linux-kernel - RE: [PATCH] ALSA: hda/hdmi - Fix duplicate unref of pci

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Date:   Tue, 10 Dec 2019 16:51:04 +0000
From:   "Deucher, Alexander" <Alexander.Deucher@....com>
To:     Takashi Iwai <tiwai@...e.de>
CC:     Lukas Wunner <lukas@...ner.de>, Jaroslav Kysela <perex@...ex.cz>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Nicholas Johnson <nicholas.johnson-opensource@...look.com.au>,
        "alsa-devel@...a-project.org" <alsa-devel@...a-project.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>
Subject: RE: [PATCH] ALSA: hda/hdmi - Fix duplicate unref of pci_dev

> -----Original Message-----
> From: Takashi Iwai <tiwai@...e.de>
> Sent: Tuesday, December 10, 2019 11:11 AM
> To: Deucher, Alexander <Alexander.Deucher@....com>
> Cc: Lukas Wunner <lukas@...ner.de>; Jaroslav Kysela <perex@...ex.cz>;
> Mika Westerberg <mika.westerberg@...ux.intel.com>; Bjorn Helgaas
> <helgaas@...nel.org>; Nicholas Johnson <nicholas.johnson-
> opensource@...look.com.au>; alsa-devel@...a-project.org; linux-
> kernel@...r.kernel.org; linux-pci@...r.kernel.org
> Subject: Re: [PATCH] ALSA: hda/hdmi - Fix duplicate unref of pci_dev
> 
> On Tue, 10 Dec 2019 16:53:20 +0100,
> Deucher, Alexander wrote:
> >
> > > -----Original Message-----
> > > From: Lukas Wunner <lukas@...ner.de>
> > > Sent: Tuesday, December 10, 2019 10:47 AM
> > > To: Deucher, Alexander <Alexander.Deucher@....com>
> > > Cc: Takashi Iwai <tiwai@...e.de>; Jaroslav Kysela <perex@...ex.cz>;
> > > Mika Westerberg <mika.westerberg@...ux.intel.com>; Bjorn Helgaas
> > > <helgaas@...nel.org>; Nicholas Johnson <nicholas.johnson-
> > > opensource@...look.com.au>; alsa-devel@...a-project.org; linux-
> > > kernel@...r.kernel.org; linux-pci@...r.kernel.org
> > > Subject: Re: [PATCH] ALSA: hda/hdmi - Fix duplicate unref of pci_dev
> > >
> > > On Tue, Dec 10, 2019 at 03:34:27PM +0000, Deucher, Alexander wrote:
> > > > > Nicholas Johnson reports a null pointer deref as well as a
> > > > > refcount underflow upon hot-removal of a Thunderbolt-attached
> AMD eGPU.
> > > > > He's bisected the issue down to commit 586bc4aab878 ("ALSA:
> > > > > hda/hdmi
> > > > > - fix vgaswitcheroo detection for AMD").
> > > > >
> > > > > The commit iterates over PCI devices using pci_get_class() and
> > > > > unreferences each device found, even though pci_get_class()
> > > > > subsequently unreferences the device as well.  Fix it.
> > > >
> > > > The pci_dev_put() a few lines above should probably be dropped as
> well.
> > >
> > > That one looks fine to me.  The refcount is already increased in the
> > > caller
> > > get_bound_vga() via pci_get_domain_bus_and_slot() and it's increased
> > > again in atpx_present() via pci_get_class().  It needs to be
> > > decremented in
> > > atpx_present() to avoid leaking a ref.
> >
> > I'm not following.  This is part of the same loop as the one you removed.  All
> we are doing is checking whether the ATPX method exists or not om the
> platform.  The pdev may not be the same one as the one in
> pci_get_domain_bus_and_slot().  The APTX method in the APU's ACPI
> namespace, not the dGPUs.
> 
> Well, the tricky part is that pci_get_class() itself does unrefeference the old
> object and reference the new object (if found).
> At the end of the loop, nothing is referenced, so it's fine.
> OTOH, if you go out of the loop in the middle, you're still keeping the pdev
> object reference, so you need to manually unreference it.
> 

Ah, I see what you are saying.  Thanks.  Patch is:
Reviewed-by: Alex Deucher <alexander.deucher@....com>

> 
> Takashi
> 
> >
> > Alex
> >
> > >
> > > Thanks,
> > >
> > > Lukas
> > >
> > > > > diff --git a/sound/pci/hda/hda_intel.c
> > > > > b/sound/pci/hda/hda_intel.c index 35b4526f0d28..b856b89378ac
> > > > > 100644
> > > > > --- a/sound/pci/hda/hda_intel.c
> > > > > +++ b/sound/pci/hda/hda_intel.c
> > > > > @@ -1419,7 +1419,6 @@ static bool atpx_present(void)
> > > > >  				return true;
> > > > >  			}
> > > > >  		}
> > > > > -		pci_dev_put(pdev);
> > > > >  	}
> > > > >  	return false;
> > > > >  }
> > > > > --
> > > > > 2.24.0
> >