[PATCH v10 5/6] arm64: entry: Enable random_kstack_offset support

[Will Deacon]

On Thu, Apr 01, 2021 at 04:23:46PM -0700, Kees Cook wrote:
> Allow for a randomized stack offset on a per-syscall basis, with roughly
> 5 bits of entropy. (And include AAPCS rationale AAPCS thanks to Mark
> Rutland.)
> 
> In order to avoid unconditional stack canaries on syscall entry (due to
> the use of alloca()), also disable stack protector to avoid triggering
> needless checks and slowing down the entry path. As there is no general
> way to control stack protector coverage with a function attribute[1],
> this must be disabled at the compilation unit level. This isn't a problem
> here, though, since stack protector was not triggered before: examining
> the resulting syscall.o, there are no changes in canary coverage (none
> before, none now).
> 
> [1] a working __attribute__((no_stack_protector)) has been added to GCC
> and Clang but has not been released in any version yet:
> https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;h=346b302d09c1e6db56d9fe69048acb32fbb97845
> https://reviews.llvm.org/rG4fbf84c1732fca596ad1d6e96015e19760eb8a9b
> 
> Signed-off-by: Kees Cook <keescook at chromium.org>
> ---
>  arch/arm64/Kconfig          |  1 +
>  arch/arm64/kernel/Makefile  |  5 +++++
>  arch/arm64/kernel/syscall.c | 16 ++++++++++++++++
>  3 files changed, 22 insertions(+)

Acked-by: Will Deacon <will at kernel.org>

Will