lkml.org 
[lkml]   [2022]   [Jan]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    From
    Subject[PATCH AUTOSEL 5.10 030/116] crypto: jitter - consider 32 LSB for APT
    Date
    From: Stephan Müller <smueller@chronox.de>

    [ Upstream commit 552d03a223eda3df84526ab2c1f4d82e15eaee7a ]

    The APT compares the current time stamp with a pre-set value. The
    current code only considered the 4 LSB only. Yet, after reviews by
    mathematicians of the user space Jitter RNG version >= 3.1.0, it was
    concluded that the APT can be calculated on the 32 LSB of the time
    delta. Thi change is applied to the kernel.

    This fixes a bug where an AMD EPYC fails this test as its RDTSC value
    contains zeros in the LSB. The most appropriate fix would have been to
    apply a GCD calculation and divide the time stamp by the GCD. Yet, this
    is a significant code change that will be considered for a future
    update. Note, tests showed that constantly the GCD always was 32 on
    these systems, i.e. the 5 LSB were always zero (thus failing the APT
    since it only considered the 4 LSB for its calculation).

    Signed-off-by: Stephan Mueller <smueller@chronox.de>
    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    crypto/jitterentropy.c | 3 +--
    1 file changed, 1 insertion(+), 2 deletions(-)

    diff --git a/crypto/jitterentropy.c b/crypto/jitterentropy.c
    index 6e147c43fc186..37c4c308339e4 100644
    --- a/crypto/jitterentropy.c
    +++ b/crypto/jitterentropy.c
    @@ -265,7 +265,6 @@ static int jent_stuck(struct rand_data *ec, __u64 current_delta)
    {
    __u64 delta2 = jent_delta(ec->last_delta, current_delta);
    __u64 delta3 = jent_delta(ec->last_delta2, delta2);
    - unsigned int delta_masked = current_delta & JENT_APT_WORD_MASK;

    ec->last_delta = current_delta;
    ec->last_delta2 = delta2;
    @@ -274,7 +273,7 @@ static int jent_stuck(struct rand_data *ec, __u64 current_delta)
    * Insert the result of the comparison of two back-to-back time
    * deltas.
    */
    - jent_apt_insert(ec, delta_masked);
    + jent_apt_insert(ec, current_delta);

    if (!current_delta || !delta2 || !delta3) {
    /* RCT with a stuck bit */
    --
    2.34.1
    \
     
     \ /
      Last update: 2022-01-18 04:10    [W:4.108 / U:0.436 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site