Re: [RFC][AArch64] Add support for system register based stack protector canary access

[James Greenhalgh <james.greenhalgh@arm.com>]

On Mon, Dec 03, 2018 at 03:55:36AM -0600, Ramana Radhakrishnan wrote:
> For quite sometime the kernel guys, (more specifically Ard) have been 
> talking about using a system register (sp_el0) and an offset from that 
> for a canary based access. This patchset adds support for a new set of
> command line options similar to how powerpc has done this.
> 
> I don't intend to change the defaults in userland, we've discussed this 
> for user-land in the past and as far as glibc and userland is concerned 
> we stick to the options as currently existing. The system register 
> option is really for the kernel to use along with an offset as they 
> control their ABI and this is a decision for them to make.
> 
> I did consider sticking this all under a mcmodel=kernel-small option but
> thought that would be a bit too aggressive. There is very little error
> checking I can do in terms of the system register being used and really
> the assembler would barf quite quickly in case things go wrong. I've
> managed to rebuild Ard's kernel tree with an additional patch that
> I will send to him. I haven't managed to boot this kernel.
> 
> There was an additional question asked about the performance 
> characteristics of this but it's a security feature and the kernel 
> doesn't have the luxury of a hidden symbol. Further since the kernel 
> uses sp_el0 for access everywhere and if they choose to use the same 
> register I don't think the performance characteristics would be too bad, 
> but that's a decision for the kernel folks to make when taking in the 
> feature into the kernel.
> 
> I still need to add some tests and documentation in invoke.texi but
> this is at the stage where it would be nice for some other folks
> to look at this.
> 
> The difference in code generated is as below.
> 
> extern void bar (char *);
> int foo (void)
> {
>    char a[100];
>    bar (&a);
> }
> 
> $GCC -O2  -fstack-protector-strong  vs 
> -mstack-protector-guard-reg=sp_el0 -mstack-protector-guard=sysreg 
> -mstack-protector-guard-offset=1024 -fstack-protector-strong
> 
> 	
> --- tst.s	2018-12-03 09:46:21.174167443 +0000
> +++ tst.s.1	2018-12-03 09:46:03.546257203 +0000
> @@ -15,15 +15,14 @@
>   	mov	x29, sp
>   	str	x19, [sp, 16]
>   	.cfi_offset 19, -128
> -	adrp	x19, __stack_chk_guard
> -	add	x19, x19, :lo12:__stack_chk_guard
> -	ldr	x0, [x19]
> -	str	x0, [sp, 136]
> -	mov	x0,0
> +	mrs	x19, sp_el0
>   	add	x0, sp, 32
> +	ldr	x1, [x19, 1024]
> +	str	x1, [sp, 136]
> +	mov	x1,0
>   	bl	bar
>   	ldr	x0, [sp, 136]
> -	ldr	x1, [x19]
> +	ldr	x1, [x19, 1024]
>   	eor	x1, x0, x1
>   	cbnz	x1, .L5
> 
> 
> 
> 
> I will be afk tomorrow and day after but this is to elicit some comments 
> and for Ard to try this out with his kernel patches.
> 
> Thoughts ?

I didn't see ananswer on list to Ard's questions about the command-line logic.
Remember to also fix up the error message concerns Florian raised.

That said, if Jakub is happy with this in Stage 4, I am too.

My biggest concern is the -mstack-protector-guard-reg interface, which
is unchecked user input and so opens up nasty ways to force the compiler
towards out of bounds accesses (e.g.
-mstack-protector-guard-reg="What memory is at %10")

Thanks,
James

> 
> regards
> Ramana
> 
> gcc/ChangeLog:
> 
> 2018-11-23  Ramana Radhakrishnan  <ramana.radhakrishnan@arm.com>
> 
>          * config/aarch64/aarch64-opts.h (enum stack_protector_guard): New
>          * config/aarch64/aarch64.c (aarch64_override_options_internal): 
> Handle
>          and put in error checks for stack protector guard options.
>          (aarch64_stack_protect_guard): New.
>          (TARGET_STACK_PROTECT_GUARD): Define.
>          * config/aarch64/aarch64.md (UNSPEC_SSP_SYSREG): New.
>          (reg_stack_protect_address<mode>): New.
>          (stack_protect_set): Adjust for SSP_GLOBAL.
>          (stack_protect_test): Likewise.
>          * config/aarch64/aarch64.opt (-mstack-protector-guard-reg): New.
>          (-mstack-protector-guard): Likewise.
>          (-mstack-protector-guard-offset): Likewise.
>          * doc/invoke.texi: Document new AArch64 options.