kernel-hardening - Re: [PATCH v7 00/11] add support for Clang's Shadow Call Stack

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Date: Tue, 11 Feb 2020 13:57:24 +0000
From: James Morse <james.morse@....com>
To: Sami Tolvanen <samitolvanen@...gle.com>
Cc: Will Deacon <will@...nel.org>, Catalin Marinas <catalin.marinas@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, Ard Biesheuvel <ard.biesheuvel@...aro.org>,
 Mark Rutland <mark.rutland@....com>, Dave Martin <Dave.Martin@....com>,
 Kees Cook <keescook@...omium.org>, Laura Abbott <labbott@...hat.com>,
 Marc Zyngier <maz@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>,
 Jann Horn <jannh@...gle.com>, Miguel Ojeda
 <miguel.ojeda.sandonis@...il.com>,
 Masahiro Yamada <yamada.masahiro@...ionext.com>,
 clang-built-linux@...glegroups.com, kernel-hardening@...ts.openwall.com,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 00/11] add support for Clang's Shadow Call Stack

Hi Sami,

On 28/01/2020 18:49, Sami Tolvanen wrote:
> This patch series adds support for Clang's Shadow Call Stack
> (SCS) mitigation, which uses a separately allocated shadow stack
> to protect against return address overwrites. More information
> can be found here:
> 
>   https://clang.llvm.org/docs/ShadowCallStack.html
> 
> SCS provides better protection against traditional buffer
> overflows than CONFIG_STACKPROTECTOR_*, but it should be noted
> that SCS security guarantees in the kernel differ from the ones
> documented for user space. The kernel must store addresses of
> shadow stacks used by inactive tasks and interrupt handlers in
> memory, which means an attacker capable reading and writing
> arbitrary memory may be able to locate them and hijack control
> flow by modifying shadow stacks that are not currently in use.
> 
> SCS is currently supported only on arm64, where the compiler
> requires the x18 register to be reserved for holding the current
> task's shadow stack pointer.

I found I had to add:
| KBUILD_CFLAGS := $(filter-out -ffixed-x18 $(CC_FLAGS_SCS), $(KBUILD_CFLAGS))

to drivers/firmware/efi/libstub/Makefile, to get this going.
I don't think there is much point supporting SCS for the EFIstub, its already isolated
from the rest of the kernel's C code by the __efistub symbol prefix machinery, and trying
to use it would expose us to buggy firmware at a point we can't handle it!

I can send a patch if its easier for you,


Thanks,

James

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