random: introduce getrandom(2) system call]

Sorry, I typo'ed the mailing list for linux-api originally....

- Ted

The getrandom(2) system call was requested by the LibreSSL Portable developers. It is analoguous to the getentropy(2) system call in OpenBSD.

The rationale of this system call is to provide resiliance against file descriptor exhaustion attacks, where the attacker consumes all available file descriptors, forcing the use of the fallback code where /dev/[u]random is not available. Since the fallback code is often not well-tested, it is better to eliminate this potential failure mode entirely.

The other feature provided by this new system call is the ability to request randomness from the /dev/urandom entropy pool, but to block until at least 128 bits of entropy has been accumulated in the /dev/urandom entropy pool. Historically, the emphasis in the /dev/urandom development has been to ensure that urandom pool is initialized as quickly as possible after system boot, and preferably before the init scripts start execution. This is because changing /dev/urandom reads to block represents an interface change that could potentially break userspace which is not acceptable. In practice, on most x86 desktop and server systems, in general the entropy pool can be initialized before it is needed (and in modern kernels, we will printk a warning message if not). However, on an embedded system, this may not be hte case. And so with a new interface, we can provide this requested functionality of blocking until the urandom pool has been initialized. Any userspace program which uses this new functionality must make sure that if it is used in early boot, that it will not cause the boot up scripts or other portions of the system startup to hang indefinitely.

SYNOPSIS

	#include <linux/random.h>

	int getrandom(void *buf, size_t buflen, unsigned int flags);

DESCRIPTION

The system call getrandom() fills the buffer pointed to by buf with up to buflen random bytes which can be used to seed user space random number generators (i.e., DRBG's) or for other cryptographic processes. It should not be used Monte Carlo simulations or for other probabilistic sampling applications.

If the GRND_RANDOM flags bit is set, then draw from the /dev/random pool instead of /dev/urandom pool. The /dev/random pool is limited based on the entropy that can be obtained from environmental noise, so if there is insufficient entropy, the requested number of bytes may not be returned. If there is no entropy available at all, getrandom(2) will either return an error with errno set to EAGAIN, or block if the GRND_BLOCK flags bit is set.

If the GRND_RANDOM flags bit is not set, then the /dev/raundom pool will be used. Unlike reading from the /dev/urandom, if the urandom pool has not been sufficiently initialized, getrandom(2) will either return an error with errno set to EGAIN, or block if the GRND_BLOCK flags bit is set.

RETURN VALUE
On success, the number of bytes that was returned is returned.

On error, -1 is returned, and errno is set appropriately

ERRORS
EINVAL The buflen value was invalid.

EFAULT buf is outside your accessible address space.

EAGAIN The requested entropy was not available, and the
getentropy(2) would have blocked if GRND_BLOCK flag was set.

---
 arch/x86/syscalls/syscall_32.tbl  |  1 +
 arch/x86/syscalls/syscall_64.tbl  |  1 +
 drivers/char/random.c             | 35 +++++++++++++++++++++++++++++++++--
 include/linux/syscalls.h          |  3 +++
 include/uapi/asm-generic/unistd.h |  4 +++-
 include/uapi/linux/random.h       |  9 +++++++++
 6 files changed, 50 insertions(+), 3 deletions(-)

index d6b8679..f484e39 100644
@@ -360,3 +360,4 @@
 351	i386	sched_setattr		sys_sched_setattr
 352	i386	sched_getattr		sys_sched_getattr
 353	i386	renameat2		sys_renameat2
+354	i386	getrandom		sys_getrandom

index ec255a1..6705032 100644
@@ -323,6 +323,7 @@
 314	common	sched_setattr		sys_sched_setattr
 315	common	sched_getattr		sys_sched_getattr
 316	common	renameat2		sys_renameat2
+317	common	getrandom		sys_getrandom
 
 #
 # x32-specific system call numbers start at 512 to avoid cache impact

index aa22fe5..76a56f6 100644
@@ -258,6 +258,8 @@
 #include <linux/kmemcheck.h>
 #include <linux/workqueue.h>
 #include <linux/irq.h>
+#include <linux/syscalls.h>
+#include <linux/completion.h>
 
 #include <asm/processor.h>
 #include <asm/uaccess.h>

@@ -469,6 +471,8 @@ static struct entropy_store nonblocking_pool = {
 					push_to_pool),
 };
 
+DECLARE_COMPLETION(urandom_initialized);
+
 static __u32 const twist_table[8] = {
 	0x00000000, 0x3b6e20c8, 0x76dc4190, 0x4db26158,
 	0xedb88320, 0xd6d6a3e8, 0x9b64c2b0, 0xa00ae278 };

@@ -657,6 +661,7 @@ retry:
 		r->entropy_total = 0;
 		if (r == &nonblocking_pool) {
 			prandom_reseed_late();
+			complete_all(&urandom_initialized);
 			pr_notice("random: %s pool is initialized\n", r->name);
 		}
 	}

@@ -1355,7 +1360,7 @@ static int arch_random_refill(void)
 }
 
 static ssize_t
-random_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
+_random_read(int nonblock, char __user *buf, size_t nbytes)
 {
 	ssize_t n;
 

@@ -1379,7 +1384,7 @@ random_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
 		if (arch_random_refill())
 			continue;
 
-		if (file->f_flags & O_NONBLOCK)
+		if (nonblock)
 			return -EAGAIN;
 
 		wait_event_interruptible(random_read_wait,

@@ -1391,6 +1396,12 @@ random_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
 }
 
 static ssize_t
+random_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
+{
+	return _random_read(file->f_flags & O_NONBLOCK, buf, nbytes);
+}
+
+static ssize_t
 urandom_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
 {
 	int ret;

@@ -1533,6 +1544,26 @@ const struct file_operations urandom_fops = {
 	.llseek = noop_llseek,
 };
 
+SYSCALL_DEFINE3(getrandom, char __user *, buf, size_t, count,
+		unsigned int, flags)
+{
+	int r;
+
+	if (count > 256)
+		return -EINVAL;
+
+	if (flags & GRND_RANDOM) {
+		return _random_read(!(flags & GRND_BLOCK), buf, count);
+	}
+	if (flags & GRND_BLOCK) {
+		r = wait_for_completion_interruptible(&urandom_initialized);
+		if (r)
+			return r;
+	} else if (!completion_done(&urandom_initialized))
+		return -EAGAIN;
+	return urandom_read(NULL, buf, count, NULL);
+}
+
 /***************************************************************
  * Random UUID interface
  *

index b0881a0..cd82f72f 100644
@@ -866,4 +866,7 @@ asmlinkage long sys_process_vm_writev(pid_t pid,
 asmlinkage long sys_kcmp(pid_t pid1, pid_t pid2, int type,
 			 unsigned long idx1, unsigned long idx2);
 asmlinkage long sys_finit_module(int fd, const char __user *uargs, int flags);
+asmlinkage long sys_getrandom(char __user * buf, size_t count,
+			      unsigned int flags);
+
 #endif

index 3336406..2926b1d 100644
@@ -699,9 +699,11 @@ __SYSCALL(__NR_sched_setattr, sys_sched_setattr)
 __SYSCALL(__NR_sched_getattr, sys_sched_getattr)
 #define __NR_renameat2 276
 __SYSCALL(__NR_renameat2, sys_renameat2)
+#define __NR_getrandom 277
+__SYSCALL(__NR_getrandom, sys_getrandom)
 
 #undef __NR_syscalls
-#define __NR_syscalls 277
+#define __NR_syscalls 278
 
 /*
  * All syscalls below here should go away really,

index fff3528..6c13442 100644
@@ -40,4 +40,13 @@ struct rand_pool_info {
 	__u32	buf[0];
 };
 
+/*
+ * Flags for getrandom(2)
+ *
+ * GAND_BLOCK 		Allow getrandom(2) to block
+ * GAND_RANDOM		Use the /dev/random pool instead of /dev/urandom
+ */
+#define GRND_BLOCK	0x0001
+#define GRND_RANDOM	0x0002
+
 #endif /* _UAPI_LINUX_RANDOM_H */

2.0.0