it will, some 0.4% of times this code is called.
> 
> Then enqueue + 1 is an invalid pointer. It will crash the kernel right
> away or load some junk which may look like a link TRB and cause the
> real link TRB to be replaced with a NOOP. This wouldn't end well.
> 
> Use a functionally equivalent test which doesn't dereference the
> pointer and always gives correct result.
> 
> Something has crashed my machine twice in recent days while playing
> with an Etron HC, and a control transfer stress test ran for
> confirmation has just crashed it again. The same test passes with
> this patch applied.
> 
> Fixes: 5e1c67abc930 ("xhci: Fix control transfer error on Etron xHCI
> host") Cc: stable@vger.kernel.org
> Signed-off-by: Michal Pecio <michal.pecio@gmail.com>

Regards,
Michal