// SPDX-License-Identifier: GPL-2.0+
/*
* Restartable sequences system call
*
* Copyright (C) 2015, Google, Inc.,
* Paul Turner <[email protected]> and Andrew Hunter <[email protected]>
* Copyright (C) 2015-2018, EfficiOS Inc.,
* Mathieu Desnoyers <[email protected]>
*/
#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>
/* The original rseq structure size (including padding) is 32 bytes. */
#define ORIG_RSEQ_SIZE 32
#define RSEQ_CS_NO_RESTART_FLAGS (RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT | \
RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL | \
RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE)
/*
*
* 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_node_id(struct task_struct *t)
{
struct rseq __user *rseq = t->rseq;
u32 cpu_id = raw_smp_processor_id();
u32 node_id = cpu_to_node(cpu_id);
u32 mm_cid = task_mm_cid(t);
WARN_ON_ONCE((int) mm_cid < 0);
if (!user_write_access_begin(rseq, t->rseq_len))
goto efault;
unsafe_put_user(cpu_id, &rseq->cpu_id_start, efault_end);
unsafe_put_user(cpu_id, &rseq->cpu_id, efault_end);
unsafe_put_user(node_id, &rseq->node_id, efault_end);
unsafe_put_user(mm_cid, &rseq->mm_cid, efault_end);
/*
* Additional feature fields added after ORIG_RSEQ_SIZE
* need to be conditionally updated only if
* t->rseq_len != ORIG_RSEQ_SIZE.
*/
user_write_access_end();
trace_rseq_update(t);
return 0;
efault_end:
user_write_access_end();
efault:
return -EFAULT;
}
static int rseq_reset_rseq_cpu_node_id(struct task_struct *t)
{
u32 cpu_id_start = 0, cpu_id = RSEQ_CPU_ID_UNINITIALIZED, node_id = 0,
mm_cid = 0;
/*
* 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;
/*
* Reset node_id to its initial state (0).
*/
if (put_user(node_id, &t->rseq->node_id))
return -EFAULT;
/*
* Reset mm_cid to its initial state (0).
*/
if (put_user(mm_cid, &t->rseq->mm_cid))
return -EFAULT;
/*
* Additional feature fields added after ORIG_RSEQ_SIZE
* need to be conditionally reset only if
* t->rseq_len != ORIG_RSEQ_SIZE.
*/
return 0;
}
static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)
{
struct rseq_cs __user *urseq_cs;
u64 ptr;
u32 __user *usig;
u32 sig;
int ret;
#ifdef CONFIG_64BIT
if (get_user(ptr, &t->rseq->rseq_cs))
return -EFAULT;
#else
if (copy_from_user(&ptr, &t->rseq->rseq_cs, sizeof(ptr)))
return -EFAULT;
#endif
if (!ptr) {
memset(rseq_cs, 0, sizeof(*rseq_cs));
return 0;
}
if (ptr >= TASK_SIZE)
return -EINVAL;
urseq_cs = (struct rseq_cs __user *)(unsigned long)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 bool rseq_warn_flags(const char *str, u32 flags)
{
u32 test_flags;
if (!flags)
return false;
test_flags = flags & RSEQ_CS_NO_RESTART_FLAGS;
if (test_flags)
pr_warn_once("Deprecated flags (%u) in %s ABI structure", test_flags, str);
test_flags = flags & ~RSEQ_CS_NO_RESTART_FLAGS;
if (test_flags)
pr_warn_once("Unknown flags (%u) in %s ABI structure", test_flags, str);
return true;
}
static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
{
u32 flags, event_mask;
int ret;
if (rseq_warn_flags("rseq_cs", cs_flags))
return -EINVAL;
/* Get thread flags. */
ret = get_user(flags, &t->rseq->flags);
if (ret)
return ret;
if (rseq_warn_flags("rseq", 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;
}
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.
*/
#ifdef CONFIG_64BIT
return put_user(0UL, &t->rseq->rseq_cs);
#else
if (clear_user(&t->rseq->rseq_cs, sizeof(t->rseq->rseq_cs)))
return -EFAULT;
return 0;
#endif
}
/*
* 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
* 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;
/*
* regs is NULL if and only if the caller is in a syscall path. Skip
* fixup and leave rseq_cs as is so that rseq_sycall() will detect and
* kill a misbehaving userspace on debug kernels.
*/
if (regs) {
ret = rseq_ip_fixup(regs);
if (unlikely(ret < 0))
goto error;
}
if (unlikely(rseq_update_cpu_node_id(t)))
goto error;
return;
error:
sig = ksig ? ksig->sig : 0;
force_sigsegv(sig);
}
#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 (rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs))
force_sig(SIGSEGV);
}
#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) {
if (flags & ~RSEQ_FLAG_UNREGISTER)
return -EINVAL;
/* Unregister rseq for current thread. */
if (current->rseq != rseq || !current->rseq)
return -EINVAL;
if (rseq_len != current->rseq_len)
return -EINVAL;
if (current->rseq_sig != sig)
return -EPERM;
ret = rseq_reset_rseq_cpu_node_id(current);
if (ret)
return ret;
current->rseq = NULL;
current->rseq_sig = 0;
current->rseq_len = 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 || rseq_len != current->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, as communcated to user-space through the ELF
* auxiliary vector AT_RSEQ_ALIGN. If rseq_len is the original rseq
* size, the required alignment is the original struct rseq alignment.
*
* In order to be valid, rseq_len is either the original rseq size, or
* large enough to contain all supported fields, as communicated to
* user-space through the ELF auxiliary vector AT_RSEQ_FEATURE_SIZE.
*/
if (rseq_len < ORIG_RSEQ_SIZE ||
(rseq_len == ORIG_RSEQ_SIZE && !IS_ALIGNED((unsigned long)rseq, ORIG_RSEQ_SIZE)) ||
(rseq_len != ORIG_RSEQ_SIZE && (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq)) ||
rseq_len < offsetof(struct rseq, end))))
return -EINVAL;
if (!access_ok(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;
}
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