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Date: Wed, 25 Jan 2017 12:10:16 -0800
From: Thomas Garnier <thgarnie@...gle.com>
To: Andy Lutomirski <luto@...capital.net>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	"H . Peter Anvin" <hpa@...or.com>, Kees Cook <keescook@...omium.org>, 
	"Rafael J . Wysocki" <rjw@...ysocki.net>, Pavel Machek <pavel@....cz>, Andy Lutomirski <luto@...nel.org>, 
	Borislav Petkov <bp@...e.de>, Christian Borntraeger <borntraeger@...ibm.com>, Brian Gerst <brgerst@...il.com>, 
	He Chen <he.chen@...ux.intel.com>, Dave Hansen <dave.hansen@...el.com>, 
	Chen Yucong <slaoub@...il.com>, Baoquan He <bhe@...hat.com>, 
	Paul Gortmaker <paul.gortmaker@...driver.com>, Joerg Roedel <joro@...tes.org>, 
	Paolo Bonzini <pbonzini@...hat.com>, Radim Krčmář <rkrcmar@...hat.com>, 
	Fenghua Yu <fenghua.yu@...el.com>, X86 ML <x86@...nel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>, kvm list <kvm@...r.kernel.org>, 
	"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v1 2/3] x86: Remap GDT tables in the Fixmap section

The kbuild bot found interesting build errors with the new
BUILD_BUG_ON on 32bit (64G mem support). I think I went a bit too far
with them given the ioremap part is just temporary on early boot.

I removed them and tested different configurations trying to use as
much fixmap as possible (DEBUG_HIGHMEM, 64G/PAE support and more).
Everything looks good and we can still support 512 processors.

Next iteration, I will remove the BUILD_BUG_ON that I added and remove
restriction to 256 back to 512. On top of all changes suggested by
Andy on the patch set.

On Fri, Jan 20, 2017 at 5:06 PM, Thomas Garnier <thgarnie@...gle.com> wrote:
> On Fri, Jan 20, 2017 at 4:57 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Fri, Jan 20, 2017 at 8:41 AM, Thomas Garnier <thgarnie@...gle.com> wrote:
>>> Each processor holds a GDT in its per-cpu structure. The sgdt
>>> instruction gives the base address of the current GDT. This address can
>>> be used to bypass KASLR memory randomization. With another bug, an
>>> attacker could target other per-cpu structures or deduce the base of
>>> the main memory section (PAGE_OFFSET).
>>>
>>> This patch relocates the GDT table for each processor inside the
>>> Fixmap section. The space is reserved based on number of supported
>>> cpus.
>>>
>>> For consistency, the remapping is done by default on 32 and 64 bit.
>>>
>>> Each processor switches to its remapped GDT at the end of
>>> initialization. For hibernation, the main processor returns with the
>>> original GDT and switches back to the remapping at completion.
>>>
>>> On 32 bit, the maximum number of processors is now 256. The Fixmap
>>> section cannot handle the original 512. Additional asserts ensure that
>>> the Fixmap section cannot grow beyond the space available.
>>>
>>> This patch was tested on both architectures. Hibernation and KVM were
>>> both tested specially for their usage of the GDT.
>>>
>>> Signed-off-by: Thomas Garnier <thgarnie@...gle.com>
>>> ---
>>> Based on next-20170119
>>> ---
>>>  arch/x86/Kconfig                 |  1 +
>>>  arch/x86/include/asm/fixmap.h    |  4 ++++
>>>  arch/x86/include/asm/processor.h |  1 +
>>>  arch/x86/kernel/cpu/common.c     | 18 +++++++++++++++++-
>>>  arch/x86/mm/init_32.c            |  2 ++
>>>  arch/x86/power/cpu.c             |  3 +++
>>>  6 files changed, 28 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>>> index f1d4e8f2131f..b4ed35db10a8 100644
>>> --- a/arch/x86/Kconfig
>>> +++ b/arch/x86/Kconfig
>>> @@ -912,6 +912,7 @@ config MAXSMP
>>>  config NR_CPUS
>>>         int "Maximum number of CPUs" if SMP && !MAXSMP
>>>         range 2 8 if SMP && X86_32 && !X86_BIGSMP
>>> +       range 2 256 if SMP && X86_32 && X86_BIGSMP
>>>         range 2 512 if SMP && !MAXSMP && !CPUMASK_OFFSTACK
>>>         range 2 8192 if SMP && !MAXSMP && CPUMASK_OFFSTACK && X86_64
>>>         default "1" if !SMP
>>> diff --git a/arch/x86/include/asm/fixmap.h b/arch/x86/include/asm/fixmap.h
>>> index c46289799b02..8b913b5e9383 100644
>>> --- a/arch/x86/include/asm/fixmap.h
>>> +++ b/arch/x86/include/asm/fixmap.h
>>> @@ -100,6 +100,10 @@ enum fixed_addresses {
>>>  #ifdef CONFIG_X86_INTEL_MID
>>>         FIX_LNW_VRTC,
>>>  #endif
>>> +       /* Fixmap entries to remap the GDTs, one per processor. */
>>> +       FIX_GDT_REMAP_BEGIN,
>>> +       FIX_GDT_REMAP_END = FIX_GDT_REMAP_BEGIN + NR_CPUS - 1,
>>> +
>>>         __end_of_permanent_fixed_addresses,
>>>
>>>         /*
>>> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
>>> index 1be64da0384e..280211ad8be9 100644
>>> --- a/arch/x86/include/asm/processor.h
>>> +++ b/arch/x86/include/asm/processor.h
>>> @@ -705,6 +705,7 @@ extern struct desc_ptr              early_gdt_descr;
>>>
>>>  extern void cpu_set_gdt(int);
>>>  extern void switch_to_new_gdt(int);
>>> +extern void load_remapped_gdt(int);
>>>  extern void load_percpu_segment(int);
>>>  extern void cpu_init(void);
>>>
>>> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
>>> index e97ffc8d29f4..7d940b0e805a 100644
>>> --- a/arch/x86/kernel/cpu/common.c
>>> +++ b/arch/x86/kernel/cpu/common.c
>>> @@ -443,6 +443,19 @@ void load_percpu_segment(int cpu)
>>>         load_stack_canary_segment();
>>>  }
>>>
>>> +/* Load a fixmap remapping of the per-cpu GDT */
>>> +void load_remapped_gdt(int cpu)
>>> +{
>>> +       struct desc_ptr gdt_descr;
>>> +       unsigned long idx = FIX_GDT_REMAP_BEGIN + cpu;
>>> +
>>> +       __set_fixmap(idx, __pa(get_cpu_gdt_table(cpu)), PAGE_KERNEL);
>>> +
>>> +       gdt_descr.address = (long)__fix_to_virt(idx);
>>> +       gdt_descr.size = GDT_SIZE - 1;
>>> +       load_gdt(&gdt_descr);
>>> +}
>>
>> IMO this should be split into two functions, one to set up the fixmap
>> entry and one to load the GDT.
>>
>
> That make sense.
>
>> Also, would it be possible to rename the various gdt helpers so that
>> their functionality is more obvious?  For example, get_cpu_gdt_table()
>> could be get_cpu_direct_gdt_table() and the function to load the gdt
>> could be load_fixmap_gdt().
>>
>
> Sure no problem.
>
>> --Andy
>
>
>
> --
> Thomas



-- 
Thomas

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