linux-kernel - Re: [PATCH v12 23/28] riscv: kernel command line option to opt out of user cfi

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Message-Id: <D8LF0RDZ6809.1I3MCCVSHRSQ2@ventanamicro.com>
Date: Thu, 20 Mar 2025 22:35:52 +0100
From: Radim Krčmář <rkrcmar@...tanamicro.com>
To: "Deepak Gupta" <debug@...osinc.com>, "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>, "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 "Vlastimil Babka" <vbabka@...e.cz>, "Lorenzo Stoakes"
 <lorenzo.stoakes@...cle.com>, "Paul Walmsley" <paul.walmsley@...ive.com>,
 "Palmer Dabbelt" <palmer@...belt.com>, "Albert Ou" <aou@...s.berkeley.edu>,
 "Conor Dooley" <conor@...nel.org>, "Rob Herring" <robh@...nel.org>,
 "Krzysztof Kozlowski" <krzk+dt@...nel.org>, "Arnd Bergmann"
 <arnd@...db.de>, "Christian Brauner" <brauner@...nel.org>, "Peter Zijlstra"
 <peterz@...radead.org>, "Oleg Nesterov" <oleg@...hat.com>, "Eric Biederman"
 <ebiederm@...ssion.com>, "Kees Cook" <kees@...nel.org>, "Jonathan Corbet"
 <corbet@....net>, "Shuah Khan" <shuah@...nel.org>, "Jann Horn"
 <jannh@...gle.com>, "Conor Dooley" <conor+dt@...nel.org>
Cc: <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>, <linux-kselftest@...r.kernel.org>,
 <alistair.francis@....com>, <richard.henderson@...aro.org>,
 <jim.shu@...ive.com>, <andybnac@...il.com>, <kito.cheng@...ive.com>,
 <charlie@...osinc.com>, <atishp@...osinc.com>, <evan@...osinc.com>,
 <cleger@...osinc.com>, <alexghiti@...osinc.com>, <samitolvanen@...gle.com>,
 <broonie@...nel.org>, <rick.p.edgecombe@...el.com>, "linux-riscv"
 <linux-riscv-bounces@...ts.infradead.org>
Subject: Re: [PATCH v12 23/28] riscv: kernel command line option to opt out
 of user cfi

2025-03-14T14:39:42-07:00, Deepak Gupta <debug@...osinc.com>:
> This commit adds a kernel command line option using which user cfi can be
> disabled.
>
> Signed-off-by: Deepak Gupta <debug@...osinc.com>
> ---
>  arch/riscv/kernel/usercfi.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
>
> diff --git a/arch/riscv/kernel/usercfi.c b/arch/riscv/kernel/usercfi.c
> index d31d89618763..813162ce4f15 100644
> --- a/arch/riscv/kernel/usercfi.c
> +++ b/arch/riscv/kernel/usercfi.c
> @@ -17,6 +17,8 @@
>  #include <asm/csr.h>
>  #include <asm/usercfi.h>
>  
> +bool disable_riscv_usercfi;
> +
>  #define SHSTK_ENTRY_SIZE sizeof(void *)
>  
>  bool is_shstk_enabled(struct task_struct *task)
> @@ -396,6 +398,9 @@ int arch_set_shadow_stack_status(struct task_struct *t, unsigned long status)
>  	unsigned long size = 0, addr = 0;
>  	bool enable_shstk = false;
>  
> +	if (disable_riscv_usercfi)
> +		return 0;
> +
>  	if (!cpu_supports_shadow_stack())
>  		return -EINVAL;
>  
> @@ -475,6 +480,9 @@ int arch_set_indir_br_lp_status(struct task_struct *t, unsigned long status)
>  {
>  	bool enable_indir_lp = false;
>  
> +	if (disable_riscv_usercfi)
> +		return 0;
> +
>  	if (!cpu_supports_indirect_br_lp_instr())
>  		return -EINVAL;
>  
> @@ -507,3 +515,16 @@ int arch_lock_indir_br_lp_status(struct task_struct *task,
>  
>  	return 0;
>  }
> +
> +static int __init setup_global_riscv_enable(char *str)
> +{
> +	if (strcmp(str, "true") == 0)
> +		disable_riscv_usercfi = true;
> +
> +	pr_info("Setting riscv usercfi to be %s\n",
> +		(disable_riscv_usercfi ? "disabled" : "enabled"));
> +
> +	return 1;
> +}
> +
> +__setup("disable_riscv_usercfi=", setup_global_riscv_enable);

I'd prefer two command line options instead.

In any case, I think we still document params in kernel-parameters.txt.