linux-kernel - Re: [PATCH] x86/boot: Move x86_cache_alignment initialization to correct spot

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Date:   Mon, 2 Oct 2023 15:24:02 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, acdunlap@...gle.com,
        ashok.raj@...el.com, bp@...en8.de, david@...hat.com,
        dionnaglaze@...gle.com, hpa@...or.com, jacobhxu@...gle.com,
        jgross@...e.com, jroedel@...e.de, khalid.elmously@...onical.com,
        kim.phillips@....com, kirill.shutemov@...ux.intel.com,
        llvm@...ts.linux.dev, luto@...nel.org, mingo@...hat.com,
        nikunj@....com, peterz@...radead.org, pgonda@...gle.com,
        rientjes@...gle.com, rppt@...nel.org, seanjc@...gle.com,
        tglx@...utronix.de, thomas.lendacky@....com,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] x86/boot: Move x86_cache_alignment initialization to
 correct spot

On Mon, Oct 02, 2023 at 03:00:45PM -0700, Dave Hansen wrote:
> c->x86_cache_alignment is initialized from c->x86_clflush_size.
> However, commit fbf6449f84bf moved c->x86_clflush_size initialization
> to later in boot without moving the c->x86_cache_alignment assignment.
> 
> This presumably left c->x86_cache_alignment set to zero for longer
> than it should be.
> 
> The result was an oops on 32-bit kernels while accessing a pointer
> at 0x20.  The 0x20 came from accessing a structure member at offset
> 0x10 (buffer->cpumask) from a ZERO_SIZE_PTR=0x10.  kmalloc() can
> evidently return ZERO_SIZE_PTR when it's given 0 as its alignment
> requirement.
> 
> Move the c->x86_cache_alignment initialization to be after
> c->x86_clflush_size has an actual value.
> 
> Fixes: fbf6449f84bf ("x86/sev-es: Set x86_virt_bits to the correct value straight away, instead of a two-phase approach")
> Cc: Adam Dunlap <acdunlap@...gle.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Jacob Xu <jacobhxu@...gle.com>
> Link: https://lore.kernel.org/all/20231002200426.GA4127272@dev-arch.thelio-3990X/

Tested-by: Nathan Chancellor <nathan@...nel.org>

Thanks for the quick fix!

> ---
>  arch/x86/kernel/cpu/common.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 8d7063e4f63c9..9c51ad5bbf319 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -1141,6 +1141,7 @@ void get_cpu_address_sizes(struct cpuinfo_x86 *c)
>  		}
>  	}
>  	c->x86_cache_bits = c->x86_phys_bits;
> +	c->x86_cache_alignment = c->x86_clflush_size;
>  }
>  
>  static void identify_cpu_without_cpuid(struct cpuinfo_x86 *c)
> @@ -1594,8 +1595,6 @@ static void __init cpu_parse_early_param(void)
>   */
>  static void __init early_identify_cpu(struct cpuinfo_x86 *c)
>  {
> -	c->x86_cache_alignment = c->x86_clflush_size;
> -
>  	memset(&c->x86_capability, 0, sizeof(c->x86_capability));
>  	c->extended_cpuid_level = 0;
>  
> -- 
> 2.34.1
>