// 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->version > 0)		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 *)(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 -EPERM;	}	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;}