Re: [PATCH v2] x86/shstk: Provide kernel command line knob to disable

[Mathias Krause]

On 02.04.26 19:36, Mathias Krause wrote:
> Provide a kernel command line option 'shstk=off' to disable CET shadow
> stacks, much like 'ibt=off' can be used to disable CET IBT.
> 
> With both set to off, it avoids setting CR4.CET on capable hardware to
> allow debugging related issues during early boot which I happened to
> have done way too many times in the recent past.
> 
> Document it along with its sibling option 'ibt' in kernel-parameters.txt
> to allow others to find it more easily.
> 
> Signed-off-by: Mathias Krause <minipli@xxxxxxxxxxxxxx>
> Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Acked-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> ---
> v2:
> - pick up Ack's
> - document the new option as well as ibt=
> - tweak changelog accordingly
> 

Ping!

Anything still to fix with this or is it ready to get merged?

Thanks,
Mathias


>  Documentation/admin-guide/kernel-parameters.txt | 14 ++++++++++++++
>  arch/x86/kernel/shstk.c                         |  9 +++++++++
>  2 files changed, 23 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 03a550630644..43bdf72f6495 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2248,6 +2248,16 @@ Kernel parameters
>  			syscalls, essentially overriding IA32_EMULATION_DEFAULT_DISABLED at
>  			boot time. When false, unconditionally disables IA32 emulation.
>  
> +	ibt=		[X86-64]
> +			Format: ibt=warn, ibt=off
> +			Changes the handling of CET IBT violations in the kernel.
> +
> +			The 'warn' setting makes CET IBT violations emit a
> +			warning only instead of being fatal while the 'off'
> +			setting completely disables CET IBT for the kernel.
> +
> +			To fully disable CET, use 'ibt=off shstk=off'.
> +
>  	icn=		[HW,ISDN]
>  			Format: <io>[,<membase>[,<icn_id>[,<icn_id2>]]]
>  
> @@ -6924,6 +6934,10 @@ Kernel parameters
>  			Specify the MCLK divider for Intel SoundWire buses in
>  			case the BIOS does not provide the clock rate properly.
>  
> +	shstk=off	[X86-64] Disable CET userspace shadow stack support.
> +
> +			To fully disable CET, use 'ibt=off shstk=off'.
> +
>  	skew_tick=	[KNL,EARLY] Offset the periodic timer tick per cpu to mitigate
>  			xtime_lock contention on larger systems, and/or RCU lock
>  			contention on all systems with CONFIG_MAXSMP set.
> diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
> index 978232b6d48d..68b46bf1540b 100644
> --- a/arch/x86/kernel/shstk.c
> +++ b/arch/x86/kernel/shstk.c
> @@ -542,6 +542,15 @@ static int shstk_disable(void)
>  	return 0;
>  }
>  
> +static int __init shstk_configure(char *str)
> +{
> +	if (!strcmp(str, "off"))
> +		setup_clear_cpu_cap(X86_FEATURE_SHSTK);
> +
> +	return 1;
> +}
> +__setup("shstk=", shstk_configure);
> +
>  SYSCALL_DEFINE3(map_shadow_stack, unsigned long, addr, unsigned long, size, unsigned int, flags)
>  {
>  	bool set_tok = flags & SHADOW_STACK_SET_TOKEN;