Re: [PATCHv2] x86/alternatives: Disable KASAN in apply_alternatives()

[Peter Zijlstra]

On Wed, Oct 11, 2023 at 09:58:49AM +0300, Kirill A. Shutemov wrote:
> Fei has reported that KASAN triggers during apply_alternatives() on
> 5-level paging machine:
> 

Urgh @ KASAN splat, can't we summarize that?

> 
> On machines with 5-level paging, cpu_feature_enabled(X86_FEATURE_LA57)
> got patched. It includes KASAN code, where KASAN_SHADOW_START depends on
> __VIRTUAL_MASK_SHIFT, which is defined with the cpu_feature_enabled().
> 
> KASAN gets confused when apply_alternatives() patches the
> KASAN_SHADOW_START users. A test patch that makes KASAN_SHADOW_START
> static, by replacing __VIRTUAL_MASK_SHIFT with 56, fixes the issue.
> 
> Disable KASAN while kernel patches alternatives.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Reported-by: Fei Yang <fei.yang@xxxxxxxxx>
> Fixes: 6657fca06e3f ("x86/mm: Allow to boot without LA57 if CONFIG_X86_5LEVEL=y")
> Cc: stable@xxxxxxxxxxxxxxx
> ---
> 
>  v2:
>   - Move kasan_disable/_enable_current() to cover whole loop, not only
>     text_poke_early();
>   - Adjust commit message.
> 
> ---
>  arch/x86/kernel/alternative.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 517ee01503be..b4cc4d7c0825 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -403,6 +403,17 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
>  	u8 insn_buff[MAX_PATCH_LEN];
>  
>  	DPRINTK(ALT, "alt table %px, -> %px", start, end);
> +
> +	/*
> +	 * In the case CONFIG_X86_5LEVEL=y, KASAN_SHADOW_START is defined using
> +	 * cpu_feature_enabled(X86_FEATURE_LA57) and is therefore patched here.
> +	 * During the process, KASAN becomes confused and triggers

	because of partial LA57 convertion ..

> +	 * a false-positive out-of-bound report.
> +	 *
> +	 * Disable KASAN until the patching is complete.
> +	 */
> +	kasan_disable_current();
> +
>  	/*
>  	 * The scan order should be from start to end. A later scanned
>  	 * alternative code can overwrite previously scanned alternative code.
> @@ -452,6 +463,8 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
>  
>  		text_poke_early(instr, insn_buff, insn_buff_sz);
>  	}
> +
> +	kasan_enable_current();
>  }

Other than that, ACK.