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random: introduce getrandom(2) system call
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 the case. And so with this new interface, we can provide the functionality of blocking until the urandom pool has been initialized. Any userspace program which uses this new functionality must take care to assure that if it is used during the boot process, that it will not cause the init 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 uses. It should not be used for Monte Carlo simulations or other programs/algorithms which are doing probabilistic sampling. If the GRND_RANDOM flags bit is set, then draw from the /dev/random pool instead of the /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 block, or return an error with errno set to EAGAIN if the GRND_NONBLOCK bit is set in flags. If the GRND_RANDOM bit is not set, then the /dev/urandom pool will be used. Unlike using read(2) to fetch data from /dev/urandom, if the urandom pool has not been sufficiently initialized, getrandom(2) will block (or return -1 with the errno set to EAGAIN if the GRND_NONBLOCK bit is set in flags). The getentropy(2) system call in OpenBSD can be emulated using the following function: int getentropy(void *buf, size_t buflen) { int ret; if (buflen > 256) goto failure; ret = getrandom(buf, buflen, 0); if (ret < 0) return ret; if (ret == buflen) return 0; failure: errno = EIO; return -1; } RETURN VALUE On success, the number of bytes that was filled in the buf is returned. This may not be all the bytes requested by the caller via buflen if insufficient entropy was present in the /dev/random pool, or if the system call was interrupted by a signal. On error, -1 is returned, and errno is set appropriately. ERRORS EINVAL An invalid flag was passed to getrandom(2) EFAULT buf is outside the accessible address space. EAGAIN The requested entropy was not available, and getentropy(2) would have blocked if the GRND_NONBLOCK flag was not set. EINTR While blocked waiting for entropy, the call was interrupted by a signal handler; see the description of how interrupted read(2) calls on "slow" devices are handled with and without the SA_RESTART flag in the signal(7) man page. NOTES For small requests (buflen <= 256) getrandom(2) will not return EINTR when reading from the urandom pool once the entropy pool has been initialized, and it will return all of the bytes that have been requested. This is the recommended way to use getrandom(2), and is designed for compatibility with OpenBSD's getentropy() system call. However, if you are using GRND_RANDOM, then getrandom(2) may block until the entropy accounting determines that sufficient environmental noise has been gathered such that getrandom(2) will be operating as a NRBG instead of a DRBG for those people who are working in the NIST SP 800-90 regime. Since it may block for a long time, these guarantees do *not* apply. The user may want to interrupt a hanging process using a signal, so blocking until all of the requested bytes are returned would be unfriendly. For this reason, the user of getrandom(2) MUST always check the return value, in case it returns some error, or if fewer bytes than requested was returned. In the case of !GRND_RANDOM and small request, the latter should never happen, but the careful userspace code (and all crypto code should be careful) should check for this anyway! Finally, unless you are doing long-term key generation (and perhaps not even then), you probably shouldn't be using GRND_RANDOM. The cryptographic algorithms used for /dev/urandom are quite conservative, and so should be sufficient for all purposes. The disadvantage of GRND_RANDOM is that it can block, and the increased complexity required to deal with partially fulfilled getrandom(2) requests. Signed-off-by: Theodore Ts'o <tytso@mit.edu> Reviewed-by: Zach Brown <zab@zabbo.net>
This commit is contained in:
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9dda727d37
commit
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6 changed files with 54 additions and 4 deletions
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@ -360,3 +360,4 @@
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351 i386 sched_setattr sys_sched_setattr
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352 i386 sched_getattr sys_sched_getattr
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353 i386 renameat2 sys_renameat2
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355 i386 getrandom sys_getrandom
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@ -323,6 +323,7 @@
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314 common sched_setattr sys_sched_setattr
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315 common sched_getattr sys_sched_getattr
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316 common renameat2 sys_renameat2
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318 common getrandom sys_getrandom
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#
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# x32-specific system call numbers start at 512 to avoid cache impact
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@ -258,6 +258,8 @@
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#include <linux/kmemcheck.h>
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#include <linux/workqueue.h>
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#include <linux/irq.h>
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#include <linux/syscalls.h>
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#include <linux/completion.h>
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#include <asm/processor.h>
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#include <asm/uaccess.h>
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@ -404,6 +406,7 @@ static struct poolinfo {
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*/
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static DECLARE_WAIT_QUEUE_HEAD(random_read_wait);
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static DECLARE_WAIT_QUEUE_HEAD(random_write_wait);
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static DECLARE_WAIT_QUEUE_HEAD(urandom_init_wait);
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static struct fasync_struct *fasync;
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/**********************************************************************
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@ -657,6 +660,7 @@ static void credit_entropy_bits(struct entropy_store *r, int nbits)
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r->entropy_total = 0;
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if (r == &nonblocking_pool) {
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prandom_reseed_late();
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wake_up_interruptible(&urandom_init_wait);
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pr_notice("random: %s pool is initialized\n", r->name);
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}
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}
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@ -1174,13 +1178,14 @@ static ssize_t extract_entropy_user(struct entropy_store *r, void __user *buf,
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{
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ssize_t ret = 0, i;
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__u8 tmp[EXTRACT_SIZE];
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int large_request = (nbytes > 256);
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trace_extract_entropy_user(r->name, nbytes, ENTROPY_BITS(r), _RET_IP_);
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xfer_secondary_pool(r, nbytes);
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nbytes = account(r, nbytes, 0, 0);
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while (nbytes) {
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if (need_resched()) {
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if (large_request && need_resched()) {
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if (signal_pending(current)) {
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if (ret == 0)
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ret = -ERESTARTSYS;
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@ -1355,7 +1360,7 @@ static int arch_random_refill(void)
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}
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static ssize_t
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random_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
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_random_read(int nonblock, char __user *buf, size_t nbytes)
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{
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ssize_t n;
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@ -1379,7 +1384,7 @@ random_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
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if (arch_random_refill())
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continue;
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if (file->f_flags & O_NONBLOCK)
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if (nonblock)
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return -EAGAIN;
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wait_event_interruptible(random_read_wait,
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@ -1390,6 +1395,12 @@ random_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
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}
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}
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static ssize_t
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random_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
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{
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return _random_read(file->f_flags & O_NONBLOCK, buf, nbytes);
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}
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static ssize_t
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urandom_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
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{
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.llseek = noop_llseek,
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};
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SYSCALL_DEFINE3(getrandom, char __user *, buf, size_t, count,
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unsigned int, flags)
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{
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if (flags & ~(GRND_NONBLOCK|GRND_RANDOM))
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return -EINVAL;
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if (count > INT_MAX)
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count = INT_MAX;
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if (flags & GRND_RANDOM)
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return _random_read(flags & GRND_NONBLOCK, buf, count);
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if (unlikely(nonblocking_pool.initialized == 0)) {
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if (flags & GRND_NONBLOCK)
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return -EAGAIN;
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wait_event_interruptible(urandom_init_wait,
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nonblocking_pool.initialized);
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if (signal_pending(current))
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return -ERESTARTSYS;
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}
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return urandom_read(NULL, buf, count, NULL);
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}
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/***************************************************************
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* Random UUID interface
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*
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@ -866,4 +866,7 @@ asmlinkage long sys_process_vm_writev(pid_t pid,
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asmlinkage long sys_kcmp(pid_t pid1, pid_t pid2, int type,
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unsigned long idx1, unsigned long idx2);
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asmlinkage long sys_finit_module(int fd, const char __user *uargs, int flags);
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asmlinkage long sys_getrandom(char __user *buf, size_t count,
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unsigned int flags);
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#endif
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@ -699,9 +699,11 @@ __SYSCALL(__NR_sched_setattr, sys_sched_setattr)
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__SYSCALL(__NR_sched_getattr, sys_sched_getattr)
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#define __NR_renameat2 276
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__SYSCALL(__NR_renameat2, sys_renameat2)
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#define __NR_getrandom 278
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__SYSCALL(__NR_getrandom, sys_getrandom)
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#undef __NR_syscalls
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#define __NR_syscalls 277
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#define __NR_syscalls 279
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/*
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* All syscalls below here should go away really,
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@ -40,4 +40,13 @@ struct rand_pool_info {
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__u32 buf[0];
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};
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/*
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* Flags for getrandom(2)
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*
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* GRND_NONBLOCK Don't block and return EAGAIN instead
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* GRND_RANDOM Use the /dev/random pool instead of /dev/urandom
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*/
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#define GRND_NONBLOCK 0x0001
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#define GRND_RANDOM 0x0002
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#endif /* _UAPI_LINUX_RANDOM_H */
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