random-util: optionally allow randomness to be generated via RDRAND

We only use this when we don't require the best randomness. The primary
usecase for this is UUID generation, as this means we don't drain
randomness from the kernel pool for them. Since UUIDs are usually not
secrets RDRAND should be goot enough for them to avoid real-life
collisions.

1 files changed, 48 insertions, 27 deletions

diff --git a/src/basic/random-util.c b/src/basic/random-util.c
index 6b08f72fc2..8eb6acfbbf 100644
--- a/src/basic/random-util.c
+++ b/src/basic/random-util.c
@@ -67,18 +67,52 @@ int rdrand64(uint64_t *ret) {
 
 int genuine_random_bytes(void *p, size_t n, RandomFlags flags) {
         static int have_syscall = -1;
-
         _cleanup_close_ int fd = -1;
+        bool got_some = false;
         int r;
 
-        /* Gathers some randomness from the kernel. This call won't block, unless the RANDOM_BLOCK flag is set. If
-         * RANDOM_DONT_DRAIN is set, an error is returned if the random pool is not initialized. Otherwise it will
-         * always return some data from the kernel, regardless of whether the random pool is fully initialized or
-         * not. */
+        /* Gathers some randomness from the kernel (or the CPU if the RANDOM_ALLOW_RDRAND flag is set). This call won't
+         * block, unless the RANDOM_BLOCK flag is set. If RANDOM_DONT_DRAIN is set, an error is returned if the random
+         * pool is not initialized. Otherwise it will always return some data from the kernel, regardless of whether
+         * the random pool is fully initialized or not. */
 
         if (n == 0)
                 return 0;
 
+        if (FLAGS_SET(flags, RANDOM_ALLOW_RDRAND))
+                /* Try x86-64' RDRAND intrinsic if we have it. We only use it if high quality randomness is not
+                 * required, as we don't trust it (who does?). Note that we only do a single iteration of RDRAND here,
+                 * even though the Intel docs suggest calling this in a tight loop of 10 invocations or so. That's
+                 * because we don't really care about the quality here. We generally prefer using RDRAND if the caller
+                 * allows us too, since this way we won't drain the kernel randomness pool if we don't need it, as the
+                 * pool's entropy is scarce. */
+                for (;;) {
+                        uint64_t u;
+                        size_t m;
+
+                        if (rdrand64(&u) < 0) {
+                                if (got_some && FLAGS_SET(flags, RANDOM_EXTEND_WITH_PSEUDO)) {
+                                        /* Fill in the remaining bytes using pseudo-random values */
+                                        pseudo_random_bytes(p, n);
+                                        return 0;
+                                }
+
+                                /* OK, this didn't work, let's go to getrandom() + /dev/urandom instead */
+                                break;
+                        }
+
+                        m = MIN(sizeof(u), n);
+                        memcpy(p, &u, m);
+
+                        p = (uint8_t*) p + m;
+                        n -= m;
+
+                        if (n == 0)
+                                return 0; /* Yay, success! */
+
+                        got_some = true;
+                }
+
         /* Use the getrandom() syscall unless we know we don't have it. */
         if (have_syscall != 0 && !HAS_FEATURE_MEMORY_SANITIZER) {
 
@@ -100,6 +134,8 @@ int genuine_random_bytes(void *p, size_t n, RandomFlags flags) {
                                         return 0;
                                 }
 
+                                got_some = true;
+
                                 /* Hmm, we didn't get enough good data but the caller insists on good data? Then try again */
                                 if (FLAGS_SET(flags, RANDOM_BLOCK))
                                         continue;
@@ -125,30 +161,15 @@ int genuine_random_bytes(void *p, size_t n, RandomFlags flags) {
                                  * but the kernel will produce some bytes for us on a best-effort basis. */
                                 have_syscall = true;
 
-                                if (FLAGS_SET(flags, RANDOM_DONT_DRAIN))
-                                        return -ENODATA;
-
-                                if (FLAGS_SET(flags, RANDOM_EXTEND_WITH_PSEUDO)) {
-                                        uint64_t u;
-                                        size_t k;
-
-                                        /* Try x86-64' RDRAND intrinsic if we have it. We only use it if high quality
-                                         * randomness is not required, as we don't trust it (who does?). Note that we only do a
-                                         * single iteration of RDRAND here, even though the Intel docs suggest calling this in
-                                         * a tight loop of 10 invocatins or so. That's because we don't really care about the
-                                         * quality here. */
-
-                                        if (rdrand64(&u) < 0)
-                                                return -ENODATA;
-
-                                        k = MIN(n, sizeof(u));
-                                        memcpy(p, &u, k);
-
-                                        /* We only get 64bit out of RDRAND, the rest let's fill up with pseudo-random crap. */
-                                        pseudo_random_bytes((uint8_t*) p + k, n - k);
+                                if (got_some && FLAGS_SET(flags, RANDOM_EXTEND_WITH_PSEUDO)) {
+                                        /* Fill in the remaining bytes using pseudorandom values */
+                                        pseudo_random_bytes(p, n);
                                         return 0;
                                 }
 
+                                if (FLAGS_SET(flags, RANDOM_DONT_DRAIN))
+                                        return -ENODATA;
+
                                 /* Use /dev/urandom instead */
                                 break;
                         } else
@@ -229,7 +250,7 @@ void pseudo_random_bytes(void *p, size_t n) {
 
 void random_bytes(void *p, size_t n) {
 
-        if (genuine_random_bytes(p, n, RANDOM_EXTEND_WITH_PSEUDO|RANDOM_DONT_DRAIN) >= 0)
+        if (genuine_random_bytes(p, n, RANDOM_EXTEND_WITH_PSEUDO|RANDOM_DONT_DRAIN|RANDOM_ALLOW_RDRAND) >= 0)
                 return;
 
         /* If for some reason some user made /dev/urandom unavailable to us, or the kernel has no entropy, use a PRNG instead. */