linux-kernel - Re: [PATCH] usb: xhci: Fix xhci_free_virt_devices_depth

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Message-ID: <20250902110730.723a48a0.michal.pecio@gmail.com>
Date: Tue, 2 Sep 2025 11:07:30 +0200
From: Michał Pecio <michal.pecio@...il.com>
To: "David Wang" <00107082@....com>
Cc: "Mathias Nyman" <mathias.nyman@...ux.intel.com>,
 WeitaoWang-oc@...oxin.com, gregkh@...uxfoundation.org,
 linux-usb@...r.kernel.org, regressions@...ts.linux.dev,
 linux-kernel@...r.kernel.org, surenb@...gle.com, kent.overstreet@...ux.dev
Subject: Re: [PATCH] usb: xhci: Fix xhci_free_virt_devices_depth_first()

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.