[RFC/RFT PATCH 0/2] arm64: per-task stack canaries

[Laura Abbott]

On 01/23/2018 05:03 AM, Ard Biesheuvel wrote:
> This is a proof of concept implementation of per-task stack canaries for
> arm64. The purpose is to reach agreement between the arm64 kernel and GCC
> maintainers on how to implement support for this in the compiler.
> 
> What these patches show is that we can support per-task stack canaries
> on arm64 using only very minor changes on the kernel side, i.e., all
> that is needed is exposing the offset of stack_canary in task_struct
> via an ELF symbol. With that in place, the compiler needs to emit the
> following sequence when -fstack-protector-guard=tls is enabled
> 
>    movz    xN, :abs_g0:__stack_chk_guard_offset
>    msr     xM, sp_el0
>    ldr     xM, [xM, xN]
> 
> Note that this does not involve per-CPU variables, and so there are no
> concurrency issues to be addressed. sp_el0 is the current task pointer,
> whose value never changes from the POV of the task, even when migrating
> to another CPU.
> 
> Patch #1 implements a GCC plugin that patches the sequence
> 
>    adrp    xN, __stack_chk_guard
>    add     xN, Xn, :lo12:__stack_chk_guard
> 
> into
> 
>    mrs     xN, sp_el0
>    add     xN, xN, :lo12:__stack_chk_guard_offset
> 
> which is a poor man's version of the movz/msr/ldr sequence above (and only
> works for small model code), but is sufficient as a proof of concept.
> 
> Patch #2 exposes the __stack_chk_guard_offset symbol and wires up the plugin
> (if enabled in Kconfig)
> 
> Again, the point is not to use GCC plugin based hacks, but to reach agreement
> on how to proceed with this for GCC.
> 
> Comments welcome.
> 

I was seeing some crashes with these when I tried to boot up on my
full Fedora system. It looked like a compiler bug with grabbing
the wrong literal but I don't think it's worth looking at it since
it's probably just something with the plugin which isn't the real
focus here. I can send along the crash if you are interested.

It looked good to me otherwise.

> Ard Biesheuvel (2):
>    gcc-plugins: add support plugin for arm64 per-task stack canaries
>    arm64: kernel: use a unique stack canary value for each task
> 
>   arch/Kconfig                                    |   4 +
>   arch/arm64/Kconfig                              |   7 ++
>   arch/arm64/include/asm/stackprotector.h         |   4 +-
>   arch/arm64/kernel/asm-offsets.c                 |   3 +
>   arch/arm64/kernel/process.c                     |   4 +
>   arch/arm64/kernel/vmlinux.lds.S                 |   8 ++
>   scripts/Makefile.gcc-plugins                    |   2 +
>   scripts/gcc-plugins/arm64_ssp_per_task_plugin.c | 121 ++++++++++++++++++++
>   8 files changed, 152 insertions(+), 1 deletion(-)
>   create mode 100644 scripts/gcc-plugins/arm64_ssp_per_task_plugin.c
>