linux-kernel - Re: [PATCH] acpi: cppc: Prevent cpc_desc

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Date:	Sat, 25 Jun 2016 01:05:25 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Hoan Tran <hotran@....com>
Cc:	Ashwin Chaugule <ashwin.chaugule@...aro.org>,
	Len Brown <lenb@...nel.org>,
	linux acpi <linux-acpi@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>, Duc Dang <dhdang@....com>,
	Loc Ho <lho@....com>
Subject: Re: [PATCH] acpi: cppc: Prevent cpc_desc_ptr points to the invalid data

On Friday, June 24, 2016 09:35:32 AM Hoan Tran wrote:
> On Tue, May 31, 2016 at 12:29 PM, Ashwin Chaugule
> <ashwin.chaugule@...aro.org> wrote:
> >
> > On 27 May 2016 at 12:41, Hoan Tran <hotran@....com> wrote:
> > > Hi Ashwin,
> >
> > Hi,
> >
> > >
> > > Yes, I saw kernel crash.
> > > As cpc_desc_ptr is not NULL, cppc_cpufreq_init() still can pass then
> > > crash during cppc_get_perf_caps() access CPPC shared memory.
> > >
> > > It's not only "PCC channel request fail" can create this issue but
> > > "acpi_get_psd() fail" also creates it
> > >
> > > Thanks
> > > Hoan
> >
> >
> > Just for future reference. :)
> >
> > https://web.archive.org/web/20080722025748/http://www.zip.com.au/~akpm/linux/patches/stuff/top-posting.txt
> >
> >
> > >
> > > On Fri, May 27, 2016 at 9:10 AM, Ashwin Chaugule
> > > <ashwin.chaugule@...aro.org> wrote:
> > >> On 25 May 2016 at 15:09, Hoan Tran <hotran@....com> wrote:
> > >>> When CPPC fails to request PCC channel, the CPC data is freed
> > >>> and cpc_desc_ptr points to the invalid data. This change prevents
> > >>> this issue by moving cpc_desc_ptr assignment after PCC channel
> > >>> request.
> > >>>
> > >>> Signed-off-by: Hoan Tran <hotran@....com>
> > >>> ---
> > >>>  drivers/acpi/cppc_acpi.c | 6 +++---
> > >>>  1 file changed, 3 insertions(+), 3 deletions(-)
> > >>>
> > >>> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
> > >>> index 8adac69..85fd8f7 100644
> > >>> --- a/drivers/acpi/cppc_acpi.c
> > >>> +++ b/drivers/acpi/cppc_acpi.c
> > >>> @@ -595,9 +595,6 @@ int acpi_cppc_processor_probe(struct acpi_processor *pr)
> > >>>         /* Store CPU Logical ID */
> > >>>         cpc_ptr->cpu_id = pr->id;
> > >>>
> > >>> -       /* Plug it into this CPUs CPC descriptor. */
> > >>> -       per_cpu(cpc_desc_ptr, pr->id) = cpc_ptr;
> > >>> -
> > >>>         /* Parse PSD data for this CPU */
> > >>>         ret = acpi_get_psd(cpc_ptr, handle);
> > >>>         if (ret)
> > >>> @@ -610,6 +607,9 @@ int acpi_cppc_processor_probe(struct acpi_processor *pr)
> > >>>                         goto out_free;
> > >>>         }
> > >>>
> > >>> +       /* Plug PSD data into this CPUs CPC descriptor. */
> > >>> +       per_cpu(cpc_desc_ptr, pr->id) = cpc_ptr;
> > >>> +
> > >>
> > >> Are you seeing a real problem without this change? I'm missing where
> > >> this pointer is dereferenced if the PCC channel request fails.
> > >>
> >
> > So, after freeing the cpc_ptr, we need to NULL the per-cpu pointer as
> > well. Alternately, not assign it until everything passes and rely on
> > the static declaration, which is the path you've taken here.
> >
> > Acked-by: Ashwin Chaugule <ashwin.chaugule@...aro.org>
> 
> Hi Rafael,
> 
> Do you have plan to apply this patch ?

I do now. :-)

I've overlooked the Ashwin's ACK, so thanks for the reminder.

Thanks,
Rafael