1// SPDX-License-Identifier: GPL-2.0
2
3#define pr_fmt(fmt) "rethook: " fmt
4
5#include <linux/bug.h>
6#include <linux/kallsyms.h>
7#include <linux/kprobes.h>
8#include <linux/preempt.h>
9#include <linux/rethook.h>
10#include <linux/slab.h>
11
12/* Return hook list (shadow stack by list) */
13
14/*
15 * This function is called from delayed_put_task_struct() when a task is
16 * dead and cleaned up to recycle any kretprobe instances associated with
17 * this task. These left over instances represent probed functions that
18 * have been called but will never return.
19 */
20void rethook_flush_task(struct task_struct *tk)
21{
22	struct rethook_node *rhn;
23	struct llist_node *node;
24
25	node = __llist_del_all(&tk->rethooks);
26	while (node) {
27		rhn = container_of(node, struct rethook_node, llist);
28		node = node->next;
29		preempt_disable();
30		rethook_recycle(rhn);
31		preempt_enable();
32	}
33}
34
35static void rethook_free_rcu(struct rcu_head *head)
36{
37	struct rethook *rh = container_of(head, struct rethook, rcu);
38	objpool_fini(&rh->pool);
39}
40
41/**
42 * rethook_stop() - Stop using a rethook.
43 * @rh: the struct rethook to stop.
44 *
45 * Stop using a rethook to prepare for freeing it. If you want to wait for
46 * all running rethook handler before calling rethook_free(), you need to
47 * call this first and wait RCU, and call rethook_free().
48 */
49void rethook_stop(struct rethook *rh)
50{
51	rcu_assign_pointer(rh->handler, NULL);
52}
53
54/**
55 * rethook_free() - Free struct rethook.
56 * @rh: the struct rethook to be freed.
57 *
58 * Free the rethook. Before calling this function, user must ensure the
59 * @rh::data is cleaned if needed (or, the handler can access it after
60 * calling this function.) This function will set the @rh to be freed
61 * after all rethook_node are freed (not soon). And the caller must
62 * not touch @rh after calling this.
63 */
64void rethook_free(struct rethook *rh)
65{
66	rethook_stop(rh);
67
68	call_rcu(&rh->rcu, rethook_free_rcu);
69}
70
71static int rethook_init_node(void *nod, void *context)
72{
73	struct rethook_node *node = nod;
74
75	node->rethook = context;
76	return 0;
77}
78
79static int rethook_fini_pool(struct objpool_head *head, void *context)
80{
81	kfree(context);
82	return 0;
83}
84
85static inline rethook_handler_t rethook_get_handler(struct rethook *rh)
86{
87	return (rethook_handler_t)rcu_dereference_check(rh->handler,
88							rcu_read_lock_any_held());
89}
90
91/**
92 * rethook_alloc() - Allocate struct rethook.
93 * @data: a data to pass the @handler when hooking the return.
94 * @handler: the return hook callback function, must NOT be NULL
95 * @size: node size: rethook node and additional data
96 * @num: number of rethook nodes to be preallocated
97 *
98 * Allocate and initialize a new rethook with @data and @handler.
99 * Return pointer of new rethook, or error codes for failures.
100 *
101 * Note that @handler == NULL means this rethook is going to be freed.
102 */
103struct rethook *rethook_alloc(void *data, rethook_handler_t handler,
104			      int size, int num)
105{
106	struct rethook *rh;
107
108	if (!handler || num <= 0 || size < sizeof(struct rethook_node))
109		return ERR_PTR(-EINVAL);
110
111	rh = kzalloc(sizeof(struct rethook), GFP_KERNEL);
112	if (!rh)
113		return ERR_PTR(-ENOMEM);
114
115	rh->data = data;
116	rcu_assign_pointer(rh->handler, handler);
117
118	/* initialize the objpool for rethook nodes */
119	if (objpool_init(&rh->pool, num, size, GFP_KERNEL, rh,
120			 rethook_init_node, rethook_fini_pool)) {
121		kfree(rh);
122		return ERR_PTR(-ENOMEM);
123	}
124	return rh;
125}
126
127static void free_rethook_node_rcu(struct rcu_head *head)
128{
129	struct rethook_node *node = container_of(head, struct rethook_node, rcu);
130	struct rethook *rh = node->rethook;
131
132	objpool_drop(node, &rh->pool);
133}
134
135/**
136 * rethook_recycle() - return the node to rethook.
137 * @node: The struct rethook_node to be returned.
138 *
139 * Return back the @node to @node::rethook. If the @node::rethook is already
140 * marked as freed, this will free the @node.
141 */
142void rethook_recycle(struct rethook_node *node)
143{
144	rethook_handler_t handler;
145
146	handler = rethook_get_handler(node->rethook);
147	if (likely(handler))
148		objpool_push(node, &node->rethook->pool);
149	else
150		call_rcu(&node->rcu, free_rethook_node_rcu);
151}
152NOKPROBE_SYMBOL(rethook_recycle);
153
154/**
155 * rethook_try_get() - get an unused rethook node.
156 * @rh: The struct rethook which pools the nodes.
157 *
158 * Get an unused rethook node from @rh. If the node pool is empty, this
159 * will return NULL. Caller must disable preemption.
160 */
161struct rethook_node *rethook_try_get(struct rethook *rh)
162{
163	rethook_handler_t handler = rethook_get_handler(rh);
164
165	/* Check whether @rh is going to be freed. */
166	if (unlikely(!handler))
167		return NULL;
168
169	/*
170	 * This expects the caller will set up a rethook on a function entry.
171	 * When the function returns, the rethook will eventually be reclaimed
172	 * or released in the rethook_recycle() with call_rcu().
173	 * This means the caller must be run in the RCU-availabe context.
174	 */
175	if (unlikely(!rcu_is_watching()))
176		return NULL;
177
178	return (struct rethook_node *)objpool_pop(&rh->pool);
179}
180NOKPROBE_SYMBOL(rethook_try_get);
181
182/**
183 * rethook_hook() - Hook the current function return.
184 * @node: The struct rethook node to hook the function return.
185 * @regs: The struct pt_regs for the function entry.
186 * @mcount: True if this is called from mcount(ftrace) context.
187 *
188 * Hook the current running function return. This must be called when the
189 * function entry (or at least @regs must be the registers of the function
190 * entry.) @mcount is used for identifying the context. If this is called
191 * from ftrace (mcount) callback, @mcount must be set true. If this is called
192 * from the real function entry (e.g. kprobes) @mcount must be set false.
193 * This is because the way to hook the function return depends on the context.
194 */
195void rethook_hook(struct rethook_node *node, struct pt_regs *regs, bool mcount)
196{
197	arch_rethook_prepare(node, regs, mcount);
198	__llist_add(&node->llist, &current->rethooks);
199}
200NOKPROBE_SYMBOL(rethook_hook);
201
202/* This assumes the 'tsk' is the current task or is not running. */
203static unsigned long __rethook_find_ret_addr(struct task_struct *tsk,
204					     struct llist_node **cur)
205{
206	struct rethook_node *rh = NULL;
207	struct llist_node *node = *cur;
208
209	if (!node)
210		node = tsk->rethooks.first;
211	else
212		node = node->next;
213
214	while (node) {
215		rh = container_of(node, struct rethook_node, llist);
216		if (rh->ret_addr != (unsigned long)arch_rethook_trampoline) {
217			*cur = node;
218			return rh->ret_addr;
219		}
220		node = node->next;
221	}
222	return 0;
223}
224NOKPROBE_SYMBOL(__rethook_find_ret_addr);
225
226/**
227 * rethook_find_ret_addr -- Find correct return address modified by rethook
228 * @tsk: Target task
229 * @frame: A frame pointer
230 * @cur: a storage of the loop cursor llist_node pointer for next call
231 *
232 * Find the correct return address modified by a rethook on @tsk in unsigned
233 * long type.
234 * The @tsk must be 'current' or a task which is not running. @frame is a hint
235 * to get the currect return address - which is compared with the
236 * rethook::frame field. The @cur is a loop cursor for searching the
237 * kretprobe return addresses on the @tsk. The '*@cur' should be NULL at the
238 * first call, but '@cur' itself must NOT NULL.
239 *
240 * Returns found address value or zero if not found.
241 */
242unsigned long rethook_find_ret_addr(struct task_struct *tsk, unsigned long frame,
243				    struct llist_node **cur)
244{
245	struct rethook_node *rhn = NULL;
246	unsigned long ret;
247
248	if (WARN_ON_ONCE(!cur))
249		return 0;
250
251	if (WARN_ON_ONCE(tsk != current && task_is_running(tsk)))
252		return 0;
253
254	do {
255		ret = __rethook_find_ret_addr(tsk, cur);
256		if (!ret)
257			break;
258		rhn = container_of(*cur, struct rethook_node, llist);
259	} while (rhn->frame != frame);
260
261	return ret;
262}
263NOKPROBE_SYMBOL(rethook_find_ret_addr);
264
265void __weak arch_rethook_fixup_return(struct pt_regs *regs,
266				      unsigned long correct_ret_addr)
267{
268	/*
269	 * Do nothing by default. If the architecture which uses a
270	 * frame pointer to record real return address on the stack,
271	 * it should fill this function to fixup the return address
272	 * so that stacktrace works from the rethook handler.
273	 */
274}
275
276/* This function will be called from each arch-defined trampoline. */
277unsigned long rethook_trampoline_handler(struct pt_regs *regs,
278					 unsigned long frame)
279{
280	struct llist_node *first, *node = NULL;
281	unsigned long correct_ret_addr;
282	rethook_handler_t handler;
283	struct rethook_node *rhn;
284
285	correct_ret_addr = __rethook_find_ret_addr(current, &node);
286	if (!correct_ret_addr) {
287		pr_err("rethook: Return address not found! Maybe there is a bug in the kernel\n");
288		BUG_ON(1);
289	}
290
291	instruction_pointer_set(regs, correct_ret_addr);
292
293	/*
294	 * These loops must be protected from rethook_free_rcu() because those
295	 * are accessing 'rhn->rethook'.
296	 */
297	preempt_disable_notrace();
298
299	/*
300	 * Run the handler on the shadow stack. Do not unlink the list here because
301	 * stackdump inside the handlers needs to decode it.
302	 */
303	first = current->rethooks.first;
304	while (first) {
305		rhn = container_of(first, struct rethook_node, llist);
306		if (WARN_ON_ONCE(rhn->frame != frame))
307			break;
308		handler = rethook_get_handler(rhn->rethook);
309		if (handler)
310			handler(rhn, rhn->rethook->data,
311				correct_ret_addr, regs);
312
313		if (first == node)
314			break;
315		first = first->next;
316	}
317
318	/* Fixup registers for returning to correct address. */
319	arch_rethook_fixup_return(regs, correct_ret_addr);
320
321	/* Unlink used shadow stack */
322	first = current->rethooks.first;
323	current->rethooks.first = node->next;
324	node->next = NULL;
325
326	while (first) {
327		rhn = container_of(first, struct rethook_node, llist);
328		first = first->next;
329		rethook_recycle(rhn);
330	}
331	preempt_enable_notrace();
332
333	return correct_ret_addr;
334}
335NOKPROBE_SYMBOL(rethook_trampoline_handler);
336