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@@ -0,0 +1,357 @@
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+// SPDX-License-Identifier: GPL-2.0+
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+/*
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+ * Restartable sequences system call
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+ *
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+ * Copyright (C) 2015, Google, Inc.,
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+ * Paul Turner <pjt@google.com> and Andrew Hunter <ahh@google.com>
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+ * Copyright (C) 2015-2018, EfficiOS Inc.,
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+ * Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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+ */
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+
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+#include <linux/sched.h>
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+#include <linux/uaccess.h>
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+#include <linux/syscalls.h>
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+#include <linux/rseq.h>
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+#include <linux/types.h>
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+#include <asm/ptrace.h>
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+
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+#define CREATE_TRACE_POINTS
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+#include <trace/events/rseq.h>
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+
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+#define RSEQ_CS_PREEMPT_MIGRATE_FLAGS (RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE | \
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+ RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT)
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+
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+/*
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+ *
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+ * Restartable sequences are a lightweight interface that allows
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+ * user-level code to be executed atomically relative to scheduler
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+ * preemption and signal delivery. Typically used for implementing
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+ * per-cpu operations.
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+ *
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+ * It allows user-space to perform update operations on per-cpu data
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+ * without requiring heavy-weight atomic operations.
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+ *
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+ * Detailed algorithm of rseq user-space assembly sequences:
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+ *
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+ * init(rseq_cs)
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+ * cpu = TLS->rseq::cpu_id_start
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+ * [1] TLS->rseq::rseq_cs = rseq_cs
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+ * [start_ip] ----------------------------
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+ * [2] if (cpu != TLS->rseq::cpu_id)
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+ * goto abort_ip;
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+ * [3] <last_instruction_in_cs>
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+ * [post_commit_ip] ----------------------------
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+ *
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+ * The address of jump target abort_ip must be outside the critical
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+ * region, i.e.:
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+ *
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+ * [abort_ip] < [start_ip] || [abort_ip] >= [post_commit_ip]
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+ *
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+ * Steps [2]-[3] (inclusive) need to be a sequence of instructions in
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+ * userspace that can handle being interrupted between any of those
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+ * instructions, and then resumed to the abort_ip.
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+ *
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+ * 1. Userspace stores the address of the struct rseq_cs assembly
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+ * block descriptor into the rseq_cs field of the registered
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+ * struct rseq TLS area. This update is performed through a single
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+ * store within the inline assembly instruction sequence.
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+ * [start_ip]
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+ *
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+ * 2. Userspace tests to check whether the current cpu_id field match
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+ * the cpu number loaded before start_ip, branching to abort_ip
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+ * in case of a mismatch.
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+ *
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+ * If the sequence is preempted or interrupted by a signal
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+ * at or after start_ip and before post_commit_ip, then the kernel
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+ * clears TLS->__rseq_abi::rseq_cs, and sets the user-space return
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+ * ip to abort_ip before returning to user-space, so the preempted
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+ * execution resumes at abort_ip.
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+ *
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+ * 3. Userspace critical section final instruction before
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+ * post_commit_ip is the commit. The critical section is
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+ * self-terminating.
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+ * [post_commit_ip]
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+ *
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+ * 4. <success>
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+ *
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+ * On failure at [2], or if interrupted by preempt or signal delivery
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+ * between [1] and [3]:
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+ *
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+ * [abort_ip]
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+ * F1. <failure>
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+ */
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+
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+static int rseq_update_cpu_id(struct task_struct *t)
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+{
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+ u32 cpu_id = raw_smp_processor_id();
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+
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+ if (__put_user(cpu_id, &t->rseq->cpu_id_start))
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+ return -EFAULT;
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+ if (__put_user(cpu_id, &t->rseq->cpu_id))
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+ return -EFAULT;
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+ trace_rseq_update(t);
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+ return 0;
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+}
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+
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+static int rseq_reset_rseq_cpu_id(struct task_struct *t)
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+{
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+ u32 cpu_id_start = 0, cpu_id = RSEQ_CPU_ID_UNINITIALIZED;
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+
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+ /*
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+ * Reset cpu_id_start to its initial state (0).
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+ */
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+ if (__put_user(cpu_id_start, &t->rseq->cpu_id_start))
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+ return -EFAULT;
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+ /*
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+ * Reset cpu_id to RSEQ_CPU_ID_UNINITIALIZED, so any user coming
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+ * in after unregistration can figure out that rseq needs to be
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+ * registered again.
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+ */
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+ if (__put_user(cpu_id, &t->rseq->cpu_id))
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+ return -EFAULT;
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+ return 0;
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+}
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+
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+static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)
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+{
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+ struct rseq_cs __user *urseq_cs;
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+ unsigned long ptr;
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+ u32 __user *usig;
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+ u32 sig;
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+ int ret;
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+
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+ ret = __get_user(ptr, &t->rseq->rseq_cs);
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+ if (ret)
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+ return ret;
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+ if (!ptr) {
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+ memset(rseq_cs, 0, sizeof(*rseq_cs));
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+ return 0;
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+ }
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+ urseq_cs = (struct rseq_cs __user *)ptr;
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+ if (copy_from_user(rseq_cs, urseq_cs, sizeof(*rseq_cs)))
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+ return -EFAULT;
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+ if (rseq_cs->version > 0)
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+ return -EINVAL;
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+
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+ /* Ensure that abort_ip is not in the critical section. */
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+ if (rseq_cs->abort_ip - rseq_cs->start_ip < rseq_cs->post_commit_offset)
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+ return -EINVAL;
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+
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+ usig = (u32 __user *)(rseq_cs->abort_ip - sizeof(u32));
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+ ret = get_user(sig, usig);
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+ if (ret)
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+ return ret;
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+
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+ if (current->rseq_sig != sig) {
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+ printk_ratelimited(KERN_WARNING
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+ "Possible attack attempt. Unexpected rseq signature 0x%x, expecting 0x%x (pid=%d, addr=%p).\n",
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+ sig, current->rseq_sig, current->pid, usig);
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+ return -EPERM;
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+ }
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+ return 0;
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+}
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+
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+static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
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+{
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+ u32 flags, event_mask;
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+ int ret;
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+
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+ /* Get thread flags. */
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+ ret = __get_user(flags, &t->rseq->flags);
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+ if (ret)
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+ return ret;
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+
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+ /* Take critical section flags into account. */
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+ flags |= cs_flags;
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+
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+ /*
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+ * Restart on signal can only be inhibited when restart on
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+ * preempt and restart on migrate are inhibited too. Otherwise,
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+ * a preempted signal handler could fail to restart the prior
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+ * execution context on sigreturn.
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+ */
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+ if (unlikely((flags & RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL) &&
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+ (flags & RSEQ_CS_PREEMPT_MIGRATE_FLAGS) !=
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+ RSEQ_CS_PREEMPT_MIGRATE_FLAGS))
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+ return -EINVAL;
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+
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+ /*
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+ * Load and clear event mask atomically with respect to
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+ * scheduler preemption.
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+ */
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+ preempt_disable();
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+ event_mask = t->rseq_event_mask;
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+ t->rseq_event_mask = 0;
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+ preempt_enable();
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+
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+ return !!(event_mask & ~flags);
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+}
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+
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+static int clear_rseq_cs(struct task_struct *t)
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+{
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+ /*
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+ * The rseq_cs field is set to NULL on preemption or signal
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+ * delivery on top of rseq assembly block, as well as on top
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+ * of code outside of the rseq assembly block. This performs
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+ * a lazy clear of the rseq_cs field.
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+ *
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+ * Set rseq_cs to NULL with single-copy atomicity.
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+ */
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+ return __put_user(0UL, &t->rseq->rseq_cs);
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+}
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+
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+/*
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+ * Unsigned comparison will be true when ip >= start_ip, and when
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+ * ip < start_ip + post_commit_offset.
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+ */
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+static bool in_rseq_cs(unsigned long ip, struct rseq_cs *rseq_cs)
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+{
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+ return ip - rseq_cs->start_ip < rseq_cs->post_commit_offset;
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+}
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+
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+static int rseq_ip_fixup(struct pt_regs *regs)
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+{
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+ unsigned long ip = instruction_pointer(regs);
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+ struct task_struct *t = current;
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+ struct rseq_cs rseq_cs;
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+ int ret;
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+
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+ ret = rseq_get_rseq_cs(t, &rseq_cs);
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+ if (ret)
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+ return ret;
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+
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+ /*
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+ * Handle potentially not being within a critical section.
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+ * If not nested over a rseq critical section, restart is useless.
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+ * Clear the rseq_cs pointer and return.
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+ */
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+ if (!in_rseq_cs(ip, &rseq_cs))
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+ return clear_rseq_cs(t);
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+ ret = rseq_need_restart(t, rseq_cs.flags);
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+ if (ret <= 0)
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+ return ret;
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+ ret = clear_rseq_cs(t);
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+ if (ret)
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+ return ret;
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+ trace_rseq_ip_fixup(ip, rseq_cs.start_ip, rseq_cs.post_commit_offset,
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+ rseq_cs.abort_ip);
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+ instruction_pointer_set(regs, (unsigned long)rseq_cs.abort_ip);
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+ return 0;
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+}
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+
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+/*
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+ * This resume handler must always be executed between any of:
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+ * - preemption,
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+ * - signal delivery,
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+ * and return to user-space.
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+ *
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+ * This is how we can ensure that the entire rseq critical section,
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+ * consisting of both the C part and the assembly instruction sequence,
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+ * will issue the commit instruction only if executed atomically with
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+ * respect to other threads scheduled on the same CPU, and with respect
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+ * to signal handlers.
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+ */
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+void __rseq_handle_notify_resume(struct pt_regs *regs)
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+{
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+ struct task_struct *t = current;
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+ int ret;
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+
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+ if (unlikely(t->flags & PF_EXITING))
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+ return;
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+ if (unlikely(!access_ok(VERIFY_WRITE, t->rseq, sizeof(*t->rseq))))
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+ goto error;
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+ ret = rseq_ip_fixup(regs);
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+ if (unlikely(ret < 0))
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+ goto error;
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+ if (unlikely(rseq_update_cpu_id(t)))
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+ goto error;
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+ return;
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+
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+error:
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+ force_sig(SIGSEGV, t);
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+}
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+
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+#ifdef CONFIG_DEBUG_RSEQ
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+
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+/*
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+ * Terminate the process if a syscall is issued within a restartable
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+ * sequence.
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+ */
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+void rseq_syscall(struct pt_regs *regs)
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+{
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+ unsigned long ip = instruction_pointer(regs);
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+ struct task_struct *t = current;
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+ struct rseq_cs rseq_cs;
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+
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+ if (!t->rseq)
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+ return;
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+ if (!access_ok(VERIFY_READ, t->rseq, sizeof(*t->rseq)) ||
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+ rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs))
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+ force_sig(SIGSEGV, t);
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+}
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+
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+#endif
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+
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+/*
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+ * sys_rseq - setup restartable sequences for caller thread.
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+ */
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+SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
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+ int, flags, u32, sig)
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+{
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+ int ret;
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+
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+ if (flags & RSEQ_FLAG_UNREGISTER) {
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+ /* Unregister rseq for current thread. */
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+ if (current->rseq != rseq || !current->rseq)
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+ return -EINVAL;
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+ if (current->rseq_len != rseq_len)
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+ return -EINVAL;
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+ if (current->rseq_sig != sig)
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+ return -EPERM;
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+ ret = rseq_reset_rseq_cpu_id(current);
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+ if (ret)
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+ return ret;
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+ current->rseq = NULL;
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+ current->rseq_len = 0;
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+ current->rseq_sig = 0;
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+ return 0;
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+ }
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+
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+ if (unlikely(flags))
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+ return -EINVAL;
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+
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+ if (current->rseq) {
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+ /*
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+ * If rseq is already registered, check whether
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+ * the provided address differs from the prior
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+ * one.
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+ */
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+ if (current->rseq != rseq || current->rseq_len != rseq_len)
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+ return -EINVAL;
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+ if (current->rseq_sig != sig)
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+ return -EPERM;
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+ /* Already registered. */
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+ return -EBUSY;
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+ }
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+
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+ /*
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+ * If there was no rseq previously registered,
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+ * ensure the provided rseq is properly aligned and valid.
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+ */
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+ if (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) ||
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+ rseq_len != sizeof(*rseq))
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+ return -EINVAL;
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+ if (!access_ok(VERIFY_WRITE, rseq, rseq_len))
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+ return -EFAULT;
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+ current->rseq = rseq;
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+ current->rseq_len = rseq_len;
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+ current->rseq_sig = sig;
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+ /*
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+ * If rseq was previously inactive, and has just been
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+ * registered, ensure the cpu_id_start and cpu_id fields
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+ * are updated before returning to user-space.
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+ */
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+ rseq_set_notify_resume(current);
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+
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+ return 0;
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+}
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Mathieu Desnoyers