/*      $NetBSD: sys_getrandom.c,v 1.2 2021/12/28 13:22:43 riastradh Exp $      */

/*-
* Copyright (c) 2020 The NetBSD Foundation, Inc.
* All rights reserved.
*
* This code is derived from software contributed to The NetBSD Foundation
* by Taylor R. Campbell.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
*    notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
*    notice, this list of conditions and the following disclaimer in the
*    documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
* ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
* TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
* BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*/

/*
* getrandom() system call
*/

#include <sys/cdefs.h>
__KERNEL_RCSID(0, "$NetBSD: sys_getrandom.c,v 1.2 2021/12/28 13:22:43 riastradh Exp $");

#include <sys/types.h>
#include <sys/param.h>

#include <sys/atomic.h>
#include <sys/cprng.h>
#include <sys/entropy.h>
#include <sys/kmem.h>
#include <sys/lwp.h>
#include <sys/proc.h>
#include <sys/random.h>
#include <sys/sched.h>
#include <sys/signalvar.h>
#include <sys/syscallargs.h>
#include <sys/uio.h>

#include <crypto/nist_hash_drbg/nist_hash_drbg.h>

#define RANDOM_BUFSIZE  512

int
dogetrandom(struct uio *uio, unsigned int flags)
{
       uint8_t seed[NIST_HASH_DRBG_SEEDLEN_BYTES] = {0};
       struct nist_hash_drbg drbg;
       uint8_t *buf;
       int extractflags = 0;
       int error;

       KASSERT((flags & ~(GRND_RANDOM|GRND_INSECURE|GRND_NONBLOCK)) == 0);
       KASSERT((flags & (GRND_RANDOM|GRND_INSECURE)) !=
           (GRND_RANDOM|GRND_INSECURE));

       /* Get a buffer for transfers.  */
       buf = kmem_alloc(RANDOM_BUFSIZE, KM_SLEEP);

       /*
        * Fast path: for short reads other than from /dev/random, if
        * seeded or if INSECURE, just draw from per-CPU cprng_strong.
        */
       if (uio->uio_resid <= RANDOM_BUFSIZE &&
           !ISSET(flags, GRND_RANDOM) &&
           (entropy_ready() || ISSET(flags, GRND_INSECURE))) {
               /* Generate data and transfer it out.  */
               cprng_strong(user_cprng, buf, uio->uio_resid, 0);
               error = uiomove(buf, uio->uio_resid, uio);
               goto out;
       }

       /*
        * Try to get a seed from the entropy pool.  Fail if we would
        * block.  If GRND_INSECURE, always return something even if it
        * is partial entropy; if !GRND_INSECURE, set ENTROPY_HARDFAIL
        * in order to tell entropy_extract not to bother drawing
        * anything from a partial pool if we can't get full entropy.
        */
       if (!ISSET(flags, GRND_NONBLOCK) && !ISSET(flags, GRND_INSECURE))
               extractflags |= ENTROPY_WAIT|ENTROPY_SIG;
       if (!ISSET(flags, GRND_INSECURE))
               extractflags |= ENTROPY_HARDFAIL;
       error = entropy_extract(seed, sizeof seed, extractflags);
       if (error && !ISSET(flags, GRND_INSECURE))
               goto out;

       /* Instantiate the DRBG.  */
       if (nist_hash_drbg_instantiate(&drbg, seed, sizeof seed, NULL, 0,
               NULL, 0))
               panic("nist_hash_drbg_instantiate");

       /* Promptly zero the seed.  */
       explicit_memset(seed, 0, sizeof seed);

       /* Generate data.  */
       error = 0;
       while (uio->uio_resid) {
               size_t n = MIN(uio->uio_resid, RANDOM_BUFSIZE);

               /*
                * Clamp /dev/random output to the entropy capacity and
                * seed size.  Programs can't rely on long reads.
                */
               if (ISSET(flags, GRND_RANDOM)) {
                       n = MIN(n, ENTROPY_CAPACITY);
                       n = MIN(n, sizeof seed);
                       /*
                        * Guarantee never to return more than one
                        * buffer in this case to minimize bookkeeping.
                        */
                       CTASSERT(ENTROPY_CAPACITY <= RANDOM_BUFSIZE);
                       CTASSERT(sizeof seed <= RANDOM_BUFSIZE);
               }

               /*
                * Try to generate a block of data, but if we've hit
                * the DRBG reseed interval, reseed.
                */
               if (nist_hash_drbg_generate(&drbg, buf, n, NULL, 0)) {
                       /*
                        * Get a fresh seed without blocking -- we have
                        * already generated some output so it is not
                        * useful to block.  This can fail only if the
                        * request is obscenely large, so it is OK for
                        * either /dev/random or /dev/urandom to fail:
                        * we make no promises about gigabyte-sized
                        * reads happening all at once.
                        */
                       error = entropy_extract(seed, sizeof seed,
                           ENTROPY_HARDFAIL);
                       if (error)
                               break;

                       /* Reseed and try again.  */
                       if (nist_hash_drbg_reseed(&drbg, seed, sizeof seed,
                               NULL, 0))
                               panic("nist_hash_drbg_reseed");

                       /* Promptly zero the seed.  */
                       explicit_memset(seed, 0, sizeof seed);

                       /* If it fails now, that's a bug.  */
                       if (nist_hash_drbg_generate(&drbg, buf, n, NULL, 0))
                               panic("nist_hash_drbg_generate");
               }

               /* Transfer n bytes out.  */
               error = uiomove(buf, n, uio);
               if (error)
                       break;

               /*
                * If this is /dev/random, stop here, return what we
                * have, and force the next read to reseed.  Programs
                * can't rely on /dev/random for long reads.
                */
               if (ISSET(flags, GRND_RANDOM)) {
                       error = 0;
                       break;
               }

               /* Now's a good time to yield if needed.  */
               preempt_point();

               /* Check for interruption after at least 256 bytes.  */
               CTASSERT(RANDOM_BUFSIZE >= 256);
               if (__predict_false(curlwp->l_flag & LW_PENDSIG) &&
                   sigispending(curlwp, 0)) {
                       error = EINTR;
                       break;
               }
       }

out:    /* Zero the buffer and free it.  */
       explicit_memset(buf, 0, RANDOM_BUFSIZE);
       kmem_free(buf, RANDOM_BUFSIZE);

       return error;
}

int
sys_getrandom(struct lwp *l, const struct sys_getrandom_args *uap,
   register_t *retval)
{
       /* {
               syscallarg(void *)      buf;
               syscallarg(size_t)      buflen;
               syscallarg(unsigned)    flags;
       } */
       void *buf = SCARG(uap, buf);
       size_t buflen = SCARG(uap, buflen);
       int flags = SCARG(uap, flags);
       int error;

       /* Set up an iov and uio to read into the user's buffer.  */
       struct iovec iov = { .iov_base = buf, .iov_len = buflen };
       struct uio uio = {
               .uio_iov = &iov,
               .uio_iovcnt = 1,
               .uio_offset = 0,
               .uio_resid = buflen,
               .uio_rw = UIO_READ,
               .uio_vmspace = curproc->p_vmspace,
       };

       /* Validate the flags.  */
       if (flags & ~(GRND_RANDOM|GRND_INSECURE|GRND_NONBLOCK)) {
               /* Unknown flags.  */
               error = EINVAL;
               goto out;
       }
       if ((flags & (GRND_RANDOM|GRND_INSECURE)) ==
           (GRND_RANDOM|GRND_INSECURE)) {
               /* Nonsensical combination.  */
               error = EINVAL;
               goto out;
       }

       /* Do it.  */
       error = dogetrandom(&uio, flags);

out:    /*
        * If we transferred anything, return the number of bytes
        * transferred and suppress error; otherwise return the error.
        */
       *retval = buflen - uio.uio_resid;
       if (*retval)
               error = 0;
       return error;
}

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