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Date: Wed, 19 Feb 2020 11:33:52 +0000
From: Mark Rutland <mark.rutland@....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>, james.morse@....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 v8 04/12] scs: disable when function graph tracing is
enabled
On Tue, Feb 18, 2020 at 04:08:09PM -0800, Sami Tolvanen wrote:
> The graph tracer hooks returns by modifying frame records on the
> (regular) stack, but with SCS the return address is taken from the
> shadow stack, and the value in the frame record has no effect. As we
> don't currently have a mechanism to determine the corresponding slot
> on the shadow stack (and to pass this through the ftrace
> infrastructure), for now let's disable SCS when the graph tracer is
> enabled.
>
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> Reviewed-by: Kees Cook <keescook@...omium.org>
> ---
> arch/Kconfig | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 66b34fd0df54..4102b8e0eea9 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -535,6 +535,7 @@ config ARCH_SUPPORTS_SHADOW_CALL_STACK
>
> config SHADOW_CALL_STACK
> bool "Clang Shadow Call Stack"
> + depends on !FUNCTION_GRAPH_TRACER
Fangrui Song has implemented `-fpatchable-function-entry` in LLVM (for
10.x onwards), so we can support this when DYNAMIC_FTRACE_WITH_REGS is
selected.
This can be:
depends on DYNAMIC_FTRACE_WITH_REGS || !FUNCTION_GRAPH_TRACER
... and we can update the commit message to something like:
| With SCS the return address is taken from the shadow stack and the
| value in the frame record has no effect. The mcount based graph tracer
| hooks returns by modifying frame records on the (regular) stack, and
| thus is not compatible. The patchable-function-entry graph tracer
| used for DYNAMIC_FTRACE_WITH_REGS modifies the LR before it is saved
| to the shadow stack, and is compatible.
|
| Modifying the mcount based graph tracer to work with SCS would require
| a mechanism to determine the corresponding slot on the shadow stack
| (and to pass this through the ftrace infrastructure), and we expect
| that everyone will eventually move to the patchable-function-entry
| based graph tracer anyway, so for now let's disable SCS when the
| mcount-based graph tracer is enabled.
|
| SCS and patchable-function-entry are both supported from LLVM 10.x.
Assuming you're happy with that:
Reviewed-by: Mark Rutland <mark.rutland@....com>
Thanks,
Mark.
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