Re: [PATCH v2 3/6] ARM: trusted_foundations: do not use naked function

[Kees Cook]

On Tue, Apr 17, 2018 at 1:11 AM, Thierry Reding <treding@xxxxxxxxxx> wrote:
> On Mon, Apr 16, 2018 at 08:21:09PM +0200, Stefan Agner wrote:
>> On 16.04.2018 18:08, Stephen Warren wrote:
>> > On 04/16/2018 09:56 AM, Stefan Agner wrote:
>> >> On 27.03.2018 14:16, Dmitry Osipenko wrote:
>> >>> On 27.03.2018 14:54, Robin Murphy wrote:
>> >>>> On 26/03/18 22:20, Dmitry Osipenko wrote:
>> >>>>> On 25.03.2018 21:09, Stefan Agner wrote:
>> >>>>>> As documented in GCC naked functions should only use Basic asm
>> >>>>>> syntax. The Extended asm or mixture of Basic asm and "C" code is
>> >>>>>> not guaranteed. Currently this works because it was hard coded
>> >>>>>> to follow and check GCC behavior for arguments and register
>> >>>>>> placement.
>> >>>>>>
>> >>>>>> Furthermore with clang using parameters in Extended asm in a
>> >>>>>> naked function is not supported:
>> >>>>>>     arch/arm/firmware/trusted_foundations.c:47:10: error: parameter
>> >>>>>>             references not allowed in naked functions
>> >>>>>>                   : "r" (type), "r" (arg1), "r" (arg2)
>> >>>>>>                          ^
>> >>>>>>
>> >>>>>> Use a regular function to be more portable. This aligns also with
>> >>>>>> the other smc call implementations e.g. in qcom_scm-32.c and
>> >>>>>> bcm_kona_smc.c.
>> >>>>>>
>> >>>>>> Cc: Dmitry Osipenko <digetx@xxxxxxxxx>
>> >>>>>> Cc: Stephen Warren <swarren@xxxxxxxxxx>
>> >>>>>> Cc: Thierry Reding <treding@xxxxxxxxxx>
>> >>>>>> Signed-off-by: Stefan Agner <stefan@xxxxxxxx>
>> >>>>>> ---
>> >>>>>> Changes in v2:
>> >>>>>> - Keep stmfd/ldmfd to avoid potential ABI issues
>> >>>>>>
>> >>>>>>    arch/arm/firmware/trusted_foundations.c | 14 +++++++++-----
>> >>>>>>    1 file changed, 9 insertions(+), 5 deletions(-)
>> >>>>>>
>> >>>>>> diff --git a/arch/arm/firmware/trusted_foundations.c
>> >>>>>> b/arch/arm/firmware/trusted_foundations.c
>> >>>>>> index 3fb1b5a1dce9..689e6565abfc 100644
>> >>>>>> --- a/arch/arm/firmware/trusted_foundations.c
>> >>>>>> +++ b/arch/arm/firmware/trusted_foundations.c
>> >>>>>> @@ -31,21 +31,25 @@
>> >>>>>>      static unsigned long cpu_boot_addr;
>> >>>>>>    -static void __naked tf_generic_smc(u32 type, u32 arg1, u32 arg2)
>> >>>>>> +static void tf_generic_smc(u32 type, u32 arg1, u32 arg2)
>> >>>>>>    {
>> >>>>>> +    register u32 r0 asm("r0") = type;
>> >>>>>> +    register u32 r1 asm("r1") = arg1;
>> >>>>>> +    register u32 r2 asm("r2") = arg2;
>> >>>>>> +
>> >>>>>>        asm volatile(
>> >>>>>>            ".arch_extension    sec\n\t"
>> >>>>>> -        "stmfd    sp!, {r4 - r11, lr}\n\t"
>> >>>>>> +        "stmfd    sp!, {r4 - r11}\n\t"
>> >>>>>>            __asmeq("%0", "r0")
>> >>>>>>            __asmeq("%1", "r1")
>> >>>>>>            __asmeq("%2", "r2")
>> >>>>>>            "mov    r3, #0\n\t"
>> >>>>>>            "mov    r4, #0\n\t"
>> >>>>>>            "smc    #0\n\t"
>> >>>>>> -        "ldmfd    sp!, {r4 - r11, pc}"
>> >>>>>> +        "ldmfd    sp!, {r4 - r11}\n\t"
>> >>>>>>            :
>> >>>>>> -        : "r" (type), "r" (arg1), "r" (arg2)
>> >>>>>> -        : "memory");
>> >>>>>> +        : "r" (r0), "r" (r1), "r" (r2)
>> >>>>>> +        : "memory", "r3", "r12", "lr");
>> >>>>>
>> >>>>> Although seems "lr" won't be affected by SMC invocation because it should be
>> >>>>> banked and hence could be omitted entirely from the code. Maybe somebody could
>> >>>>> confirm this.
>> >>>> Strictly per the letter of the architecture, the SMC could be trapped to Hyp
>> >>>> mode, and a hypervisor might clobber LR_usr in the process of forwarding the
>> >>>> call to the firmware secure monitor (since Hyp doesn't have a banked LR of its
>> >>>> own). Admittedly there are probably no real systems with the appropriate
>> >>>> hardware/software combination to hit that, but on the other hand if this gets
>> >>>> inlined where the compiler has already created a stack frame then an LR clobber
>> >>>> is essentially free, so I reckon we're better off keeping it for reassurance.
>> >>>> This isn't exactly a critical fast path anyway.
>> >>>
>> >>> Okay, thank you for the clarification.
>> >>
>> >> So it seems this change is fine?
>> >>
>> >> Stephen, you picked up changes for this driver before, is this patch
>> >> going through your tree?
>> >
>> > You had best ask Thierry; he's taken over Tegra maintenance upstream.
>> > But that said, don't files in arch/arm go through Russell?
>>
>> I think the last patches applied to that file went through your tree.
>>
>> Thierry, Russel, any preferences?
>
> I don't mind picking this up into the Tegra tree. Might be a good idea
> to move this into drivers/firmware, though, since that's where all the
> other firmware-related drivers reside.
>
> Firmware code, such as the BPMP driver, usually goes through ARM-SoC
> these days. I think this is in the same category.
>
> Russell, any objections to me picking this patch up and moving it into
> drivers/firmware?

Please take this -- without it I'm seeing build failures on the arm
allmodconfig under gcc 7.3.0:

/tmp/ccKdsC59.s: Assembler messages:
/tmp/ccKdsC59.s:35: Error: .err encountered
/tmp/ccKdsC59.s:36: Error: .err encountered
/tmp/ccKdsC59.s:37: Error: .err encountered
scripts/Makefile.build:317: recipe for target
'arch/arm/firmware/trusted_foundations.o' failed

The above patch fixes it for me.

Thanks!

-Kees

-- 
Kees Cook
Pixel Security