linux-kernel - Re: [PATCH 3.3.0-rc3] Fix use-after-free in acpi_map

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Date:	Thu, 08 Mar 2012 14:01:15 -0700
From:	Toshi Kani <toshi.kani@...com>
To:	Alok Kataria <akataria@...are.com>
Cc:	Petr Vandrovec <petr@...are.com>,
	the arch/x86 maintainers <x86@...nel.org>, lenb@...nel.org,
	linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
	dcovelli@...are.com
Subject: Re: [PATCH 3.3.0-rc3] Fix use-after-free in acpi_map_lsapic

On Wed, 2012-03-07 at 11:48 -0800, Alok Kataria wrote:
> Len, anyone else, 
> 
> Any comments on this one ? This fixes a important bug during cpu hotadd
> where the kernel fails to recognize all the newly added cpus.
> 
> Thanks,
> Alok
> 
> 
> On Wed, 2012-02-15 at 16:06 -0800, Petr Vandrovec wrote:
> > From: Petr Vandrovec <petr@...are.com>
> > 
> > When processor is being hot-added to the system, acpi_map_lsapic invokes
> > ACPI _MAT method to find APIC ID and flags, verifies that returned structure
> > is indeed ACPI's local APIC structure, and that flags contain MADT_ENABLED
> > bit.  Then saves APIC ID, frees structure - and accesses structure when
> > computing arguments for acpi_register_lapic call.  Which sometime leads
> > to acpi_register_lapic call being made with second argument zero, failing
> > to bring processor online with error 'Unable to map lapic to logical cpu
> > number'.
> > 
> > As lapic->lapic_flags & ACPI_MADT_ENABLED was already confirmed to be non-zero
> > few lines above, we can just pass unconditional ACPI_MADT_ENABLED to the
> > acpi_register_lapic.
> > 
> > Thanks, Petr
> > 
> > Signed-off-by: Petr Vandrovec <petr@...are.com>
> > Signed-off-by: Alok Kataria <akataria@...are.com>
> > 
> > 
> > diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
> > index ce664f3..a4a0901 100644
> > --- a/arch/x86/kernel/acpi/boot.c
> > +++ b/arch/x86/kernel/acpi/boot.c
> > @@ -650,7 +650,7 @@ static int __cpuinit _acpi_map_lsapic(acpi_handle handle, int *pcpu)
> >  		goto free_tmp_map;
> >  
> >  	cpumask_copy(tmp_map, cpu_present_mask);
> > -	acpi_register_lapic(physid, lapic->lapic_flags & ACPI_MADT_ENABLED);
> > +	acpi_register_lapic(physid, ACPI_MADT_ENABLED);

The change looks good. I suggest you also add the following line to
prevent such bug in future.

        kfree(buffer.pointer);
        buffer.length = ACPI_ALLOCATE_BUFFER;
        buffer.pointer = NULL;
+       lapic = NULL;

Thanks,
-Toshi


> >  
> >  	/*
> >  	 * If mp_register_lapic successfully generates a new logical cpu
> 
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