[PATCH] arch/powerpc/crypto/crc-vpmsum_test: Use cheaper random numbers for self-test

[PATCH] arch/powerpc/crypto/crc-vpmsum_test: Use cheaper random numbers for self-test

[George Spelvin]

This code was filling a 64K buffer from /dev/urandom in order to
compute a CRC over (on average half of) it by two different methods,
comparing the CRCs, and repeating.

This is not a remotely security-critical application, so use the far
faster and cheaper prandom_u32() generator.

And, while we're at it, only fill as much of the buffer as we plan to use.

Signed-off-by: George Spelvin <lkml@sdf.org>
Cc: Daniel Axtens <dja@axtens.net>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
---
 arch/powerpc/crypto/crc-vpmsum_test.c | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/arch/powerpc/crypto/crc-vpmsum_test.c b/arch/powerpc/crypto/crc-vpmsum_test.c
index 0153a9c6f4af..98ea4f4d3dde 100644
--- a/arch/powerpc/crypto/crc-vpmsum_test.c
+++ b/arch/powerpc/crypto/crc-vpmsum_test.c
@@ -78,16 +78,12 @@ static int __init crc_test_init(void)
 
 		pr_info("crc-vpmsum_test begins, %lu iterations\n", iterations);
 		for (i=0; i<iterations; i++) {
-			size_t len, offset;
+			size_t offset = prandom_u32_max(16);
+			size_t len = prandom_u32_max(MAX_CRC_LENGTH);
 
-			get_random_bytes(data, MAX_CRC_LENGTH);
-			get_random_bytes(&len, sizeof(len));
-			get_random_bytes(&offset, sizeof(offset));
-
-			len %= MAX_CRC_LENGTH;
-			offset &= 15;
 			if (len <= offset)
 				continue;
+			prandom_bytes(data, len);
 			len -= offset;
 
 			crypto_shash_update(crct10dif_shash, data+offset, len);
-- 
2.20.1

Re: [PATCH] arch/powerpc/crypto/crc-vpmsum_test: Use cheaper random numbers for self-test

[Daniel Axtens]

Hi George,

> This code was filling a 64K buffer from /dev/urandom in order to
> compute a CRC over (on average half of) it by two different methods,
> comparing the CRCs, and repeating.
>
> This is not a remotely security-critical application, so use the far
> faster and cheaper prandom_u32() generator.
>

I've had a quick look at the prandom_u32 generator and I agree that it's
suitable for this. 

> And, while we're at it, only fill as much of the buffer as we plan to use.

This also looks good to me.

Acked-by: Daniel Axtens <dja@axtens.net>

Regards,
Daniel

>
> Signed-off-by: George Spelvin <lkml@sdf.org>
> Cc: Daniel Axtens <dja@axtens.net>
> Cc: Herbert Xu <herbert@gondor.apana.org.au>
> Cc: linuxppc-dev@lists.ozlabs.org
> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Cc: Paul Mackerras <paulus@samba.org>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> ---
>  arch/powerpc/crypto/crc-vpmsum_test.c | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
>
> diff --git a/arch/powerpc/crypto/crc-vpmsum_test.c b/arch/powerpc/crypto/crc-vpmsum_test.c
> index 0153a9c6f4af..98ea4f4d3dde 100644
> --- a/arch/powerpc/crypto/crc-vpmsum_test.c
> +++ b/arch/powerpc/crypto/crc-vpmsum_test.c
> @@ -78,16 +78,12 @@ static int __init crc_test_init(void)
>  
>  		pr_info("crc-vpmsum_test begins, %lu iterations\n", iterations);
>  		for (i=0; i<iterations; i++) {
> -			size_t len, offset;
> +			size_t offset = prandom_u32_max(16);
> +			size_t len = prandom_u32_max(MAX_CRC_LENGTH);
>  
> -			get_random_bytes(data, MAX_CRC_LENGTH);
> -			get_random_bytes(&len, sizeof(len));
> -			get_random_bytes(&offset, sizeof(offset));
> -
> -			len %= MAX_CRC_LENGTH;
> -			offset &= 15;
>  			if (len <= offset)
>  				continue;
> +			prandom_bytes(data, len);
>  			len -= offset;
>  
>  			crypto_shash_update(crct10dif_shash, data+offset, len);
> -- 
> 2.20.1

Re: arch/powerpc/crypto/crc-vpmsum_test: Use cheaper random numbers for self-test

[Michael Ellerman]

On Thu, 2019-03-21 at 10:42:22 UTC, George Spelvin wrote:
> This code was filling a 64K buffer from /dev/urandom in order to
> compute a CRC over (on average half of) it by two different methods,
> comparing the CRCs, and repeating.
> 
> This is not a remotely security-critical application, so use the far
> faster and cheaper prandom_u32() generator.
> 
> And, while we're at it, only fill as much of the buffer as we plan to use.
> 
> Signed-off-by: George Spelvin <lkml@sdf.org>
> Cc: Daniel Axtens <dja@axtens.net>
> Cc: Herbert Xu <herbert@gondor.apana.org.au>
> Cc: linuxppc-dev@lists.ozlabs.org
> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Cc: Paul Mackerras <paulus@samba.org>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Acked-by: Daniel Axtens <dja@axtens.net>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/80d04b7fabe161a23d143b3bfcfca1b0

cheers