[linux-2.6-block.git] / kernel / rseq.c

// SPDX-License-Identifier: GPL-2.0+
/*
 * Restartable sequences system call
 *
 * Copyright (C) 2015, Google, Inc.,
 * Paul Turner <pjt@google.com> and Andrew Hunter <ahh@google.com>
 * Copyright (C) 2015-2018, EfficiOS Inc.,
 * Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
 */

#include <linux/sched.h>
#include <linux/uaccess.h>
#include <linux/syscalls.h>
#include <linux/rseq.h>
#include <linux/types.h>
#include <asm/ptrace.h>

#define CREATE_TRACE_POINTS
#include <trace/events/rseq.h>

#define RSEQ_CS_PREEMPT_MIGRATE_FLAGS (RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE | \
				       RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT)

/*
 *
 * Restartable sequences are a lightweight interface that allows
 * user-level code to be executed atomically relative to scheduler
 * preemption and signal delivery. Typically used for implementing
 * per-cpu operations.
 *
 * It allows user-space to perform update operations on per-cpu data
 * without requiring heavy-weight atomic operations.
 *
 * Detailed algorithm of rseq user-space assembly sequences:
 *
 *                     init(rseq_cs)
 *                     cpu = TLS->rseq::cpu_id_start
 *   [1]               TLS->rseq::rseq_cs = rseq_cs
 *   [start_ip]        ----------------------------
 *   [2]               if (cpu != TLS->rseq::cpu_id)
 *                             goto abort_ip;
 *   [3]               <last_instruction_in_cs>
 *   [post_commit_ip]  ----------------------------
 *
 *   The address of jump target abort_ip must be outside the critical
 *   region, i.e.:
 *
 *     [abort_ip] < [start_ip]  || [abort_ip] >= [post_commit_ip]
 *
 *   Steps [2]-[3] (inclusive) need to be a sequence of instructions in
 *   userspace that can handle being interrupted between any of those
 *   instructions, and then resumed to the abort_ip.
 *
 *   1.  Userspace stores the address of the struct rseq_cs assembly
 *       block descriptor into the rseq_cs field of the registered
 *       struct rseq TLS area. This update is performed through a single
 *       store within the inline assembly instruction sequence.
 *       [start_ip]
 *
 *   2.  Userspace tests to check whether the current cpu_id field match
 *       the cpu number loaded before start_ip, branching to abort_ip
 *       in case of a mismatch.
 *
 *       If the sequence is preempted or interrupted by a signal
 *       at or after start_ip and before post_commit_ip, then the kernel
 *       clears TLS->__rseq_abi::rseq_cs, and sets the user-space return
 *       ip to abort_ip before returning to user-space, so the preempted
 *       execution resumes at abort_ip.
 *
 *   3.  Userspace critical section final instruction before
 *       post_commit_ip is the commit. The critical section is
 *       self-terminating.
 *       [post_commit_ip]
 *
 *   4.  <success>
 *
 *   On failure at [2], or if interrupted by preempt or signal delivery
 *   between [1] and [3]:
 *
 *       [abort_ip]
 *   F1. <failure>
 */

static int rseq_update_cpu_id(struct task_struct *t)
{
	u32 cpu_id = raw_smp_processor_id();

	if (put_user(cpu_id, &t->rseq->cpu_id_start))
		return -EFAULT;
	if (put_user(cpu_id, &t->rseq->cpu_id))
		return -EFAULT;
	trace_rseq_update(t);
	return 0;
}

static int rseq_reset_rseq_cpu_id(struct task_struct *t)
{
	u32 cpu_id_start = 0, cpu_id = RSEQ_CPU_ID_UNINITIALIZED;

	/*
	 * Reset cpu_id_start to its initial state (0).
	 */
	if (put_user(cpu_id_start, &t->rseq->cpu_id_start))
		return -EFAULT;
	/*
	 * Reset cpu_id to RSEQ_CPU_ID_UNINITIALIZED, so any user coming
	 * in after unregistration can figure out that rseq needs to be
	 * registered again.
	 */
	if (put_user(cpu_id, &t->rseq->cpu_id))
		return -EFAULT;
	return 0;
}

static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)
{
	struct rseq_cs __user *urseq_cs;
	unsigned long ptr;
	u32 __user *usig;
	u32 sig;
	int ret;

	ret = get_user(ptr, &t->rseq->rseq_cs);
	if (ret)
		return ret;
	if (!ptr) {
		memset(rseq_cs, 0, sizeof(*rseq_cs));
		return 0;
	}
	urseq_cs = (struct rseq_cs __user *)ptr;
	if (copy_from_user(rseq_cs, urseq_cs, sizeof(*rseq_cs)))
		return -EFAULT;

	if (rseq_cs->start_ip >= TASK_SIZE ||
	    rseq_cs->start_ip + rseq_cs->post_commit_offset >= TASK_SIZE ||
	    rseq_cs->abort_ip >= TASK_SIZE ||
	    rseq_cs->version > 0)
		return -EINVAL;
	/* Check for overflow. */
	if (rseq_cs->start_ip + rseq_cs->post_commit_offset < rseq_cs->start_ip)
		return -EINVAL;
	/* Ensure that abort_ip is not in the critical section. */
	if (rseq_cs->abort_ip - rseq_cs->start_ip < rseq_cs->post_commit_offset)
		return -EINVAL;

	usig = (u32 __user *)(unsigned long)(rseq_cs->abort_ip - sizeof(u32));
	ret = get_user(sig, usig);
	if (ret)
		return ret;

	if (current->rseq_sig != sig) {
		printk_ratelimited(KERN_WARNING
			"Possible attack attempt. Unexpected rseq signature 0x%x, expecting 0x%x (pid=%d, addr=%p).\n",
			sig, current->rseq_sig, current->pid, usig);
		return -EINVAL;
	}
	return 0;
}

static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
{
	u32 flags, event_mask;
	int ret;

	/* Get thread flags. */
	ret = get_user(flags, &t->rseq->flags);
	if (ret)
		return ret;

	/* Take critical section flags into account. */
	flags |= cs_flags;

	/*
	 * Restart on signal can only be inhibited when restart on
	 * preempt and restart on migrate are inhibited too. Otherwise,
	 * a preempted signal handler could fail to restart the prior
	 * execution context on sigreturn.
	 */
	if (unlikely((flags & RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL) &&
		     (flags & RSEQ_CS_PREEMPT_MIGRATE_FLAGS) !=
		     RSEQ_CS_PREEMPT_MIGRATE_FLAGS))
		return -EINVAL;

	/*
	 * Load and clear event mask atomically with respect to
	 * scheduler preemption.
	 */
	preempt_disable();
	event_mask = t->rseq_event_mask;
	t->rseq_event_mask = 0;
	preempt_enable();

	return !!(event_mask & ~flags);
}

static int clear_rseq_cs(struct task_struct *t)
{
	/*
	 * The rseq_cs field is set to NULL on preemption or signal
	 * delivery on top of rseq assembly block, as well as on top
	 * of code outside of the rseq assembly block. This performs
	 * a lazy clear of the rseq_cs field.
	 *
	 * Set rseq_cs to NULL with single-copy atomicity.
	 */
	return put_user(0UL, &t->rseq->rseq_cs);
}

/*
 * Unsigned comparison will be true when ip >= start_ip, and when
 * ip < start_ip + post_commit_offset.
 */
static bool in_rseq_cs(unsigned long ip, struct rseq_cs *rseq_cs)
{
	return ip - rseq_cs->start_ip < rseq_cs->post_commit_offset;
}

static int rseq_ip_fixup(struct pt_regs *regs)
{
	unsigned long ip = instruction_pointer(regs);
	struct task_struct *t = current;
	struct rseq_cs rseq_cs;
	int ret;

	ret = rseq_get_rseq_cs(t, &rseq_cs);
	if (ret)
		return ret;

	/*
	 * Handle potentially not being within a critical section.
	 * If not nested over a rseq critical section, restart is useless.
	 * Clear the rseq_cs pointer and return.
	 */
	if (!in_rseq_cs(ip, &rseq_cs))
		return clear_rseq_cs(t);
	ret = rseq_need_restart(t, rseq_cs.flags);
	if (ret <= 0)
		return ret;
	ret = clear_rseq_cs(t);
	if (ret)
		return ret;
	trace_rseq_ip_fixup(ip, rseq_cs.start_ip, rseq_cs.post_commit_offset,
			    rseq_cs.abort_ip);
	instruction_pointer_set(regs, (unsigned long)rseq_cs.abort_ip);
	return 0;
}

/*
 * This resume handler must always be executed between any of:
 * - preemption,
 * - signal delivery,
 * and return to user-space.
 *
 * This is how we can ensure that the entire rseq critical section,
 * consisting of both the C part and the assembly instruction sequence,
 * will issue the commit instruction only if executed atomically with
 * respect to other threads scheduled on the same CPU, and with respect
 * to signal handlers.
 */
void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
{
	struct task_struct *t = current;
	int ret, sig;

	if (unlikely(t->flags & PF_EXITING))
		return;
	if (unlikely(!access_ok(VERIFY_WRITE, t->rseq, sizeof(*t->rseq))))
		goto error;
	ret = rseq_ip_fixup(regs);
	if (unlikely(ret < 0))
		goto error;
	if (unlikely(rseq_update_cpu_id(t)))
		goto error;
	return;

error:
	sig = ksig ? ksig->sig : 0;
	force_sigsegv(sig, t);
}

#ifdef CONFIG_DEBUG_RSEQ

/*
 * Terminate the process if a syscall is issued within a restartable
 * sequence.
 */
void rseq_syscall(struct pt_regs *regs)
{
	unsigned long ip = instruction_pointer(regs);
	struct task_struct *t = current;
	struct rseq_cs rseq_cs;

	if (!t->rseq)
		return;
	if (!access_ok(VERIFY_READ, t->rseq, sizeof(*t->rseq)) ||
	    rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs))
		force_sig(SIGSEGV, t);
}

#endif

/*
 * sys_rseq - setup restartable sequences for caller thread.
 */
SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
		int, flags, u32, sig)
{
	int ret;

	if (flags & RSEQ_FLAG_UNREGISTER) {
		/* Unregister rseq for current thread. */
		if (current->rseq != rseq || !current->rseq)
			return -EINVAL;
		if (current->rseq_len != rseq_len)
			return -EINVAL;
		if (current->rseq_sig != sig)
			return -EPERM;
		ret = rseq_reset_rseq_cpu_id(current);
		if (ret)
			return ret;
		current->rseq = NULL;
		current->rseq_len = 0;
		current->rseq_sig = 0;
		return 0;
	}

	if (unlikely(flags))
		return -EINVAL;

	if (current->rseq) {
		/*
		 * If rseq is already registered, check whether
		 * the provided address differs from the prior
		 * one.
		 */
		if (current->rseq != rseq || current->rseq_len != rseq_len)
			return -EINVAL;
		if (current->rseq_sig != sig)
			return -EPERM;
		/* Already registered. */
		return -EBUSY;
	}

	/*
	 * If there was no rseq previously registered,
	 * ensure the provided rseq is properly aligned and valid.
	 */
	if (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) ||
	    rseq_len != sizeof(*rseq))
		return -EINVAL;
	if (!access_ok(VERIFY_WRITE, rseq, rseq_len))
		return -EFAULT;
	current->rseq = rseq;
	current->rseq_len = rseq_len;
	current->rseq_sig = sig;
	/*
	 * If rseq was previously inactive, and has just been
	 * registered, ensure the cpu_id_start and cpu_id fields
	 * are updated before returning to user-space.
	 */
	rseq_set_notify_resume(current);

	return 0;
}
Commit	Line	Data
d7822b1e MD	1	// SPDX-License-Identifier: GPL-2.0+
	2	/*
	3	* Restartable sequences system call
	4	*
	5	* Copyright (C) 2015, Google, Inc.,
	6	* Paul Turner <pjt@google.com> and Andrew Hunter <ahh@google.com>
	7	* Copyright (C) 2015-2018, EfficiOS Inc.,
	8	* Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
	9	*/
	10
	11	#include <linux/sched.h>
	12	#include <linux/uaccess.h>
	13	#include <linux/syscalls.h>
	14	#include <linux/rseq.h>
	15	#include <linux/types.h>
	16	#include <asm/ptrace.h>
	17
	18	#define CREATE_TRACE_POINTS
	19	#include <trace/events/rseq.h>
	20
	21	#define RSEQ_CS_PREEMPT_MIGRATE_FLAGS (RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE \| \
	22	RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT)
	23
	24	/*
	25	*
	26	* Restartable sequences are a lightweight interface that allows
	27	* user-level code to be executed atomically relative to scheduler
	28	* preemption and signal delivery. Typically used for implementing
	29	* per-cpu operations.
	30	*
	31	* It allows user-space to perform update operations on per-cpu data
	32	* without requiring heavy-weight atomic operations.
	33	*
	34	* Detailed algorithm of rseq user-space assembly sequences:
	35	*
	36	* init(rseq_cs)
	37	* cpu = TLS->rseq::cpu_id_start
	38	* [1] TLS->rseq::rseq_cs = rseq_cs
	39	* [start_ip] ----------------------------
	40	* [2] if (cpu != TLS->rseq::cpu_id)
	41	* goto abort_ip;
	42	* [3] <last_instruction_in_cs>
	43	* [post_commit_ip] ----------------------------
	44	*
	45	* The address of jump target abort_ip must be outside the critical
	46	* region, i.e.:
	47	*
	48	* [abort_ip] < [start_ip] \|\| [abort_ip] >= [post_commit_ip]
	49	*
	50	* Steps [2]-[3] (inclusive) need to be a sequence of instructions in
	51	* userspace that can handle being interrupted between any of those
	52	* instructions, and then resumed to the abort_ip.
	53	*
	54	* 1. Userspace stores the address of the struct rseq_cs assembly
	55	* block descriptor into the rseq_cs field of the registered
	56	* struct rseq TLS area. This update is performed through a single
	57	* store within the inline assembly instruction sequence.
	58	* [start_ip]
	59	*
	60	* 2. Userspace tests to check whether the current cpu_id field match
	61	* the cpu number loaded before start_ip, branching to abort_ip
	62	* in case of a mismatch.
	63	*
	64	* If the sequence is preempted or interrupted by a signal
65	* at or after start_ip and before post_commit_ip, then the kernel
66	* clears TLS->__rseq_abi::rseq_cs, and sets the user-space return
67	* ip to abort_ip before returning to user-space, so the preempted
68	* execution resumes at abort_ip.
69	*
70	* 3. Userspace critical section final instruction before
71	* post_commit_ip is the commit. The critical section is
72	* self-terminating.
73	* [post_commit_ip]
74	*
75	* 4. <success>
76	*
77	* On failure at [2], or if interrupted by preempt or signal delivery
78	* between [1] and [3]:
79	*
80	* [abort_ip]
81	* F1. <failure>
82	*/
83
84	static int rseq_update_cpu_id(struct task_struct *t)
85	{
86	u32 cpu_id = raw_smp_processor_id();
87
8f281770	88	if (put_user(cpu_id, &t->rseq->cpu_id_start))
d7822b1e	89	return -EFAULT;
8f281770	90	if (put_user(cpu_id, &t->rseq->cpu_id))
d7822b1e MD	91	return -EFAULT;
	92	trace_rseq_update(t);
	93	return 0;
	94	}
	95
	96	static int rseq_reset_rseq_cpu_id(struct task_struct *t)
	97	{
	98	u32 cpu_id_start = 0, cpu_id = RSEQ_CPU_ID_UNINITIALIZED;
	99
	100	/*
	101	* Reset cpu_id_start to its initial state (0).
	102	*/
8f281770	103	if (put_user(cpu_id_start, &t->rseq->cpu_id_start))
d7822b1e MD	104	return -EFAULT;
	105	/*
	106	* Reset cpu_id to RSEQ_CPU_ID_UNINITIALIZED, so any user coming
	107	* in after unregistration can figure out that rseq needs to be
	108	* registered again.
	109	*/
8f281770	110	if (put_user(cpu_id, &t->rseq->cpu_id))
d7822b1e MD	111	return -EFAULT;
	112	return 0;
	113	}
	114
	115	static int rseq_get_rseq_cs(struct task_struct t, struct rseq_cs rseq_cs)
	116	{
	117	struct rseq_cs __user *urseq_cs;
	118	unsigned long ptr;
	119	u32 __user *usig;
	120	u32 sig;
	121	int ret;
	122
8f281770	123	ret = get_user(ptr, &t->rseq->rseq_cs);
d7822b1e MD	124	if (ret)
	125	return ret;
	126	if (!ptr) {
	127	memset(rseq_cs, 0, sizeof(*rseq_cs));
	128	return 0;
	129	}
	130	urseq_cs = (struct rseq_cs __user *)ptr;
	131	if (copy_from_user(rseq_cs, urseq_cs, sizeof(*rseq_cs)))
	132	return -EFAULT;
d7822b1e	133
e96d7135 MD	134	if (rseq_cs->start_ip >= TASK_SIZE \|\|
	135	rseq_cs->start_ip + rseq_cs->post_commit_offset >= TASK_SIZE \|\|
	136	rseq_cs->abort_ip >= TASK_SIZE \|\|
	137	rseq_cs->version > 0)
	138	return -EINVAL;
	139	/* Check for overflow. */
	140	if (rseq_cs->start_ip + rseq_cs->post_commit_offset < rseq_cs->start_ip)
	141	return -EINVAL;
d7822b1e MD	142	/* Ensure that abort_ip is not in the critical section. */
	143	if (rseq_cs->abort_ip - rseq_cs->start_ip < rseq_cs->post_commit_offset)
	144	return -EINVAL;
	145
e96d7135	146	usig = (u32 __user *)(unsigned long)(rseq_cs->abort_ip - sizeof(u32));
d7822b1e MD	147	ret = get_user(sig, usig);
	148	if (ret)
	149	return ret;
	150
	151	if (current->rseq_sig != sig) {
	152	printk_ratelimited(KERN_WARNING
	153	"Possible attack attempt. Unexpected rseq signature 0x%x, expecting 0x%x (pid=%d, addr=%p).\n",
	154	sig, current->rseq_sig, current->pid, usig);
e96d7135	155	return -EINVAL;
d7822b1e MD	156	}
	157	return 0;
	158	}
	159
	160	static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
	161	{
	162	u32 flags, event_mask;
	163	int ret;
	164
	165	/* Get thread flags. */
8f281770	166	ret = get_user(flags, &t->rseq->flags);
d7822b1e MD	167	if (ret)
	168	return ret;
	169
	170	/* Take critical section flags into account. */
	171	flags \|= cs_flags;
	172
	173	/*
	174	* Restart on signal can only be inhibited when restart on
	175	* preempt and restart on migrate are inhibited too. Otherwise,
	176	* a preempted signal handler could fail to restart the prior
	177	* execution context on sigreturn.
	178	*/
	179	if (unlikely((flags & RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL) &&
	180	(flags & RSEQ_CS_PREEMPT_MIGRATE_FLAGS) !=
	181	RSEQ_CS_PREEMPT_MIGRATE_FLAGS))
	182	return -EINVAL;
	183
	184	/*
	185	* Load and clear event mask atomically with respect to
	186	* scheduler preemption.
	187	*/
	188	preempt_disable();
	189	event_mask = t->rseq_event_mask;
	190	t->rseq_event_mask = 0;
	191	preempt_enable();
	192
	193	return !!(event_mask & ~flags);
	194	}
	195
	196	static int clear_rseq_cs(struct task_struct *t)
	197	{
	198	/*
	199	* The rseq_cs field is set to NULL on preemption or signal
	200	* delivery on top of rseq assembly block, as well as on top
	201	* of code outside of the rseq assembly block. This performs
	202	* a lazy clear of the rseq_cs field.
	203	*
	204	* Set rseq_cs to NULL with single-copy atomicity.
	205	*/
8f281770	206	return put_user(0UL, &t->rseq->rseq_cs);
d7822b1e MD	207	}
	208
	209	/*
	210	* Unsigned comparison will be true when ip >= start_ip, and when
	211	* ip < start_ip + post_commit_offset.
	212	*/
	213	static bool in_rseq_cs(unsigned long ip, struct rseq_cs *rseq_cs)
	214	{
	215	return ip - rseq_cs->start_ip < rseq_cs->post_commit_offset;
	216	}
	217
	218	static int rseq_ip_fixup(struct pt_regs *regs)
	219	{
	220	unsigned long ip = instruction_pointer(regs);
	221	struct task_struct *t = current;
	222	struct rseq_cs rseq_cs;
	223	int ret;
	224
	225	ret = rseq_get_rseq_cs(t, &rseq_cs);
	226	if (ret)
	227	return ret;
	228
	229	/*
	230	* Handle potentially not being within a critical section.
	231	* If not nested over a rseq critical section, restart is useless.
	232	* Clear the rseq_cs pointer and return.
	233	*/
	234	if (!in_rseq_cs(ip, &rseq_cs))
	235	return clear_rseq_cs(t);
	236	ret = rseq_need_restart(t, rseq_cs.flags);
	237	if (ret <= 0)
	238	return ret;
	239	ret = clear_rseq_cs(t);
	240	if (ret)
	241	return ret;
	242	trace_rseq_ip_fixup(ip, rseq_cs.start_ip, rseq_cs.post_commit_offset,
	243	rseq_cs.abort_ip);
	244	instruction_pointer_set(regs, (unsigned long)rseq_cs.abort_ip);
	245	return 0;
	246	}
	247
	248	/*
	249	* This resume handler must always be executed between any of:
	250	* - preemption,
	251	* - signal delivery,
	252	* and return to user-space.
	253	*
	254	* This is how we can ensure that the entire rseq critical section,
	255	* consisting of both the C part and the assembly instruction sequence,
	256	* will issue the commit instruction only if executed atomically with
	257	* respect to other threads scheduled on the same CPU, and with respect
	258	* to signal handlers.
	259	*/
784e0300	260	void __rseq_handle_notify_resume(struct ksignal ksig, struct pt_regs regs)
d7822b1e MD	261	{
d7822b1e MD	262	struct task_struct *t = current;
784e0300	263	int ret, sig;
d7822b1e MD	264
	265	if (unlikely(t->flags & PF_EXITING))
	266	return;
	267	if (unlikely(!access_ok(VERIFY_WRITE, t->rseq, sizeof(*t->rseq))))
	268	goto error;
	269	ret = rseq_ip_fixup(regs);
	270	if (unlikely(ret < 0))
	271	goto error;
	272	if (unlikely(rseq_update_cpu_id(t)))
	273	goto error;
	274	return;
	275
	276	error:
784e0300 WD	277	sig = ksig ? ksig->sig : 0;
784e0300 WD	278	force_sigsegv(sig, t);
d7822b1e MD	279	}
	280
	281	#ifdef CONFIG_DEBUG_RSEQ
	282
	283	/*
	284	* Terminate the process if a syscall is issued within a restartable
	285	* sequence.
	286	*/
	287	void rseq_syscall(struct pt_regs *regs)
	288	{
	289	unsigned long ip = instruction_pointer(regs);
	290	struct task_struct *t = current;
	291	struct rseq_cs rseq_cs;
	292
	293	if (!t->rseq)
	294	return;
	295	if (!access_ok(VERIFY_READ, t->rseq, sizeof(*t->rseq)) \|\|
	296	rseq_get_rseq_cs(t, &rseq_cs) \|\| in_rseq_cs(ip, &rseq_cs))
	297	force_sig(SIGSEGV, t);
	298	}
	299
	300	#endif
	301
	302	/*
	303	* sys_rseq - setup restartable sequences for caller thread.
	304	*/
	305	SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
	306	int, flags, u32, sig)
	307	{
	308	int ret;
	309
	310	if (flags & RSEQ_FLAG_UNREGISTER) {
	311	/* Unregister rseq for current thread. */
	312	if (current->rseq != rseq \|\| !current->rseq)
	313	return -EINVAL;
	314	if (current->rseq_len != rseq_len)
	315	return -EINVAL;
	316	if (current->rseq_sig != sig)
	317	return -EPERM;
	318	ret = rseq_reset_rseq_cpu_id(current);
	319	if (ret)
	320	return ret;
	321	current->rseq = NULL;
	322	current->rseq_len = 0;
	323	current->rseq_sig = 0;
	324	return 0;
	325	}
	326
	327	if (unlikely(flags))
	328	return -EINVAL;
	329
	330	if (current->rseq) {
	331	/*
	332	* If rseq is already registered, check whether
	333	* the provided address differs from the prior
	334	* one.
	335	*/
	336	if (current->rseq != rseq \|\| current->rseq_len != rseq_len)
	337	return -EINVAL;
	338	if (current->rseq_sig != sig)
	339	return -EPERM;
	340	/* Already registered. */
	341	return -EBUSY;
	342	}
343
344	/*
345	* If there was no rseq previously registered,
346	* ensure the provided rseq is properly aligned and valid.
347	*/
348	if (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) \|\|
349	rseq_len != sizeof(*rseq))
350	return -EINVAL;
351	if (!access_ok(VERIFY_WRITE, rseq, rseq_len))
352	return -EFAULT;
353	current->rseq = rseq;
354	current->rseq_len = rseq_len;
355	current->rseq_sig = sig;
356	/*
357	* If rseq was previously inactive, and has just been
358	* registered, ensure the cpu_id_start and cpu_id fields
359	* are updated before returning to user-space.
360	*/
361	rseq_set_notify_resume(current);
362
363	return 0;
364	}