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Date:   Wed, 30 Aug 2023 16:47:07 +0100
From:   Sudeep Holla <sudeep.holla@arm.com>
To:     Radu Rendec <rrendec@redhat.com>
Cc:     Ricardo Neri <ricardo.neri-calderon@linux.intel.com>,
        x86@kernel.org, Andreas Herrmann <aherrmann@suse.com>,
        Sudeep Holla <sudeep.holla@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Chen Yu <yu.c.chen@intel.com>, Len Brown <len.brown@intel.com>,
        Pierre Gondois <Pierre.Gondois@arm.com>,
        Pu Wen <puwen@hygon.cn>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        Will Deacon <will@kernel.org>, Zhang Rui <rui.zhang@intel.com>,
        stable@vger.kernel.org, Ricardo Neri <ricardo.neri@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v3 1/3] cacheinfo: Allocate memory for memory if not done
 from the primary CPU
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On Wed, Aug 30, 2023 at 08:13:09AM -0400, Radu Rendec wrote:
> On Wed, 2023-08-30 at 12:49 +0100, Sudeep Holla wrote:
> > On Fri, Aug 04, 2023 at 06:24:19PM -0700, Ricardo Neri wrote:
> > > Commit 5944ce092b97 ("arch_topology: Build cacheinfo from primary CPU")
> > > adds functionality that architectures can use to optionally allocate and
> > > build cacheinfo early during boot. Commit 6539cffa9495 ("cacheinfo: Add
> > > arch specific early level initializer") lets secondary CPUs correct (and
> > > reallocate memory) cacheinfo data if needed.
> > > 
> > > If the early build functionality is not used and cacheinfo does not need
> > > correction, memory for cacheinfo is never allocated. x86 does not use the
> > > early build functionality. Consequently, during the cacheinfo CPU hotplug
> > > callback, last_level_cache_is_valid() attempts to dereference a NULL
> > > pointer:
> > > 
> > > ???? BUG: kernel NULL pointer dereference, address: 0000000000000100
> > > ???? #PF: supervisor read access in kernel mode
> > > ???? #PF: error_code(0x0000) - not present page
> > > ???? PGD 0 P4D 0
> > > ???? Oops: 0000 [#1] PREEPMT SMP NOPTI
> > > ???? CPU: 0 PID 19 Comm: cpuhp/0 Not tainted 6.4.0-rc2 #1
> > > ???? RIP: 0010: last_level_cache_is_valid+0x95/0xe0a
> > > 
> > > Allocate memory for cacheinfo during the cacheinfo CPU hotplug callback if
> > > not done earlier.
> > > 
> > > Cc: Andreas Herrmann <aherrmann@suse.com>
> > > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > > Cc: Chen Yu <yu.c.chen@intel.com>
> > > Cc: Len Brown <len.brown@intel.com>
> > > Cc: Radu Rendec <rrendec@redhat.com>
> > > Cc: Pierre Gondois <Pierre.Gondois@arm.com>
> > > Cc: Pu Wen <puwen@hygon.cn>
> > > Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
> > > Cc: Sudeep Holla <sudeep.holla@arm.com>
> > > Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
> > > Cc: Will Deacon <will@kernel.org>
> > > Cc: Zhang Rui <rui.zhang@intel.com>
> > > Cc: linux-arm-kernel@lists.infradead.org
> > > Cc: stable@vger.kernel.org
> > > Acked-by: Len Brown <len.brown@intel.com>
> > > Fixes: 6539cffa9495 ("cacheinfo: Add arch specific early level initializer")
> > 
> > Not sure if we strictly need this(details below), but I am fine either way.
> > 
> > > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> > > ---
> > > The motivation for commit 5944ce092b97 was to prevent a BUG splat in
> > > PREEMPT_RT kernels during memory allocation. This splat is not observed on
> > > x86 because the memory allocation for cacheinfo happens in
> > > detect_cache_attributes() from the cacheinfo CPU hotplug callback.
> > > 
> > > The dereference of a NULL pointer is not observed today because
> > > cache_leaves(cpu) is zero until after init_cache_level() is called (also
> > > during the CPU hotplug callback). Patch2 will set it earlier and the NULL-
> > > pointer dereference will be observed.
> > 
> > Right, this is the information I have been asking in the previous versions.
> > This clarifies a lot. The trigger is in the patch 2/3 which is why it didn't
> > make complete sense to me without it when you posted this patch independently.
> > Thanks for posting it together and sorry for the delay(both reviewing this
> > and in understanding the issue).
> > 
> > Given the trigger for NULL pointer dereference is in 2/3, I am not sure
> > if it is really worth applying this to all the stable kernels with the
> > commit 5944ce092b97 ("arch_topology: Build cacheinfo from primary CPU").
> > That is the reason why I asked to drop fixes tag if you agree with me.
> > It is simple fix, so I am OK if you prefer to see that in the stable kernels
> > as well.
> 
> Thanks for reviewing, Sudeep. Since my previous commit 6539cffa9495
> ("cacheinfo: Add arch specific early level initializer") opens a door
> for the NULL pointer dereference, I would sleep better at night if the
> fix was included in the stable kernels :) But seriously, I am concerned
> that with the fix applied in mainline and not in stable, something else
> could be backported to the stable in the future, that could trigger the
> NULL pointer dereference there. Ricardo's patch 2/3 is one way to
> trigger it, but you never know what other patch lands in mainline in
> the future that assumes it's safe to set the cache leaves earlier.
> 

Fair enough. I agree with you, so please retain the fixes tag as is.
Please work with x86 maintainers to get it merged along with other patches.
Let me know if you have other plans.

-- 
Regards,
Sudeep