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    Subject[PATCH 4/6] crypto: DRBG - make reseeding from get_random_bytes() synchronous
    Date
    get_random_bytes() usually hasn't full entropy available by the time DRBG
    instances are first getting seeded from it during boot. Thus, the DRBG
    implementation registers random_ready_callbacks which would in turn
    schedule some work for reseeding the DRBGs once get_random_bytes() has
    sufficient entropy available.

    For reference, the relevant history around handling DRBG (re)seeding in
    the context of a not yet fully seeded get_random_bytes() is:

    commit 16b369a91d0d ("random: Blocking API for accessing
    nonblocking_pool")
    commit 4c7879907edd ("crypto: drbg - add async seeding operation")

    commit 205a525c3342 ("random: Add callback API for random pool
    readiness")
    commit 57225e679788 ("crypto: drbg - Use callback API for random
    readiness")
    commit c2719503f5e1 ("random: Remove kernel blocking API")

    However, some time later, the initialization state of get_random_bytes()
    has been made queryable via rng_is_initialized() introduced with commit
    9a47249d444d ("random: Make crng state queryable"). This primitive now
    allows for streamlining the DRBG reseeding from get_random_bytes() by
    replacing that aforementioned asynchronous work scheduling from
    random_ready_callbacks with some simpler, synchronous code in
    drbg_generate() next to the related logic already present therein. Apart
    from improving overall code readability, this change will also enable DRBG
    users to rely on wait_for_random_bytes() for ensuring that the initial
    seeding has completed, if desired.

    The previous patches already laid the grounds by making drbg_seed() to
    record at each DRBG instance whether it was being seeded at a time when
    rng_is_initialized() still had been false as indicated by
    ->seeded == DRBG_SEED_STATE_PARTIAL.

    All that remains to be done now is to make drbg_generate() check for this
    condition, determine whether rng_is_initialized() has flipped to true in
    the meanwhile and invoke a reseed from get_random_bytes() if so.

    Make this move:
    - rename the former drbg_async_seed() work handler, i.e. the one in charge
    of reseeding a DRBG instance from get_random_bytes(), to
    "drbg_seed_from_random()",
    - change its signature as appropriate, i.e. make it take a struct
    drbg_state rather than a work_struct and change its return type from
    "void" to "int" in order to allow for passing error information from
    e.g. its __drbg_seed() invocation onwards to callers,
    - make drbg_generate() invoke this drbg_seed_from_random() once it
    encounters a DRBG instance with ->seeded == DRBG_SEED_STATE_PARTIAL by
    the time rng_is_initialized() has flipped to true and
    - prune everything related to the former, random_ready_callback based
    mechanism.

    As drbg_seed_from_random() is now getting invoked from drbg_generate() with
    the ->drbg_mutex being held, it must not attempt to recursively grab it
    once again. Remove the corresponding mutex operations from what is now
    drbg_seed_from_random(). Furthermore, as drbg_seed_from_random() can now
    report errors directly to its caller, there's no need for it to temporarily
    switch the DRBG's ->seeded state to DRBG_SEED_STATE_UNSEEDED so that a
    failure of the subsequently invoked __drbg_seed() will get signaled to
    drbg_generate(). Don't do it then.

    Signed-off-by: Nicolai Stange <nstange@suse.de>
    ---
    crypto/drbg.c | 64 +++++++++----------------------------------
    include/crypto/drbg.h | 2 --
    2 files changed, 13 insertions(+), 53 deletions(-)

    diff --git a/crypto/drbg.c b/crypto/drbg.c
    index 6aad210f101a..d9f7dddfd683 100644
    --- a/crypto/drbg.c
    +++ b/crypto/drbg.c
    @@ -1087,12 +1087,10 @@ static inline int drbg_get_random_bytes(struct drbg_state *drbg,
    return 0;
    }

    -static void drbg_async_seed(struct work_struct *work)
    +static int drbg_seed_from_random(struct drbg_state *drbg)
    {
    struct drbg_string data;
    LIST_HEAD(seedlist);
    - struct drbg_state *drbg = container_of(work, struct drbg_state,
    - seed_work);
    unsigned int entropylen = drbg_sec_strength(drbg->core->flags);
    unsigned char entropy[32];
    int ret;
    @@ -1103,23 +1101,17 @@ static void drbg_async_seed(struct work_struct *work)
    drbg_string_fill(&data, entropy, entropylen);
    list_add_tail(&data.list, &seedlist);

    - mutex_lock(&drbg->drbg_mutex);
    -
    ret = drbg_get_random_bytes(drbg, entropy, entropylen);
    if (ret)
    - goto unlock;
    -
    - /* Reset ->seeded so that if __drbg_seed fails the next
    - * generate call will trigger a reseed.
    - */
    - drbg->seeded = DRBG_SEED_STATE_UNSEEDED;
    -
    - __drbg_seed(drbg, &seedlist, true, DRBG_SEED_STATE_FULL);
    + goto out;

    -unlock:
    - mutex_unlock(&drbg->drbg_mutex);
    + ret = __drbg_seed(drbg, &seedlist, true, DRBG_SEED_STATE_FULL);
    + if (ret)
    + goto out;

    +out:
    memzero_explicit(entropy, entropylen);
    + return ret;
    }

    /*
    @@ -1422,6 +1414,11 @@ static int drbg_generate(struct drbg_state *drbg,
    goto err;
    /* 9.3.1 step 7.4 */
    addtl = NULL;
    + } else if (rng_is_initialized() &&
    + drbg->seeded == DRBG_SEED_STATE_PARTIAL) {
    + len = drbg_seed_from_random(drbg);
    + if (len)
    + goto err;
    }

    if (addtl && 0 < addtl->len)
    @@ -1514,45 +1511,15 @@ static int drbg_generate_long(struct drbg_state *drbg,
    return 0;
    }

    -static void drbg_schedule_async_seed(struct random_ready_callback *rdy)
    -{
    - struct drbg_state *drbg = container_of(rdy, struct drbg_state,
    - random_ready);
    -
    - schedule_work(&drbg->seed_work);
    -}
    -
    static int drbg_prepare_hrng(struct drbg_state *drbg)
    {
    - int err;
    -
    /* We do not need an HRNG in test mode. */
    if (list_empty(&drbg->test_data.list))
    return 0;

    drbg->jent = crypto_alloc_rng("jitterentropy_rng", 0, 0);

    - INIT_WORK(&drbg->seed_work, drbg_async_seed);
    -
    - drbg->random_ready.owner = THIS_MODULE;
    - drbg->random_ready.func = drbg_schedule_async_seed;
    -
    - err = add_random_ready_callback(&drbg->random_ready);
    -
    - switch (err) {
    - case 0:
    - break;
    -
    - case -EALREADY:
    - err = 0;
    - fallthrough;
    -
    - default:
    - drbg->random_ready.func = NULL;
    - return err;
    - }
    -
    - return err;
    + return 0;
    }

    /*
    @@ -1646,11 +1613,6 @@ static int drbg_instantiate(struct drbg_state *drbg, struct drbg_string *pers,
    */
    static int drbg_uninstantiate(struct drbg_state *drbg)
    {
    - if (drbg->random_ready.func) {
    - del_random_ready_callback(&drbg->random_ready);
    - cancel_work_sync(&drbg->seed_work);
    - }
    -
    if (!IS_ERR_OR_NULL(drbg->jent))
    crypto_free_rng(drbg->jent);
    drbg->jent = NULL;
    diff --git a/include/crypto/drbg.h b/include/crypto/drbg.h
    index 3ebdb1effe74..a6c3b8e7deb6 100644
    --- a/include/crypto/drbg.h
    +++ b/include/crypto/drbg.h
    @@ -137,12 +137,10 @@ struct drbg_state {
    bool pr; /* Prediction resistance enabled? */
    bool fips_primed; /* Continuous test primed? */
    unsigned char *prev; /* FIPS 140-2 continuous test value */
    - struct work_struct seed_work; /* asynchronous seeding support */
    struct crypto_rng *jent;
    const struct drbg_state_ops *d_ops;
    const struct drbg_core *core;
    struct drbg_string test_data;
    - struct random_ready_callback random_ready;
    };

    static inline __u8 drbg_statelen(struct drbg_state *drbg)
    --
    2.26.2
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