Re: [PATCH] usb: xhci: Fix xhci_free_virt_devices_depth_first()

[Mathias Nyman <mathias.nyman@linux.intel.com>]

On 2.9.2025 12.07, Michał Pecio wrote:
> On Tue, 2 Sep 2025 10:46:30 +0200, Michał Pecio wrote:
>> On Tue, 2 Sep 2025 16:30:48 +0800 (CST), David Wang wrote:
>>> About the change from "<" to "<=", I did not observe any difference on my system. Is it because my system does not use up all slots?
>>
>> This too, you would need to fiddle with devices (or connect enough
>> of them) to reach Slot ID 255 (probably the highest on most systems),
>> depending on the xHCI controller and its ID allocation policy.
> 
> This made me wonder what those policies are. I'm too lazy for thorough
> testing, but I plugged and unplugged the same device a few times.
> 
> Most HCs kept assigning ID 1, so they likely always pick the lowest.
> 
> My AMD chipset, two ASMedia USB 3.1 controllers and a Fresco FL1100
> kept assigning sequentially increasing IDs, so I suppose I could pump
> it up near the top, connect two high speed hubs and trigger this bug.
> 
>> But also as explained, this bug doesn't make things go boom just yet.
>>
>> Except if combined with your bug in an obscure edge case:
>>
>> 1. A high speed hub has slot ID HCS_MAX_SLOTS-1 and some TT children.
>> 2. Another high speed hub has slot ID HCS_MAX_SLOTS.
>> 3. We start with freeing the second hub.
>> 4. The loop is entered and leaves vdev pointing at the first hub.
>> 5. The first hub is freed instead of the second one.
>> 6. Then its children are freed and UAF its tt_info.

I'm not sure I follow the above.

I agree that changing the "<" to "<=" makes sense, but fortunately for us there shouldn't be any
issue with current implementation as xhci_free_virt_devices_depth_first() is called with highest possible
slot_id value first:

in xhci-memm.c:
  for (i = HCS_MAX_SLOTS(xhci->hcs_params1); i > 0; i--)
                 xhci_free_virt_devices_depth_first(xhci, i);

if HCS_MAX_SLOTS slot_id is a hs-hub then all its children have slot_id < HCS_MAX_SLOTS,
and loop works fine.

If HCS_MXA_SLOT slot_id is a leaf node then it is freed first.

Thanks
Mathias