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Re: [Xen-devel] [RFC 16/22] x86/percpu: Adapt percpu for PIE support

On Wed, Jul 19, 2017 at 4:33 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> On 07/19/17 11:26, Thomas Garnier wrote:
>> On Tue, Jul 18, 2017 at 8:08 PM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
>>> On Tue, Jul 18, 2017 at 6:33 PM, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
>>>> Perpcu uses a clever design where the .percu ELF section has a virtual
>>>> address of zero and the relocation code avoid relocating specific
>>>> symbols. It makes the code simple and easily adaptable with or without
>>>> SMP support.
>>>> This design is incompatible with PIE because generated code always try to
>>>> access the zero virtual address relative to the default mapping address.
>>>> It becomes impossible when KASLR is configured to go below -2G. This
>>>> patch solves this problem by removing the zero mapping and adapting the GS
>>>> base to be relative to the expected address. These changes are done only
>>>> when PIE is enabled. The original implementation is kept as-is
>>>> by default.
>>> The reason the per-cpu section is zero-based on x86-64 is to
>>> workaround GCC hardcoding the stack protector canary at %gs:40.  So
>>> this patch is incompatible with CONFIG_STACK_PROTECTOR.
>> Ok, that make sense. I don't want this feature to not work with
>> CONFIG_CC_STACKPROTECTOR*. One way to fix that would be adding a GDT
>> entry for gs so gs:40 points to the correct memory address and
>> gs:[rip+XX] works correctly through the MSR.
> What are you talking about?  A GDT entry and the MSR do the same thing,
> except that a GDT entry is limited to an offset of 0-0xffffffff (which
> doesn't work for us, obviously.)

A GDT entry would allow gs:0x40 to be valid while all gs:[rip+XX]
addresses uses the MSR.

I didn't tested it but that was used on the RFG mitigation [1]. The fs
segment register was used for both thread storage and shadow stack.

[1] http://xlab.tencent.com/en/2016/11/02/return-flow-guard/

>> Given the separate
>> discussion on mcmodel, I am going first to check if we can move from
>> PIE to PIC with a mcmodel=small or medium that would remove the percpu
>> change requirement. I tried before without success but I understand
>> better percpu and other components so maybe I can make it work.
>>> This is silly.  The right thing is for PIE is to be explicitly absolute,
>>> without (%rip).  The use of (%rip) memory references for percpu is just
>>> an optimization.
>> I agree that it is odd but that's how the compiler generates code. I
>> will re-explore PIC options with mcmodel=small or medium, as mentioned
>> on other threads.
> Why should the way compiler generates code affect the way we do things
> in assembly?
> That being said, the compiler now has support for generating this kind
> of code explicitly via the __seg_gs pointer modifier.  That should let
> us drop the __percpu_prefix and just use variables directly.  I suspect
> we want to declare percpu variables as "volatile __seg_gs" to account
> for the possibility of CPU switches.
> Older compilers won't be able to work with this, of course, but I think
> that it is acceptable for those older compilers to not be able to
> support PIE.
>         -hpa


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