getentropy(3): Fallback to kern.arandom sysctl on older kernels

On older kernels, when userspace program disables SIGSYS, catch ENOSYS and
emulate getrandom(2) syscall with the kern.arandom sysctl (via existing
arc4_sysctl wrapper).

Special care is taken to faithfully emulate EFAULT on NULL pointers, because
sysctl(3) as used by kern.arandom ignores NULL oldp.  (This was caught by
getentropy(3) ATF tests.)

Reported by:	kib
Reviewed by:	kib
Discussed with:	delphij
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D14785

lib/libc/gen/arc4random.c

@@ -111,8 +111,8 @@ arc4_addrandom(u_char *dat, int datlen)
 	rs.j = rs.i;
 }
 
-static size_t
-arc4_sysctl(u_char *buf, size_t size)
+size_t
+__arc4_sysctl(u_char *buf, size_t size)
 {
 	int mib[2];
 	size_t len, done;
@@ -143,7 +143,7 @@ arc4_stir(void)
 		arc4_init();
 		rs_initialized = 1;
 	}
-	if (arc4_sysctl(rdat, KEYSIZE) != KEYSIZE) {
+	if (__arc4_sysctl(rdat, KEYSIZE) != KEYSIZE) {
 		/*
 		 * The sysctl cannot fail. If it does fail on some FreeBSD
 		 * derivative or after some future change, just abort so that

lib/libc/gen/getentropy.c

@@ -37,6 +37,39 @@ __FBSDID("$FreeBSD$");
 
 #include "libc_private.h"
 
+/*
+ * If a newer libc is accidentally installed on an older kernel, provide high
+ * quality random data anyway.  The sysctl interface is not as fast and does
+ * not block by itself, but is provided by even very old kernels.
+ */
+static int
+getentropy_fallback(void *buf, size_t buflen)
+{
+	/*
+	 * oldp (buf) == NULL has a special meaning for sysctl that results in
+	 * no EFAULT.  For compatibility with the kernel getrandom(2), detect
+	 * this case and return the appropriate error.
+	 */
+	if (buf == NULL && buflen > 0) {
+		errno = EFAULT;
+		return (-1);
+	}
+	if (__arc4_sysctl(buf, buflen) != buflen) {
+		if (errno == EFAULT)
+			return (-1);
+		/*
+		 * This cannot happen.  _arc4_sysctl() spins until the random
+		 * device is seeded and then repeatedly reads until the full
+		 * request is satisfied.  The only way for this to return a zero
+		 * byte or short read is if sysctl(2) on the kern.arandom MIB
+		 * fails.  In this case, exceping the user-provided-a-bogus-
+		 * buffer EFAULT, give up (like for arc4random(3)'s arc4_stir).
+		 */
+		abort();
+	}
+	return (0);
+}
+
 int
 getentropy(void *buf, size_t buflen)
 {
@@ -53,7 +86,7 @@ getentropy(void *buf, size_t buflen)
 			if (errno == EINTR)
 				continue;
 			else if (errno == ENOSYS)
-				abort();
+				return (getentropy_fallback(buf, buflen));
 			else
 				return (-1);
 		}

lib/libc/include/libc_private.h

@@ -405,6 +405,8 @@ int		__sys_futimens(int fd, const struct timespec *times) __hidden;
 int		__sys_utimensat(int fd, const char *path,
 		    const struct timespec *times, int flag) __hidden;
 
+__size_t __arc4_sysctl(unsigned char *, __size_t);
+
 /* execve() with PATH processing to implement posix_spawnp() */
 int _execvpe(const char *, char * const *, char * const *);
 

lib/libc/sys/Symbol.map

@@ -628,8 +628,6 @@ FBSDprivate_1.0 {
 	__sys_getppid;
 	_getpriority;
 	__sys_getpriority;
-	_getrandom;
-	__sys_getrandom;
 	_getresgid;
 	__sys_getresgid;
 	_getresuid;

lib/libc/tests/gen/getentropy_test.c

@@ -31,6 +31,7 @@ __FBSDID("$FreeBSD$");
 
 #include <sys/param.h>
 #include <errno.h>
+#include <signal.h>
 #include <unistd.h>
 
 #include <atf-c.h>
@@ -77,6 +78,8 @@ ATF_TC_BODY(getentropy_sizes, tc)
 ATF_TP_ADD_TCS(tp)
 {
 
+	signal(SIGSYS, SIG_IGN);
+
 	ATF_TP_ADD_TC(tp, getentropy_count);
 	ATF_TP_ADD_TC(tp, getentropy_fault);
 	ATF_TP_ADD_TC(tp, getentropy_sizes);