1 files changed, 212 insertions, 0 deletions
diff --git a/arch/x86/kernel/static_call.c b/arch/x86/kernel/static_call.c
new file mode 100644
index 000000000..3fbb49168
--- /dev/null
+++ b/arch/x86/kernel/static_call.c
@@ -0,0 +1,212 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/static_call.h>
+#include <linux/memory.h>
+#include <linux/bug.h>
+#include <asm/text-patching.h>
+
+enum insn_type {
+	CALL = 0, /* site call */
+	NOP = 1,  /* site cond-call */
+	JMP = 2,  /* tramp / site tail-call */
+	RET = 3,  /* tramp / site cond-tail-call */
+	JCC = 4,
+};
+
+/*
+ * ud1 %esp, %ecx - a 3 byte #UD that is unique to trampolines, chosen such
+ * that there is no false-positive trampoline identification while also being a
+ * speculation stop.
+ */
+static const u8 tramp_ud[] = { 0x0f, 0xb9, 0xcc };
+
+/*
+ * cs cs cs xorl %eax, %eax - a single 5 byte instruction that clears %[er]ax
+ */
+static const u8 xor5rax[] = { 0x2e, 0x2e, 0x2e, 0x31, 0xc0 };
+
+static const u8 retinsn[] = { RET_INSN_OPCODE, 0xcc, 0xcc, 0xcc, 0xcc };
+
+static u8 __is_Jcc(u8 *insn) /* Jcc.d32 */
+{
+	u8 ret = 0;
+
+	if (insn[0] == 0x0f) {
+		u8 tmp = insn[1];
+		if ((tmp & 0xf0) == 0x80)
+			ret = tmp;
+	}
+
+	return ret;
+}
+
+extern void __static_call_return(void);
+
+asm (".global __static_call_return\n\t"
+     ".type __static_call_return, @function\n\t"
+     "__static_call_return:\n\t"
+     ANNOTATE_NOENDBR
+     ANNOTATE_RETPOLINE_SAFE
+     "ret; int3\n\t"
+     ".size __static_call_return, . - __static_call_return \n\t");
+
+static void __ref __static_call_transform(void *insn, enum insn_type type,
+					  void *func, bool modinit)
+{
+	const void *emulate = NULL;
+	int size = CALL_INSN_SIZE;
+	const void *code;
+	u8 op, buf[6];
+
+	if ((type == JMP || type == RET) && (op = __is_Jcc(insn)))
+		type = JCC;
+
+	switch (type) {
+	case CALL:
+		code = text_gen_insn(CALL_INSN_OPCODE, insn, func);
+		if (func == &__static_call_return0) {
+			emulate = code;
+			code = &xor5rax;
+		}
+
+		break;
+
+	case NOP:
+		code = x86_nops[5];
+		break;
+
+	case JMP:
+		code = text_gen_insn(JMP32_INSN_OPCODE, insn, func);
+		break;
+
+	case RET:
+		if (cpu_feature_enabled(X86_FEATURE_RETHUNK))
+			code = text_gen_insn(JMP32_INSN_OPCODE, insn, &__x86_return_thunk);
+		else
+			code = &retinsn;
+		break;
+
+	case JCC:
+		if (!func) {
+			func = __static_call_return;
+			if (cpu_feature_enabled(X86_FEATURE_RETHUNK))
+				func = __x86_return_thunk;
+		}
+
+		buf[0] = 0x0f;
+		__text_gen_insn(buf+1, op, insn+1, func, 5);
+		code = buf;
+		size = 6;
+
+		break;
+	}
+
+	if (memcmp(insn, code, size) == 0)
+		return;
+
+	if (system_state == SYSTEM_BOOTING || modinit)
+		return text_poke_early(insn, code, size);
+
+	text_poke_bp(insn, code, size, emulate);
+}
+
+static void __static_call_validate(u8 *insn, bool tail, bool tramp)
+{
+	u8 opcode = insn[0];
+
+	if (tramp && memcmp(insn+5, tramp_ud, 3)) {
+		pr_err("trampoline signature fail");
+		BUG();
+	}
+
+	if (tail) {
+		if (opcode == JMP32_INSN_OPCODE ||
+		    opcode == RET_INSN_OPCODE ||
+		    __is_Jcc(insn))
+			return;
+	} else {
+		if (opcode == CALL_INSN_OPCODE ||
+		    !memcmp(insn, x86_nops[5], 5) ||
+		    !memcmp(insn, xor5rax, 5))
+			return;
+	}
+
+	/*
+	 * If we ever trigger this, our text is corrupt, we'll probably not live long.
+	 */
+	pr_err("unexpected static_call insn opcode 0x%x at %pS\n", opcode, insn);
+	BUG();
+}
+
+static inline enum insn_type __sc_insn(bool null, bool tail)
+{
+	/*
+	 * Encode the following table without branches:
+	 *
+	 *	tail	null	insn
+	 *	-----+-------+------
+	 *	  0  |   0   |  CALL
+	 *	  0  |   1   |  NOP
+	 *	  1  |   0   |  JMP
+	 *	  1  |   1   |  RET
+	 */
+	return 2*tail + null;
+}
+
+void arch_static_call_transform(void *site, void *tramp, void *func, bool tail)
+{
+	mutex_lock(&text_mutex);
+
+	if (tramp) {
+		__static_call_validate(tramp, true, true);
+		__static_call_transform(tramp, __sc_insn(!func, true), func, false);
+	}
+
+	if (IS_ENABLED(CONFIG_HAVE_STATIC_CALL_INLINE) && site) {
+		__static_call_validate(site, tail, false);
+		__static_call_transform(site, __sc_insn(!func, tail), func, false);
+	}
+
+	mutex_unlock(&text_mutex);
+}
+EXPORT_SYMBOL_GPL(arch_static_call_transform);
+
+#ifdef CONFIG_RETHUNK
+/*
+ * This is called by apply_returns() to fix up static call trampolines,
+ * specifically ARCH_DEFINE_STATIC_CALL_NULL_TRAMP which is recorded as
+ * having a return trampoline.
+ *
+ * The problem is that static_call() is available before determining
+ * X86_FEATURE_RETHUNK and, by implication, running alternatives.
+ *
+ * This means that __static_call_transform() above can have overwritten the
+ * return trampoline and we now need to fix things up to be consistent.
+ */
+bool __static_call_fixup(void *tramp, u8 op, void *dest)
+{
+	unsigned long addr = (unsigned long)tramp;
+	/*
+	 * Not all .return_sites are a static_call trampoline (most are not).
+	 * Check if the 3 bytes after the return are still kernel text, if not,
+	 * then this definitely is not a trampoline and we need not worry
+	 * further.
+	 *
+	 * This avoids the memcmp() below tripping over pagefaults etc..
+	 */
+	if (((addr >> PAGE_SHIFT) != ((addr + 7) >> PAGE_SHIFT)) &&
+	    !kernel_text_address(addr + 7))
+		return false;
+
+	if (memcmp(tramp+5, tramp_ud, 3)) {
+		/* Not a trampoline site, not our problem. */
+		return false;
+	}
+
+	mutex_lock(&text_mutex);
+	if (op == RET_INSN_OPCODE || dest == &__x86_return_thunk)
+		__static_call_transform(tramp, RET, NULL, true);
+	mutex_unlock(&text_mutex);
+
+	return true;
+}
+#endif