linux-kernel - Re: [PATCH v24 25/28] riscv: create a config for shadow stack and landing pad instr support

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Message-ID: <aTMjS-Ok-DrJJjQY@debug.ba.rivosinc.com>
Date: Fri, 5 Dec 2025 10:24:11 -0800
From: Deepak Gupta <debug@...osinc.com>
To: Randy Dunlap <rdunlap@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
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	Benno Lossin <lossin@...nel.org>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-riscv@...ts.infradead.org, devicetree@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
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	rick.p.edgecombe@...el.com, rust-for-linux@...r.kernel.org,
	Zong Li <zong.li@...ive.com>,
	Andreas Korb <andreas.korb@...ec.fraunhofer.de>,
	Valentin Haudiquet <valentin.haudiquet@...onical.com>
Subject: Re: [PATCH v24 25/28] riscv: create a config for shadow stack and
 landing pad instr support

On Thu, Dec 04, 2025 at 02:17:27PM -0800, Randy Dunlap wrote:
>
>
>On 12/4/25 12:04 PM, Deepak Gupta wrote:
>> This patch creates a config for shadow stack support and landing pad instr
>> support. Shadow stack support and landing instr support can be enabled by
>> selecting `CONFIG_RISCV_USER_CFI`. Selecting `CONFIG_RISCV_USER_CFI` wires
>> up path to enumerate CPU support and if cpu support exists, kernel will
>> support cpu assisted user mode cfi.
>>
>> If CONFIG_RISCV_USER_CFI is selected, select `ARCH_USES_HIGH_VMA_FLAGS`,
>> `ARCH_HAS_USER_SHADOW_STACK` and DYNAMIC_SIGFRAME for riscv.
>>
>> Reviewed-by: Zong Li <zong.li@...ive.com>
>> Tested-by: Andreas Korb <andreas.korb@...ec.fraunhofer.de>
>> Tested-by: Valentin Haudiquet <valentin.haudiquet@...onical.com>
>> Signed-off-by: Deepak Gupta <debug@...osinc.com>
>> ---
>>  arch/riscv/Kconfig                  | 22 ++++++++++++++++++++++
>>  arch/riscv/configs/hardening.config |  4 ++++
>>  2 files changed, 26 insertions(+)
>>
>> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
>> index 0c6038dc5dfd..f5574c6f66d8 100644
>> --- a/arch/riscv/Kconfig
>> +++ b/arch/riscv/Kconfig
>> @@ -1146,6 +1146,28 @@ config RANDOMIZE_BASE
>>
>>            If unsure, say N.
>>
>> +config RISCV_USER_CFI
>> +	def_bool y
>> +	bool "riscv userspace control flow integrity"
>> +	depends on 64BIT && \
>> +		$(cc-option,-mabi=lp64 -march=rv64ima_zicfiss_zicfilp -fcf-protection=full)
>> +	depends on RISCV_ALTERNATIVE
>> +	select RISCV_SBI
>> +	select ARCH_HAS_USER_SHADOW_STACK
>> +	select ARCH_USES_HIGH_VMA_FLAGS
>> +	select DYNAMIC_SIGFRAME
>> +	help
>> +	  Provides CPU assisted control flow integrity to userspace tasks.
>
>	           CPU-assisted
>
>> +	  Control flow integrity is provided by implementing shadow stack for
>> +	  backward edge and indirect branch tracking for forward edge in program.
>> +	  Shadow stack protection is a hardware feature that detects function
>> +	  return address corruption. This helps mitigate ROP attacks.
>> +	  Indirect branch tracking enforces that all indirect branches must land
>> +	  on a landing pad instruction else CPU will fault. This mitigates against
>> +	  JOP / COP attacks. Applications must be enabled to use it, and old user-
>> +	  space does not get protection "for free".
>> +	  default y.
>
>	  Default is y if hardware supports it.
>?

No default Y means support is built in the kernel for cfi.
If hardware doesn't support CFI instructions, then kernel will do following

- prctls to manage shadow stack/landing pad enable/disable will fail.
- vDSO will not have shadow stack instructions in it.


>
>> +
>>  endmenu # "Kernel features"
>
>
>-- 
>~Randy
>