linux-kernel - Re: [PATCH 2/2] x86/random: Issue a warning if RDRAND or RDSEED fails

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Date: Tue, 30 Jan 2024 07:50:40 -0800
From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
 Theodore Ts'o <tytso@....edu>, "Jason A. Donenfeld" <Jason@...c4.com>
Cc: Elena Reshetova <elena.reshetova@...el.com>,
 Jun Nakajima <jun.nakajima@...el.com>, Tom Lendacky
 <thomas.lendacky@....com>, "Kalra, Ashish" <ashish.kalra@....com>,
 Sean Christopherson <seanjc@...gle.com>, linux-coco@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] x86/random: Issue a warning if RDRAND or RDSEED fails

On 1/30/24 12:30 AM, Kirill A. Shutemov wrote:
> RDRAND and RDSEED instructions rarely fail. Ten retries should be
> sufficient to account for occasional failures.
>
> If the instruction fails more than ten times, it is likely that the
> hardware is broken or someone is attempting to exceed the rate at which
> the random number generator hardware can provide random numbers.
>
> Issue a warning if ten retries were not enough.

Did you come across a case where it fails? Wondering why add this
warning now?

>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>  arch/x86/include/asm/archrandom.h | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/arch/x86/include/asm/archrandom.h b/arch/x86/include/asm/archrandom.h
> index 918c5880de9e..fc8d837fb3b9 100644
> --- a/arch/x86/include/asm/archrandom.h
> +++ b/arch/x86/include/asm/archrandom.h
> @@ -13,6 +13,12 @@
>  #include <asm/processor.h>
>  #include <asm/cpufeature.h>
>  
> +#ifdef KASLR_COMPRESSED_BOOT
> +#define rd_warn(msg) warn(msg)

Why not use warn_once in both cases?

> +#else
> +#define rd_warn(msg) WARN_ONCE(1, msg)
> +#endif
> +
>  #define RDRAND_RETRY_LOOPS	10
>  
>  /* Unconditional execution of RDRAND and RDSEED */
> @@ -28,6 +34,9 @@ static inline bool __must_check rdrand_long(unsigned long *v)
>  		if (ok)
>  			return true;
>  	} while (--retry);
> +
> +	rd_warn("RDRAND failed\n");
> +
>  	return false;
>  }
>  
> @@ -45,6 +54,8 @@ static inline bool __must_check rdseed_long(unsigned long *v)
>  			return true;
>  	} while (--retry);
>  
> +	rd_warn("RDSEED failed\n");
> +
>  	return false;
>  }
>  

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer