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Message-ID: <0371ffc5-f6c9-4352-89d5-0e98afa9ad7f@infradead.org>
Date: Sat, 3 Feb 2024 17:33:02 -0800
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Subject: Re: [PATCH RFT v5 1/7] Documentation: userspace-api: Add shadow stack
 API documentation
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To: Mark Brown <broonie@kernel.org>,
 "Rick P. Edgecombe" <rick.p.edgecombe@intel.com>,
 Deepak Gupta <debug@rivosinc.com>, Szabolcs Nagy <Szabolcs.Nagy@arm.com>,
 "H.J. Lu" <hjl.tools@gmail.com>, Florian Weimer <fweimer@redhat.com>,
 Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
 Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>,
 x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
 Peter Zijlstra <peterz@infradead.org>, Juri Lelli <juri.lelli@redhat.com>,
 Vincent Guittot <vincent.guittot@linaro.org>,
 Dietmar Eggemann <dietmar.eggemann@arm.com>,
 Steven Rostedt <rostedt@goodmis.org>, Ben Segall <bsegall@google.com>,
 Mel Gorman <mgorman@suse.de>, Daniel Bristot de Oliveira
 <bristot@redhat.com>, Valentin Schneider <vschneid@redhat.com>,
 Christian Brauner <brauner@kernel.org>, Shuah Khan <shuah@kernel.org>
Cc: linux-kernel@vger.kernel.org, Catalin Marinas <catalin.marinas@arm.com>,
 Will Deacon <will@kernel.org>, Kees Cook <keescook@chromium.org>,
 jannh@google.com, linux-kselftest@vger.kernel.org, linux-api@vger.kernel.org
References: <20240203-clone3-shadow-stack-v5-0-322c69598e4b@kernel.org>
 <20240203-clone3-shadow-stack-v5-1-322c69598e4b@kernel.org>
From: Randy Dunlap <rdunlap@infradead.org>
In-Reply-To: <20240203-clone3-shadow-stack-v5-1-322c69598e4b@kernel.org>
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Hi,

On 2/2/24 16:04, Mark Brown wrote:
> There are a number of architectures with shadow stack features which we are
> presenting to userspace with as consistent an API as we can (though there
> are some architecture specifics). Especially given that there are some
> important considerations for userspace code interacting directly with the
> feature let's provide some documentation covering the common aspects.
> 
> Signed-off-by: Mark Brown <broonie@kernel.org>
> ---
>  Documentation/userspace-api/index.rst        |  1 +
>  Documentation/userspace-api/shadow_stack.rst | 41 ++++++++++++++++++++++++++++
>  2 files changed, 42 insertions(+)
> 


> diff --git a/Documentation/userspace-api/shadow_stack.rst b/Documentation/userspace-api/shadow_stack.rst
> new file mode 100644
> index 000000000000..c6e5ab795b60
> --- /dev/null
> +++ b/Documentation/userspace-api/shadow_stack.rst
> @@ -0,0 +1,41 @@
> +=============
> +Shadow Stacks
> +=============
> +
> +Introduction
> +============
> +
> +Several architectures have features which provide backward edge
> +control flow protection through a hardware maintained stack, only
> +writeable by userspace through very limited operations.  This feature
> +is referred to as shadow stacks on Linux, on x86 it is part of Intel

                                   on Linux. On x86

> +Control Enforcement Technology (CET), on arm64 it is Guarded Control
> +Stacks feature (FEAT_GCS) and for RISC-V it is the Zicfiss extension.> +It is expected that this feature will normally be managed by the
> +system dynamic linker and libc in ways broadly transparent to
> +application code, this document covers interfaces and considerations

               code. This                                considerations.

> +
> +
> +Enabling
> +========
> +
> +Shadow stacks default to disabled when a userspace process is
> +executed, they can be enabled for the current thread with a syscall:

   executed. They

> +
> + - For x86 the ARCH_SHSTK_ENABLE arch_prctl()
> +
> +It is expected that this will normally be done by the dynamic linker.
> +Any new threads created by a thread with shadow stacks enabled will
> +themsleves have shadow stacks enabled.

   themselves

> +
> +
> +Enablement considerations
> +=========================
> +
> +- Returning from the function that enables shadow stacks without first
> +  disabling them will cause a shadow stack exception.  This includes
> +  any syscall wrapper or other library functions, the syscall will need

                                          functions. The

> +  to be inlined.
> +- A lock feature allows userspace to prevent disabling of shadow stacks.
> +- This that change the stack context like longjmp() or use of ucontext

     Those
?

> +  changes on signal return will need support from libc.
> 

-- 
#Randy