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Subject[PATCH, RFC] random: introduce getrandom(2) system call
Date
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.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
---
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(-)
diff --git a/arch/x86/syscalls/syscall_32.tbl b/arch/x86/syscalls/syscall_32.tbl
index d6b8679..f484e39 100644
--- a/arch/x86/syscalls/syscall_32.tbl
+++ b/arch/x86/syscalls/syscall_32.tbl
@@ -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
diff --git a/arch/x86/syscalls/syscall_64.tbl b/arch/x86/syscalls/syscall_64.tbl
index ec255a1..6705032 100644
--- a/arch/x86/syscalls/syscall_64.tbl
+++ b/arch/x86/syscalls/syscall_64.tbl
@@ -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
diff --git a/drivers/char/random.c b/drivers/char/random.c
index aa22fe5..76a56f6 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -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
*
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index b0881a0..cd82f72f 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -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
diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index 3336406..2926b1d 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -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,
diff --git a/include/uapi/linux/random.h b/include/uapi/linux/random.h
index fff3528..6c13442 100644
--- a/include/uapi/linux/random.h
+++ b/include/uapi/linux/random.h
@@ -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


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