Replace PostmasterRandom() with a stronger source, second attempt.

This adds a new routine, pg_strong_random() for generating random bytes,
for use in both frontend and backend. At the moment, it's only used in
the backend, but the upcoming SCRAM authentication patches need strong
random numbers in libpq as well.

pg_strong_random() is based on, and replaces, the existing implementation
in pgcrypto. It can acquire strong random numbers from a number of sources,
depending on what's available:

- OpenSSL RAND_bytes(), if built with OpenSSL
- On Windows, the native cryptographic functions are used
- /dev/urandom

Unlike the current pgcrypto function, the source is chosen by configure.
That makes it easier to test different implementations, and ensures that
we don't accidentally fall back to a less secure implementation, if the
primary source fails. All of those methods are quite reliable, it would be
pretty surprising for them to fail, so we'd rather find out by failing
hard.

If no strong random source is available, we fall back to using erand48(),
seeded from current timestamp, like PostmasterRandom() was. That isn't
cryptographically secure, but allows us to still work on platforms that
don't have any of the above stronger sources. Because it's not very secure,
the built-in implementation is only used if explicitly requested with
--disable-strong-random.

This replaces the more complicated Fortuna algorithm we used to have in
pgcrypto, which is unfortunate, but all modern platforms have /dev/urandom,
so it doesn't seem worth the maintenance effort to keep that. pgcrypto
functions that require strong random numbers will be disabled with
--disable-strong-random.

Original patch by Magnus Hagander, tons of further work by Michael Paquier
and me.

Discussion: https://www.postgresql.org/message-id/CAB7nPqRy3krN8quR9XujMVVHYtXJ0_60nqgVc6oUk8ygyVkZsA@mail.gmail.com
Discussion: https://www.postgresql.org/message-id/CAB7nPqRWkNYRRPJA7-cF+LfroYV10pvjdz6GNvxk-Eee9FypKA@mail.gmail.com

1 files changed, 158 insertions, 0 deletions

diff --git a/src/backend/utils/misc/backend_random.c b/src/backend/utils/misc/backend_random.c
new file mode 100644
index 00000000000..1bc239d1ddc
--- /dev/null
+++ b/src/backend/utils/misc/backend_random.c
@@ -0,0 +1,158 @@
+/*-------------------------------------------------------------------------
+ *
+ * backend_random.c
+ *	  Backend random number generation routine.
+ *
+ * pg_backend_random() function fills a buffer with random bytes. Normally,
+ * it is just a thin wrapper around pg_strong_random(), but when compiled
+ * with --disable-strong-random, we provide a built-in implementation.
+ *
+ * This function is used for generating nonces in authentication, and for
+ * random salt generation in pgcrypto. The built-in implementation is not
+ * cryptographically strong, but if the user asked for it, we'll go ahead
+ * and use it anyway.
+ *
+ * The built-in implementation uses the standard erand48 algorithm, with
+ * a seed shared between all backends.
+ *
+ * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/utils/misc/backend_random.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include <sys/time.h>
+
+#include "miscadmin.h"
+#include "storage/lwlock.h"
+#include "storage/shmem.h"
+#include "utils/backend_random.h"
+#include "utils/timestamp.h"
+
+#ifdef HAVE_STRONG_RANDOM
+
+Size
+BackendRandomShmemSize(void)
+{
+	return 0;
+}
+
+void
+BackendRandomShmemInit(void)
+{
+	/* do nothing */
+}
+
+bool
+pg_backend_random(char *dst, int len)
+{
+	/* should not be called in postmaster */
+	Assert (IsUnderPostmaster || !IsPostmasterEnvironment);
+
+	return pg_strong_random(dst, len);
+}
+
+#else
+
+/*
+ * Seed for the PRNG, stored in shared memory.
+ *
+ * Protected by BackendRandomLock.
+ */
+typedef struct
+{
+	bool		initialized;
+	unsigned short seed[3];
+} BackendRandomShmemStruct;
+
+static BackendRandomShmemStruct *BackendRandomShmem;
+
+Size
+BackendRandomShmemSize(void)
+{
+	return sizeof(BackendRandomShmemStruct);
+}
+
+void
+BackendRandomShmemInit(void)
+{
+	bool		found;
+
+	BackendRandomShmem = (BackendRandomShmemStruct *)
+		ShmemInitStruct("Backend PRNG state",
+						BackendRandomShmemSize(),
+						&found);
+
+	if (!IsUnderPostmaster)
+	{
+		Assert(!found);
+
+		BackendRandomShmem->initialized = false;
+	}
+	else
+		Assert(found);
+}
+
+bool
+pg_backend_random(char *dst, int len)
+{
+	int			i;
+	char	   *end = dst + len;
+
+	/* should not be called in postmaster */
+	Assert (IsUnderPostmaster || !IsPostmasterEnvironment);
+
+	LWLockAcquire(BackendRandomLock, LW_EXCLUSIVE);
+
+	/*
+	 * Seed the PRNG on the first use.
+	 */
+	if (!BackendRandomShmem->initialized)
+	{
+		struct timeval now;
+
+		gettimeofday(&now, NULL);
+
+		BackendRandomShmem->seed[0] = now.tv_sec;
+		BackendRandomShmem->seed[1] = (unsigned short) (now.tv_usec);
+		BackendRandomShmem->seed[2] = (unsigned short) (now.tv_usec >> 16);
+
+		/*
+		 * Mix in the cancel key, generated by the postmaster. This adds
+		 * what little entropy the postmaster had to the seed.
+		 */
+		BackendRandomShmem->seed[0] ^= (MyCancelKey);
+		BackendRandomShmem->seed[1] ^= (MyCancelKey >> 16);
+
+		BackendRandomShmem->initialized = true;
+	}
+
+	for (i = 0; dst < end; i++)
+	{
+		uint32		r;
+		int			j;
+
+		/*
+		 * pg_jrand48 returns a 32-bit integer. Fill the next 4 bytes from it.
+		 */
+		r  = (uint32) pg_jrand48(BackendRandomShmem->seed);
+
+		for (j = 0; j < 4 && dst < end; j++)
+		{
+			*(dst++) = (char) (r & 0xFF);
+			r >>= 8;
+		}
+	}
+	LWLockRelease(BackendRandomLock);
+
+	return true;
+}
+
+
+#endif /* HAVE_STRONG_RANDOM */