// 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;
}
