[libvirt] [PATCH 08/10] virrandom: Make virRandomBits better
Now that we have strong PRNG generator implemented in
virRandomBytes() let's use that instead of gnulib's random_r.
Problem with the latter is in way we seed it: current UNIX time
and libvirtd's PID are not that random as one might think.
Imagine two hosts booting at the same time. There's a fair chance
that those hosts spawn libvirtds at the same time and with the
same PID. This will result in both daemons generating the same
sequence of say MAC addresses [1].
1: https://www.redhat.com/archives/libvirt-users/2018-May/msg00097.html
Signed-off-by: Michal Privoznik <mprivozn(a)redhat.com>
---
src/util/virrandom.c | 63 ++--------------------------------------------------
1 file changed, 2 insertions(+), 61 deletions(-)
diff --git a/src/util/virrandom.c b/src/util/virrandom.c
index 444b0f9802..01cc82a052 100644
--- a/src/util/virrandom.c
+++ b/src/util/virrandom.c
@@ -49,53 +49,6 @@ VIR_LOG_INIT("util.random");
#define RANDOM_SOURCE "/dev/urandom"
-/* The algorithm of virRandomBits relies on gnulib's guarantee that
- * 'random_r' matches the POSIX requirements on 'random' of being
- * evenly distributed among exactly [0, 2**31) (that is, we always get
- * exactly 31 bits). While this happens to be the value of RAND_MAX
- * on glibc, note that POSIX only requires RAND_MAX to be tied to the
- * weaker 'rand', so there are platforms where RAND_MAX is smaller
- * than the range of 'random_r'. For the results to be evenly
- * distributed among up to 64 bits, we also rely on the period of
- * 'random_r' to be at least 2**64, which POSIX only guarantees for
- * 'random' if you use 256 bytes of state. */
-enum {
- RANDOM_BITS_PER_ITER = 31,
- RANDOM_BITS_MASK = (1U << RANDOM_BITS_PER_ITER) - 1,
- RANDOM_STATE_SIZE = 256,
-};
-
-static char randomState[RANDOM_STATE_SIZE];
-static struct random_data randomData;
-static virMutex randomLock = VIR_MUTEX_INITIALIZER;
-
-
-static int
-virRandomOnceInit(void)
-{
- unsigned int seed = time(NULL) ^ getpid();
-
-#if 0
- /* Normally we want a decent seed. But if reproducible debugging
- * of a fixed pseudo-random sequence is ever required, uncomment
- * this block to let an environment variable force the seed. */
- const char *debug = virGetEnvBlockSUID("VIR_DEBUG_RANDOM_SEED");
-
- if (debug && virStrToLong_ui(debug, NULL, 0, &seed) < 0)
- return -1;
-#endif
-
- if (initstate_r(seed,
- randomState,
- sizeof(randomState),
- &randomData) < 0)
- return -1;
-
- return 0;
-}
-
-VIR_ONCE_GLOBAL_INIT(virRandom)
-
/**
* virRandomBits:
* @nbits: Number of bits of randommess required
@@ -108,26 +61,14 @@ VIR_ONCE_GLOBAL_INIT(virRandom)
uint64_t virRandomBits(int nbits)
{
uint64_t ret = 0;
- int32_t bits;
- if (virRandomInitialize() < 0) {
+ if (virRandomBytes((unsigned char *) &ret, sizeof(ret)) < 0) {
/* You're already hosed, so this particular non-random value
* isn't any worse. */
return 0;
}
- virMutexLock(&randomLock);
-
- while (nbits > RANDOM_BITS_PER_ITER) {
- random_r(&randomData, &bits);
- ret = (ret << RANDOM_BITS_PER_ITER) | (bits & RANDOM_BITS_MASK);
- nbits -= RANDOM_BITS_PER_ITER;
- }
-
- random_r(&randomData, &bits);
- ret = (ret << nbits) | (bits & ((1 << nbits) - 1));
-
- virMutexUnlock(&randomLock);
+ ret &= (1U << nbits) - 1;
return ret;
}
--
2.16.1