1 files changed, 357 insertions, 0 deletions

diff --git a/kernel/rseq.c b/kernel/rseq.c
new file mode 100644
index 000000000000..ae306f90c514
--- /dev/null
+++ b/kernel/rseq.c
@@ -0,0 +1,357 @@
+// 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 pt_regs *regs)
+{
+        struct task_struct *t = current;
+        int ret;
+
+        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:
+        force_sig(SIGSEGV, 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;
+}