[v4,0/1] implement TLS register based stack canary for ARM

Bugzilla: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102352

In the Linux kernel, user processes calling into the kernel are
essentially threads running in the same address space, of a program that
never terminates. This means that using a global variable for the stack
protector canary value is problematic on SMP systems, as we can never
change it unless we reboot the system. (Processes that sleep for any
reason will do so on a call into the kernel, which means that there will
always be live kernel stack frames carrying copies of the canary taken
when the function was entered)

AArch64 implements -mstack-protector-guard=sysreg for this purpose, as
this permits the kernel to use different memory addresses for the stack
canary for each CPU, and context switch the chosen system register with
the rest of the process, allowing each process to use its own unique
value for the stack canary.

This patch implements something similar, but for the 32-bit ARM kernel,
which will start using the user space TLS register TPIDRURO to index
per-process metadata while running in the kernel. This means we can just
add an offset to TPIDRURO to obtain the address from which to load the
canary value.

Changes since v3:
- force a reload of the TLS register before performing the stack
  protector check, so that we never rely on the stack for the address of
  the canary 
Changes since v2:
- fix the template for stack_protect_test_tls so it correctly conveys
  the fact that it sets the Z flag

Comments/suggestions welcome.

Cc: Keith Packard <keithpac@amazon.com>
Cc: thomas.preudhomme@celest.fr
Cc: adhemerval.zanella@linaro.org
Cc: Qing Zhao <qing.zhao@oracle.com>
Cc: Richard Sandiford <richard.sandiford@arm.com>
Cc: gcc-patches@gcc.gnu.org

Ard Biesheuvel (1):
  [ARM] Add support for TLS register based stack protector canary access

 gcc/config/arm/arm-opts.h   |  6 ++
 gcc/config/arm/arm-protos.h |  2 +
 gcc/config/arm/arm.c        | 55 +++++++++++++++
 gcc/config/arm/arm.md       | 71 +++++++++++++++++++-
 gcc/config/arm/arm.opt      | 22 ++++++
 gcc/doc/invoke.texi         |  9 +++
 6 files changed, 163 insertions(+), 2 deletions(-)

On Thu, 28 Oct 2021 at 13:27, Ard Biesheuvel <ardb@kernel.org> wrote:
>
> Bugzilla: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102352
>
> In the Linux kernel, user processes calling into the kernel are
> essentially threads running in the same address space, of a program that
> never terminates. This means that using a global variable for the stack
> protector canary value is problematic on SMP systems, as we can never
> change it unless we reboot the system. (Processes that sleep for any
> reason will do so on a call into the kernel, which means that there will
> always be live kernel stack frames carrying copies of the canary taken
> when the function was entered)
>
> AArch64 implements -mstack-protector-guard=sysreg for this purpose, as
> this permits the kernel to use different memory addresses for the stack
> canary for each CPU, and context switch the chosen system register with
> the rest of the process, allowing each process to use its own unique
> value for the stack canary.
>
> This patch implements something similar, but for the 32-bit ARM kernel,
> which will start using the user space TLS register TPIDRURO to index
> per-process metadata while running in the kernel. This means we can just
> add an offset to TPIDRURO to obtain the address from which to load the
> canary value.
>
> Changes since v3:
> - force a reload of the TLS register before performing the stack
>   protector check, so that we never rely on the stack for the address of
>   the canary
> Changes since v2:
> - fix the template for stack_protect_test_tls so it correctly conveys
>   the fact that it sets the Z flag
>
> Comments/suggestions welcome.
>
> Cc: Keith Packard <keithpac@amazon.com>
> Cc: thomas.preudhomme@celest.fr
> Cc: adhemerval.zanella@linaro.org
> Cc: Qing Zhao <qing.zhao@oracle.com>
> Cc: Richard Sandiford <richard.sandiford@arm.com>
> Cc: gcc-patches@gcc.gnu.org
>

Note to reviewers: this feature has been accepted in LLVM/Clang, and
so the exact command line options introduced by this patch to enable
this feature can no longer be changed easily. I don't expect this to
be an issue, given that they are the same as x86, but I thought I
should note it nonetheless.