Re: [PATCH 1/5] arm64: KVM: Do not use stack-protector to compile EL2 code

Marc Zyngier
2017-05-11T16:42:41Z

On 11/05/17 17:36, Ard Biesheuvel wrote:
> On 11 May 2017 at 17:11, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
>> On 11 May 2017 at 17:02, Marc Zyngier <marc.zyngier@arm.com> wrote:
>>> On 02/05/17 15:50, Marc Zyngier wrote:
>>>> On 02/05/17 15:40, Catalin Marinas wrote:
>>>>> On Tue, May 02, 2017 at 02:30:37PM +0100, Marc Zyngier wrote:
>>>>>> We like living dangerously. Nothing explicitely forbids stack-protector
>>>>>> to be used in the EL2 code, while distributions routinely compile their
>>>>>> kernel with it. We're just lucky that no code actually triggers the
>>>>>> instrumentation.
>>>>>>
>>>>>> Let's not try our luck for much longer, and disable stack-protector
>>>>>> for code living at EL2.
>>>>>>
>>>>>> Cc: stable@vger.kernel.org
>>>>>> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
>>>>>> ---
>>>>>> arch/arm64/kvm/hyp/Makefile | 2 ++
>>>>>> 1 file changed, 2 insertions(+)
>>>>>>
>>>>>> diff --git a/arch/arm64/kvm/hyp/Makefile b/arch/arm64/kvm/hyp/Makefile
>>>>>> index aaf42ae8d8c3..14c4e3b14bcb 100644
>>>>>> --- a/arch/arm64/kvm/hyp/Makefile
>>>>>> +++ b/arch/arm64/kvm/hyp/Makefile
>>>>>> @@ -2,6 +2,8 @@
>>>>>> # Makefile for Kernel-based Virtual Machine module, HYP part
>>>>>> #
>>>>>>
>>>>>> +ccflags-y += -fno-stack-protector
>>>>>> +
>>>>>
>>>>> While you are at it, should we have a -fpic here as well? The hyp code
>>>>> runs at a different location than the rest of the kernel.
>>>>
>>>> We definitely should. I've just tried this, and this doesn't seem to
>>>> work very well. At least this seems to break our jump label
>>>> implementation. I need to page in that part of the code base and see
>>>> what happens.
>>>
>>> So here's the issue:
>>>
>>> CC arch/arm64/kvm/hyp/../../../../virt/kvm/arm/hyp/vgic-v3-sr.o
>>> In file included from ./include/linux/jump_label.h:120:0,
>>> from ./include/linux/dynamic_debug.h:5,
>>> from ./include/linux/printk.h:329,
>>> from ./include/linux/kernel.h:13,
>>> from ./include/asm-generic/bug.h:15,
>>> from ./arch/arm64/include/asm/bug.h:66,
>>> from ./include/linux/bug.h:4,
>>> from ./include/linux/mmdebug.h:4,
>>> from ./include/linux/mm.h:8,
>>> from ./arch/arm64/include/asm/cacheflush.h:22,
>>> from ./arch/arm64/include/asm/arch_gicv3.h:27,
>>> from ./include/linux/irqchip/arm-gic-v3.h:453,
>>> from arch/arm64/kvm/hyp/../../../../virt/kvm/arm/hyp/vgic-v3-sr.c:19:
>>> ./arch/arm64/include/asm/jump_label.h: In function '__vgic_v3_save_state':
>>> ./arch/arm64/include/asm/jump_label.h:31:2: warning: asm operand 0 probably doesn't match constraints
>>> asm goto("1: nop\n\t"
>>> ^~~
>>> ./arch/arm64/include/asm/jump_label.h:31:2: error: impossible constraint in 'asm'
>>> ./arch/arm64/include/asm/jump_label.h: In function '__vgic_v3_restore_state':
>>> ./arch/arm64/include/asm/jump_label.h:31:2: warning: asm operand 0 probably doesn't match constraints
>>> asm goto("1: nop\n\t"
>>> ^~~
>>> scripts/Makefile.build:294: recipe for target 'arch/arm64/kvm/hyp/../../../../virt/kvm/arm/hyp/vgic-v3-sr.o' failed
>>> make[1]: *** [arch/arm64/kvm/hyp/../../../../virt/kvm/arm/hyp/vgic-v3-sr.o] Error 1
>>> Makefile:1664: recipe for target 'arch/arm64/kvm/hyp/' failed
>>>
>>> The corresponding code does this:
>>>
>>> static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
>>> {
>>> asm goto("1: nop\n\t"
>>> ".pushsection __jump_table, \"aw\"\n\t"
>>> ".align 3\n\t"
>>> ".quad 1b, %l[l_yes], %c0\n\t"
>>> ".popsection\n\t"
>>> : : "i"(&((char *)key)[branch]) : : l_yes);
>>>
>>> return false;
>>> l_yes:
>>> return true;
>>> }
>>>
>>> and the problem lies in the evaluation of "key", which probably
>>> cannot be guaranteed a constant at that point. There is also the
>>> issue that even if it was known, the branch cannot be easily
>>> patched in from the rest of the kernel (how is the l_yes address
>>> represented?).
>>>
>>> It looks to me like we either need to rewrite the whole of our
>>> static key infrastructure to cope with PIC, or switch over to
>>> the hyp_alternate_select() hack, which I'd rather avoid spreading
>>> further.
>>>
>>> In the end, I wonder if that's even worth it...
>>>
>>
>> Could you check if it builds with
>>
>>> ".long 1b - ., %l[l_yes] - ., %c0 - .\n\t"
>>
>> instead? We'd still need to update the code that interprets the
>> __jump_table fields, but it changes the references into relative ones,
>> which also reduces the size as a bonus.
>
> OK, strike that, this is more tricky than I thought. I am failing to
> reproduce this locally, though. Which gcc and tree are you using?

That's current mainline + a number of patches which I don't think are
relevant to this discussion, and -fPIC added to
arch/arm64/kvm/hyp/Makefile. You should see it exploding in timer-sr.c
because of the has_vhe() helper.

GCC is "aarch64-linux-gnu-gcc (Linaro GCC 6.2-2016.11) 6.2.1 20161016".

Thanks,

M.
--
Jazz is not dead. It just smells funny...