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Date:   Sun, 5 Jul 2020 13:40:30 -0700
From:   Kees Cook <keescook@...omium.org>
To:     guoren@...nel.org
Cc:     linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-csky@...r.kernel.org, Guo Ren <guoren@...ux.alibaba.com>,
        Ramana Radhakrishnan <ramana.radhakrishnan@....com>
Subject: Re: [RFC PATCH] riscv: enable per-task stack canaries

On Sun, Jul 05, 2020 at 02:13:17PM +0000, guoren@...nel.org wrote:
> From: Guo Ren <guoren@...ux.alibaba.com>
> 
> After compare arm64 and x86 implementations, seems arm64's is more
> flexible and readable. The key point is how gcc get the offset of
> stack_canary from gs/el0_sp.
> 
> x86: Use a fix offset from gs, not flexible.
> 
> struct fixed_percpu_data {
>         /*
>          * GCC hardcodes the stack canary as %gs:40.  Since the
>          * irq_stack is the object at %gs:0, we reserve the bottom
>          * 48 bytes of the irq stack for the canary.
>          */
>         char            gs_base[40]; // :(
>         unsigned long   stack_canary;
> };

Yes, x86's compiler's implementation of "thread local" stack canary
isn't great for the kernel.

> arm64: Use -mstack-protector-guard-offset & guard-reg
> 
> ifeq ($(CONFIG_STACKPROTECTOR_PER_TASK),y)
> prepare: stack_protector_prepare
> stack_protector_prepare: prepare0
>        $(eval KBUILD_CFLAGS += -mstack-protector-guard=sysreg            \
>                                -mstack-protector-guard-reg=sp_el0        \
>                                -mstack-protector-guard-offset=$(shell    \
>                        awk '{if ($$2 == "TSK_STACK_CANARY") print $$3;}' \
>                                        include/generated/asm-offsets.h))
> endif
> 
> I prefer arm64, but x86 percpu_data design needs to be considered ?

I don't know riscv internals, so I leave that to y'all! :)

> After the discussion, let's continue the work for riscv gcc
> stack-protector.

I think you'll need some buy-in from GCC before this kernel patch can
land.

> Here is arm64 gcc's work [1].
> 
> [1] https://github.com/gcc-mirror/gcc/commit/cd0b2d361df82c848dc7e1c3078651bb0624c3c6

Can this kind of thing be made general-purposes, instead of having to
reimplement it each time there's a new arch wanting to do it?

> 
> Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
> Cc: Kees Cook <keescook@...omium.org>
> ---
>  arch/riscv/Kconfig                      |  7 +++++++
>  arch/riscv/Makefile                     | 10 ++++++++++
>  arch/riscv/include/asm/stackprotector.h |  3 ++-
>  arch/riscv/kernel/asm-offsets.c         |  3 +++
>  arch/riscv/kernel/process.c             |  2 +-
>  5 files changed, 23 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 4b0e308..4b4e833 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -394,6 +394,13 @@ config CMDLINE_FORCE
>  
>  endchoice
>  
> +config CC_HAVE_STACKPROTECTOR_SYSREG
> +	def_bool $(cc-option,-mstack-protector-guard=gpr -mstack-protector-guard-reg=tp -mstack-protector-guard-offset=0)

And, as I'm sure you realize, it's not supported by the riscv backend
yet:

riscv64-unknown-linux-gnu-gcc: error: unrecognized command line option '-mstack-protector-guard=gpr'; did you mean '-fstack-protector-strong'?
riscv64-unknown-linux-gnu-gcc: error: unrecognized command line option '-mstack-protector-guard-reg=tp'; did you mean '-fstack-protector-strong'?
riscv64-unknown-linux-gnu-gcc: error: unrecognized command line option '-mstack-protector-guard-offset=0'; did you mean '-fstack-protector-strong'?

-- 
Kees Cook

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