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Re: [Xen-devel] [RFC 16/22] x86/percpu: Adapt percpu for PIE support
- To: Brian Gerst <brgerst@xxxxxxxxx>
- From: Thomas Garnier <thgarnie@xxxxxxxxxx>
- Date: Wed, 19 Jul 2017 11:26:32 -0700
- Cc: Michal Hocko <mhocko@xxxxxxxx>, kvm list <kvm@xxxxxxxxxxxxxxx>, Radim Krčmář <rkrcmar@xxxxxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, Catalin Marinas <catalin.marinas@xxxxxxx>, Christopher Li <sparse@xxxxxxxxxxx>, the arch/x86 maintainers <x86@xxxxxxxxxx>, Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>, Pavel Machek <pavel@xxxxxx>, "H . Peter Anvin" <hpa@xxxxxxxxx>, Kernel Hardening <kernel-hardening@xxxxxxxxxxxxxxxxxx>, Christoph Lameter <cl@xxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Chris Metcalf <cmetcalf@xxxxxxxxxxxx>, linux-arch <linux-arch@xxxxxxxxxxxxxxx>, Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>, Daniel Borkmann <daniel@xxxxxxxxxxxxx>, Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>, Joerg Roedel <joro@xxxxxxxxxx>, Peter Foley <pefoley2@xxxxxxxxxxx>, Christian Borntraeger <borntraeger@xxxxxxxxxx>, linux-sparse@xxxxxxxxxxxxxxx, Matthias Kaehlcke <mka@xxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, Borislav Petkov <bp@xxxxxxx>, Markus Trippelsdorf <markus@xxxxxxxxxxxxxxx>, Len Brown <len.brown@xxxxxxxxx>, Kees Cook <keescook@xxxxxxxxxxxx>, Arnd Bergmann <arnd@xxxxxxxx>, linux-pm <linux-pm@xxxxxxxxxxxxxxx>, "H . J . Lu" <hjl.tools@xxxxxxxxx>, Steven Rostedt <rostedt@xxxxxxxxxxx>, Borislav Petkov <bp@xxxxxxxxx>, Andy Lutomirski <luto@xxxxxxxxxx>, Josh Poimboeuf <jpoimboe@xxxxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, Dou Liyang <douly.fnst@xxxxxxxxxxxxxx>, Paul Bolle <pebolle@xxxxxxxxxx>, "Paul E . McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>, Baoquan He <bhe@xxxxxxxxxx>, Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>, Jiri Kosina <jkosina@xxxxxxx>, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>, "Rafael J . Wysocki" <rjw@xxxxxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Daniel Micay <danielmicay@xxxxxxxxx>, "linux-crypto@xxxxxxxxxxxxxxx" <linux-crypto@xxxxxxxxxxxxxxx>, Rob Landley <rob@xxxxxxxxxxx>, Tejun Heo <tj@xxxxxxxxxx>, Paolo Bonzini <pbonzini@xxxxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>, "David S . Miller" <davem@xxxxxxxxxxxxx>, "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
- Delivery-date: Wed, 19 Jul 2017 18:26:38 +0000
- List-id: Xen developer discussion <xen-devel.lists.xen.org>
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. 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.
Thanks a lot for the feedback.
>
> --
> Brian Gerst
--
Thomas
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