Re: [PATCH] drm/logicvc: Avoid use-after-free with devm_kzalloc()

[Romain Gantois <romain.gantois@bootlin.com>]

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On Monday, 8 June 2026 18:19:04 CEST Maxime Ripard wrote:
> On Mon, Jun 08, 2026 at 05:41:11PM +0200, Romain Gantois wrote:
> > Hi Maxime,
> > 
...
> > > You're only partially fixing the issue. You also need to protect any
> > > device resource (register mapping, clocks, etc) are no longer accessed
> > > after the device has been removed, and this is typically done using
> > > drm_dev_enter/exit.
> > 
> > Sorry there's something which I don't quite understand: is this a new
> > issue
> > which is specifically introduced by my changes in this series, or a
> > different issue in this driver which isn't handled by my series?
> 
> A bit of both I guess ? :)
> 
> My point was that while your commit log claims you avoid use-after-free,
> and your patch definitely avoids some, you can still trivially trigger
> some.
> 
> Whether you want to fix them all at once or prefer to defer it to a
> later point in time is equally fine by me, but you need to be aware that
> it's not done, and you probably want to have it in the commit log
> somewhere too?
> 

Ah ok, I didn't mean to claim that I was fixing any potential UAF in the 
driver. But if there's another one then I'm definitely interested in fixing it 
as well :).

> > IIUC all I'm doing here is just letting the drmm code handle cleaning up
> > the plane, crtc, etc. objects instead of doing it "by hand" with
> > devm_kzalloc. Why does this make it necessary to add additional
> > protection of driver resources?
> 
> It's not necessary, but it's also kind of the same issue. The reason we
> need to have drmm over devm is that the driver stays around longer than
> its device, so devm-allocated memory would have been freed.
> 
> But that's also the case for *any* devm resource, or more generally any
> resource linked to that device, so register mappings, clocks,
> interrupts, etc.
> 
> So yeah, it's a different symptom of the same underlying cause.

Okay, I'll assess the issue and probably make another patch in the same series 
to fix that.

Thanks,

-- 
Romain Gantois, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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