1// SPDX-License-Identifier: GPL-2.0
  2#include <linux/static_call.h>
  3#include <linux/memory.h>
  4#include <linux/bug.h>
  5#include <asm/text-patching.h>
  6
  7enum insn_type {
  8	CALL = 0, /* site call */
  9	NOP = 1,  /* site cond-call */
 10	JMP = 2,  /* tramp / site tail-call */
 11	RET = 3,  /* tramp / site cond-tail-call */
 12	JCC = 4,
 13};
 14
 15/*
 16 * ud1 %esp, %ecx - a 3 byte #UD that is unique to trampolines, chosen such
 17 * that there is no false-positive trampoline identification while also being a
 18 * speculation stop.
 19 */
 20static const u8 tramp_ud[] = { 0x0f, 0xb9, 0xcc };
 21
 22/*
 23 * cs cs cs xorl %eax, %eax - a single 5 byte instruction that clears %[er]ax
 24 */
 25static const u8 xor5rax[] = { 0x2e, 0x2e, 0x2e, 0x31, 0xc0 };
 26
 27static const u8 retinsn[] = { RET_INSN_OPCODE, 0xcc, 0xcc, 0xcc, 0xcc };
 28
 29static u8 __is_Jcc(u8 *insn) /* Jcc.d32 */
 30{
 31	u8 ret = 0;
 32
 33	if (insn[0] == 0x0f) {
 34		u8 tmp = insn[1];
 35		if ((tmp & 0xf0) == 0x80)
 36			ret = tmp;
 37	}
 38
 39	return ret;
 40}
 41
 42extern void __static_call_return(void);
 43
 44asm (".global __static_call_return\n\t"
 45     ".type __static_call_return, @function\n\t"
 46     ASM_FUNC_ALIGN "\n\t"
 47     "__static_call_return:\n\t"
 48     ANNOTATE_NOENDBR
 49     ANNOTATE_RETPOLINE_SAFE
 50     "ret; int3\n\t"
 51     ".size __static_call_return, . - __static_call_return \n\t");
 52
 53static void __ref __static_call_transform(void *insn, enum insn_type type,
 54					  void *func, bool modinit)
 55{
 56	const void *emulate = NULL;
 57	int size = CALL_INSN_SIZE;
 58	const void *code;
 59	u8 op, buf[6];
 60
 61	if ((type == JMP || type == RET) && (op = __is_Jcc(insn)))
 62		type = JCC;
 63
 64	switch (type) {
 65	case CALL:
 66		func = callthunks_translate_call_dest(func);
 67		code = text_gen_insn(CALL_INSN_OPCODE, insn, func);
 68		if (func == &__static_call_return0) {
 69			emulate = code;
 70			code = &xor5rax;
 71		}
 72
 73		break;
 74
 75	case NOP:
 76		code = x86_nops[5];
 77		break;
 78
 79	case JMP:
 80		code = text_gen_insn(JMP32_INSN_OPCODE, insn, func);
 81		break;
 82
 83	case RET:
 84		if (cpu_feature_enabled(X86_FEATURE_RETHUNK))
 85			code = text_gen_insn(JMP32_INSN_OPCODE, insn, x86_return_thunk);
 86		else
 87			code = &retinsn;
 88		break;
 89
 90	case JCC:
 91		if (!func) {
 92			func = __static_call_return;
 93			if (cpu_feature_enabled(X86_FEATURE_RETHUNK))
 94				func = x86_return_thunk;
 95		}
 96
 97		buf[0] = 0x0f;
 98		__text_gen_insn(buf+1, op, insn+1, func, 5);
 99		code = buf;
100		size = 6;
101
102		break;
103	}
104
105	if (memcmp(insn, code, size) == 0)
106		return;
107
108	if (system_state == SYSTEM_BOOTING || modinit)
109		return text_poke_early(insn, code, size);
110
111	text_poke_bp(insn, code, size, emulate);
112}
113
114static void __static_call_validate(u8 *insn, bool tail, bool tramp)
115{
116	u8 opcode = insn[0];
117
118	if (tramp && memcmp(insn+5, tramp_ud, 3)) {
119		pr_err("trampoline signature fail");
120		BUG();
121	}
122
123	if (tail) {
124		if (opcode == JMP32_INSN_OPCODE ||
125		    opcode == RET_INSN_OPCODE ||
126		    __is_Jcc(insn))
127			return;
128	} else {
129		if (opcode == CALL_INSN_OPCODE ||
130		    !memcmp(insn, x86_nops[5], 5) ||
131		    !memcmp(insn, xor5rax, 5))
132			return;
133	}
134
135	/*
136	 * If we ever trigger this, our text is corrupt, we'll probably not live long.
137	 */
138	pr_err("unexpected static_call insn opcode 0x%x at %pS\n", opcode, insn);
139	BUG();
140}
141
142static inline enum insn_type __sc_insn(bool null, bool tail)
143{
144	/*
145	 * Encode the following table without branches:
146	 *
147	 *	tail	null	insn
148	 *	-----+-------+------
149	 *	  0  |   0   |  CALL
150	 *	  0  |   1   |  NOP
151	 *	  1  |   0   |  JMP
152	 *	  1  |   1   |  RET
153	 */
154	return 2*tail + null;
155}
156
157void arch_static_call_transform(void *site, void *tramp, void *func, bool tail)
158{
159	mutex_lock(&text_mutex);
160
161	if (tramp) {
162		__static_call_validate(tramp, true, true);
163		__static_call_transform(tramp, __sc_insn(!func, true), func, false);
164	}
165
166	if (IS_ENABLED(CONFIG_HAVE_STATIC_CALL_INLINE) && site) {
167		__static_call_validate(site, tail, false);
168		__static_call_transform(site, __sc_insn(!func, tail), func, false);
169	}
170
171	mutex_unlock(&text_mutex);
172}
173EXPORT_SYMBOL_GPL(arch_static_call_transform);
174
175noinstr void __static_call_update_early(void *tramp, void *func)
176{
177	BUG_ON(system_state != SYSTEM_BOOTING);
178	BUG_ON(static_call_initialized);
179	__text_gen_insn(tramp, JMP32_INSN_OPCODE, tramp, func, JMP32_INSN_SIZE);
180	sync_core();
181}
182
183#ifdef CONFIG_MITIGATION_RETHUNK
184/*
185 * This is called by apply_returns() to fix up static call trampolines,
186 * specifically ARCH_DEFINE_STATIC_CALL_NULL_TRAMP which is recorded as
187 * having a return trampoline.
188 *
189 * The problem is that static_call() is available before determining
190 * X86_FEATURE_RETHUNK and, by implication, running alternatives.
191 *
192 * This means that __static_call_transform() above can have overwritten the
193 * return trampoline and we now need to fix things up to be consistent.
194 */
195bool __static_call_fixup(void *tramp, u8 op, void *dest)
196{
197	unsigned long addr = (unsigned long)tramp;
198	/*
199	 * Not all .return_sites are a static_call trampoline (most are not).
200	 * Check if the 3 bytes after the return are still kernel text, if not,
201	 * then this definitely is not a trampoline and we need not worry
202	 * further.
203	 *
204	 * This avoids the memcmp() below tripping over pagefaults etc..
205	 */
206	if (((addr >> PAGE_SHIFT) != ((addr + 7) >> PAGE_SHIFT)) &&
207	    !kernel_text_address(addr + 7))
208		return false;
209
210	if (memcmp(tramp+5, tramp_ud, 3)) {
211		/* Not a trampoline site, not our problem. */
212		return false;
213	}
214
215	mutex_lock(&text_mutex);
216	if (op == RET_INSN_OPCODE || dest == &__x86_return_thunk)
217		__static_call_transform(tramp, RET, NULL, true);
218	mutex_unlock(&text_mutex);
219
220	return true;
221}
222#endif