Re: [PATCH] crypto: ansi_cprng: avoid incorrect extra call to_get_more_prng_bytes

[Neil Horman]

On Wed, Nov 12, 2008 at 02:52:59PM -0500, Jarod Wilson wrote:
> While working with some FIPS RNGVS test vectors yesterday, I discovered a
> little bug in the way the ansi_cprng code works right now.
> 
> For example, the following test vector (complete with expected result)
> from http://csrc.nist.gov/groups/STM/cavp/documents/rng/RNGVS.pdf ...
> 
> Key = f3b1666d13607242ed061cabb8d46202
> DT = e6b3be782a23fa62d71d4afbb0e922fc
> V = f0000000000000000000000000000000
> R = 88dda456302423e5f69da57e7b95c73a
> 
> ...when run through ansi_cprng, yields an incorrect R value
> of e2afe0d794120103d6e86a2b503bdfaa.
> 
> If I load up ansi_cprng w/dbg=1 though, it was fairly obvious what was
> going wrong:
> 
> ----8<----
> getting 16 random bytes for context ffff810033fb2b10
> Calling _get_more_prng_bytes for context ffff810033fb2b10
> Input DT: 00000000: e6 b3 be 78 2a 23 fa 62 d7 1d 4a fb b0 e9 22 fc 
> Input I: 00000000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
> Input V: 00000000: f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
> tmp stage 0: 00000000: e6 b3 be 78 2a 23 fa 62 d7 1d 4a fb b0 e9 22 fc 
> tmp stage 1: 00000000: f4 8e cb 25 94 3e 8c 31 d6 14 cd 8a 23 f1 3f 84 
> tmp stage 2: 00000000: 8c 53 6f 73 a4 1a af d4 20 89 68 f4 58 64 f8 be 
> Returning new block for context ffff810033fb2b10
> Output DT: 00000000: e7 b3 be 78 2a 23 fa 62 d7 1d 4a fb b0 e9 22 fc 
> Output I: 00000000: 04 8e cb 25 94 3e 8c 31 d6 14 cd 8a 23 f1 3f 84 
> Output V: 00000000: 48 89 3b 71 bc e4 00 b6 5e 21 ba 37 8a 0a d5 70 
> New Random Data: 00000000: 88 dd a4 56 30 24 23 e5 f6 9d a5 7e 7b 95 c7 3a 
> Calling _get_more_prng_bytes for context ffff810033fb2b10
> Input DT: 00000000: e7 b3 be 78 2a 23 fa 62 d7 1d 4a fb b0 e9 22 fc 
> Input I: 00000000: 04 8e cb 25 94 3e 8c 31 d6 14 cd 8a 23 f1 3f 84 
> Input V: 00000000: 48 89 3b 71 bc e4 00 b6 5e 21 ba 37 8a 0a d5 70 
> tmp stage 0: 00000000: e7 b3 be 78 2a 23 fa 62 d7 1d 4a fb b0 e9 22 fc 
> tmp stage 1: 00000000: 80 6b 3a 8c 23 ae 8f 53 be 71 4c 16 fc 13 b2 ea 
> tmp stage 2: 00000000: 2a 4d e1 2a 0b 58 8e e6 36 b8 9c 0a 26 22 b8 30 
> Returning new block for context ffff810033fb2b10
> Output DT: 00000000: e8 b3 be 78 2a 23 fa 62 d7 1d 4a fb b0 e9 22 fc 
> Output I: 00000000: c8 e2 01 fd 9f 4a 8f e5 e0 50 f6 21 76 19 67 9a 
> Output V: 00000000: ba 98 e3 75 c0 1b 81 8d 03 d6 f8 e2 0c c6 54 4b 
> New Random Data: 00000000: e2 af e0 d7 94 12 01 03 d6 e8 6a 2b 50 3b df aa 
> returning 16 from get_prng_bytes in context ffff810033fb2b10
> ----8<----
> 
> The expected result is there, in the first "New Random Data", but we're
> incorrectly making a second call to _get_more_prng_bytes, due to some checks
> that are slightly off, which resulted in our original bytes never being
> returned anywhere.
> 
> One approach to fixing this would be to alter some byte_count checks in
> get_prng_bytes, but it would mean the last DEFAULT_BLK_SZ bytes would be
> copied a byte at a time, rather than in a single memcpy, so a slightly more
> involved, equally functional, and ultimately more efficient way of fixing this
> was suggested to me by Neil, which I'm submitting here. All of the RNGVS ANSI
> X9.31 AES128 VST test vectors I've passed through ansi_cprng are now returning
> the expected results with this change.
> 
> ---
> 

I went over this with Jarod.  Its a good fix.

Acked-by: Neil Horman <nhorman@xxxxxxxxxxxxx>


Neil

-- 
/****************************************************
 * Neil Horman <nhorman@xxxxxxxxxxxxx>
 * Software Engineer, Red Hat
 ****************************************************/
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