x86/alternative: Support relocations in alternatives

A little while ago someone (Kirill) ran into the whole 'alternatives don't
do relocations nonsense' again and I got annoyed enough to actually look
at the code.

Since the whole alternative machinery already fully decodes the
instructions it is simple enough to adjust immediates and displacement
when needed. Specifically, the immediates for IP modifying instructions
(JMP, CALL, Jcc) and the displacement for RIP-relative instructions.

  [ bp: Massage comment some more and get rid of third loop in
    apply_relocation(). ]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230208171431.313857925@infradead.org

Author: Peter Zijlstra

arch/x86/kernel/alternative.c

@@ -134,71 +134,6 @@ extern struct alt_instr __alt_instructions[], __alt_instructions_end[];
 extern s32 __smp_locks[], __smp_locks_end[];
 void text_poke_early(void *addr, const void *opcode, size_t len);
 
-/*
- * Are we looking at a near JMP with a 1 or 4-byte displacement.
- */
-static inline bool is_jmp(const u8 opcode)
-{
-	return opcode == 0xeb || opcode == 0xe9;
-}
-
-static void __init_or_module
-recompute_jump(struct alt_instr *a, u8 *orig_insn, u8 *repl_insn, u8 *insn_buff)
-{
-	u8 *next_rip, *tgt_rip;
-	s32 n_dspl, o_dspl;
-	int repl_len;
-
-	if (a->replacementlen != 5)
-		return;
-
-	o_dspl = *(s32 *)(insn_buff + 1);
-
-	/* next_rip of the replacement JMP */
-	next_rip = repl_insn + a->replacementlen;
-	/* target rip of the replacement JMP */
-	tgt_rip  = next_rip + o_dspl;
-	n_dspl = tgt_rip - orig_insn;
-
-	DPRINTK(ALT, "target RIP: %px, new_displ: 0x%x", tgt_rip, n_dspl);
-
-	if (tgt_rip - orig_insn >= 0) {
-		if (n_dspl - 2 <= 127)
-			goto two_byte_jmp;
-		else
-			goto five_byte_jmp;
-	/* negative offset */
-	} else {
-		if (((n_dspl - 2) & 0xff) == (n_dspl - 2))
-			goto two_byte_jmp;
-		else
-			goto five_byte_jmp;
-	}
-
-two_byte_jmp:
-	n_dspl -= 2;
-
-	insn_buff[0] = 0xeb;
-	insn_buff[1] = (s8)n_dspl;
-	add_nops(insn_buff + 2, 3);
-
-	repl_len = 2;
-	goto done;
-
-five_byte_jmp:
-	n_dspl -= 5;
-
-	insn_buff[0] = 0xe9;
-	*(s32 *)&insn_buff[1] = n_dspl;
-
-	repl_len = 5;
-
-done:
-
-	DPRINTK(ALT, "final displ: 0x%08x, JMP 0x%lx",
-		n_dspl, (unsigned long)orig_insn + n_dspl + repl_len);
-}
-
 /*
  * optimize_nops_range() - Optimize a sequence of single byte NOPs (0x90)
  *
@@ -265,6 +200,139 @@ static void __init_or_module noinline optimize_nops(u8 *instr, size_t len)
 	}
 }
 
+/*
+ * In this context, "source" is where the instructions are placed in the
+ * section .altinstr_replacement, for example during kernel build by the
+ * toolchain.
+ * "Destination" is where the instructions are being patched in by this
+ * machinery.
+ *
+ * The source offset is:
+ *
+ *   src_imm = target - src_next_ip                  (1)
+ *
+ * and the target offset is:
+ *
+ *   dst_imm = target - dst_next_ip                  (2)
+ *
+ * so rework (1) as an expression for target like:
+ *
+ *   target = src_imm + src_next_ip                  (1a)
+ *
+ * and substitute in (2) to get:
+ *
+ *   dst_imm = (src_imm + src_next_ip) - dst_next_ip (3)
+ *
+ * Now, since the instruction stream is 'identical' at src and dst (it
+ * is being copied after all) it can be stated that:
+ *
+ *   src_next_ip = src + ip_offset
+ *   dst_next_ip = dst + ip_offset                   (4)
+ *
+ * Substitute (4) in (3) and observe ip_offset being cancelled out to
+ * obtain:
+ *
+ *   dst_imm = src_imm + (src + ip_offset) - (dst + ip_offset)
+ *           = src_imm + src - dst + ip_offset - ip_offset
+ *           = src_imm + src - dst                   (5)
+ *
+ * IOW, only the relative displacement of the code block matters.
+ */
+
+#define apply_reloc_n(n_, p_, d_)				\
+	do {							\
+		s32 v = *(s##n_ *)(p_);				\
+		v += (d_);					\
+		BUG_ON((v >> 31) != (v >> (n_-1)));		\
+		*(s##n_ *)(p_) = (s##n_)v;			\
+	} while (0)
+
+
+static __always_inline
+void apply_reloc(int n, void *ptr, uintptr_t diff)
+{
+	switch (n) {
+	case 1: apply_reloc_n(8, ptr, diff); break;
+	case 2: apply_reloc_n(16, ptr, diff); break;
+	case 4: apply_reloc_n(32, ptr, diff); break;
+	default: BUG();
+	}
+}
+
+static __always_inline
+bool need_reloc(unsigned long offset, u8 *src, size_t src_len)
+{
+	u8 *target = src + offset;
+	/*
+	 * If the target is inside the patched block, it's relative to the
+	 * block itself and does not need relocation.
+	 */
+	return (target < src || target > src + src_len);
+}
+
+static void __init_or_module noinline
+apply_relocation(u8 *buf, size_t len, u8 *dest, u8 *src, size_t src_len)
+{
+	for (int next, i = 0; i < len; i = next) {
+		struct insn insn;
+
+		if (WARN_ON_ONCE(insn_decode_kernel(&insn, &buf[i])))
+			return;
+
+		next = i + insn.length;
+
+		switch (insn.opcode.bytes[0]) {
+		case 0x0f:
+			if (insn.opcode.bytes[1] < 0x80 ||
+			    insn.opcode.bytes[1] > 0x8f)
+				break;
+
+			fallthrough;	/* Jcc.d32 */
+		case 0x70 ... 0x7f:	/* Jcc.d8 */
+		case JMP8_INSN_OPCODE:
+		case JMP32_INSN_OPCODE:
+		case CALL_INSN_OPCODE:
+			if (need_reloc(next + insn.immediate.value, src, src_len)) {
+				apply_reloc(insn.immediate.nbytes,
+					    buf + i + insn_offset_immediate(&insn),
+					    src - dest);
+			}
+
+			/*
+			 * Where possible, convert JMP.d32 into JMP.d8.
+			 */
+			if (insn.opcode.bytes[0] == JMP32_INSN_OPCODE) {
+				s32 imm = insn.immediate.value;
+				imm += src - dest;
+				imm += JMP32_INSN_SIZE - JMP8_INSN_SIZE;
+				if ((imm >> 31) == (imm >> 7)) {
+					buf[i+0] = JMP8_INSN_OPCODE;
+					buf[i+1] = (s8)imm;
+
+					memset(&buf[i+2], INT3_INSN_OPCODE, insn.length - 2);
+				}
+			}
+			break;
+		}
+
+		if (insn_rip_relative(&insn)) {
+			if (need_reloc(next + insn.displacement.value, src, src_len)) {
+				apply_reloc(insn.displacement.nbytes,
+					    buf + i + insn_offset_displacement(&insn),
+					    src - dest);
+			}
+		}
+
+
+		/*
+		 * See if this and any potentially following NOPs can be
+		 * optimized.
+		 */
+		if (insn.length == 1 && insn.opcode.bytes[0] == 0x90)
+			next = i + optimize_nops_range(buf, len, i);
+	}
+}
+
 /*
  * Replace instructions with better alternatives for this CPU type. This runs
  * before SMP is initialized to avoid SMP problems with self modifying code.
@@ -306,8 +374,10 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
 		 * - feature not present but ALT_FLAG_NOT is set to mean,
 		 *   patch if feature is *NOT* present.
 		 */
-		if (!boot_cpu_has(a->cpuid) == !(a->flags & ALT_FLAG_NOT))
-			goto next;
+		if (!boot_cpu_has(a->cpuid) == !(a->flags & ALT_FLAG_NOT)) {
+			optimize_nops(instr, a->instrlen);
+			continue;
+		}
 
 		DPRINTK(ALT, "feat: %s%d*32+%d, old: (%pS (%px) len: %d), repl: (%px, len: %d)",
 			(a->flags & ALT_FLAG_NOT) ? "!" : "",
@@ -316,37 +386,19 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
 			instr, instr, a->instrlen,
 			replacement, a->replacementlen);
 
-		DUMP_BYTES(ALT, instr, a->instrlen, "%px:   old_insn: ", instr);
-		DUMP_BYTES(ALT, replacement, a->replacementlen, "%px:   rpl_insn: ", replacement);
-
 		memcpy(insn_buff, replacement, a->replacementlen);
 		insn_buff_sz = a->replacementlen;
 
-		/*
-		 * 0xe8 is a relative jump; fix the offset.
-		 *
-		 * Instruction length is checked before the opcode to avoid
-		 * accessing uninitialized bytes for zero-length replacements.
-		 */
-		if (a->replacementlen == 5 && *insn_buff == 0xe8) {
-			*(s32 *)(insn_buff + 1) += replacement - instr;
-			DPRINTK(ALT, "Fix CALL offset: 0x%x, CALL 0x%lx",
-				*(s32 *)(insn_buff + 1),
-				(unsigned long)instr + *(s32 *)(insn_buff + 1) + 5);
-		}
-
-		if (a->replacementlen && is_jmp(replacement[0]))
-			recompute_jump(a, instr, replacement, insn_buff);
-
 		for (; insn_buff_sz < a->instrlen; insn_buff_sz++)
 			insn_buff[insn_buff_sz] = 0x90;
 
+		apply_relocation(insn_buff, a->instrlen, instr, replacement, a->replacementlen);
+
+		DUMP_BYTES(ALT, instr, a->instrlen, "%px:   old_insn: ", instr);
+		DUMP_BYTES(ALT, replacement, a->replacementlen, "%px:   rpl_insn: ", replacement);
 		DUMP_BYTES(ALT, insn_buff, insn_buff_sz, "%px: final_insn: ", instr);
 
 		text_poke_early(instr, insn_buff, insn_buff_sz);
-
-next:
-		optimize_nops(instr, a->instrlen);
 	}
 }
 
@@ -1344,6 +1396,35 @@ static noinline void __init int3_selftest(void)
 	unregister_die_notifier(&int3_exception_nb);
 }
 
+static __initdata int __alt_reloc_selftest_addr;
+
+__visible noinline void __init __alt_reloc_selftest(void *arg)
+{
+	WARN_ON(arg != &__alt_reloc_selftest_addr);
+}
+
+static noinline void __init alt_reloc_selftest(void)
+{
+	/*
+	 * Tests apply_relocation().
+	 *
+	 * This has a relative immediate (CALL) in a place other than the first
+	 * instruction and additionally on x86_64 we get a RIP-relative LEA:
+	 *
+	 *   lea    0x0(%rip),%rdi  # 5d0: R_X86_64_PC32    .init.data+0x5566c
+	 *   call   +0              # 5d5: R_X86_64_PLT32   __alt_reloc_selftest-0x4
+	 *
+	 * Getting this wrong will either crash and burn or tickle the WARN
+	 * above.
+	 */
+	asm_inline volatile (
+		ALTERNATIVE("", "lea %[mem], %%" _ASM_ARG1 "; call __alt_reloc_selftest;", X86_FEATURE_ALWAYS)
+		: /* output */
+		: [mem] "m" (__alt_reloc_selftest_addr)
+		: _ASM_ARG1
+	);
+}
+
 void __init alternative_instructions(void)
 {
 	int3_selftest();
@@ -1431,6 +1512,8 @@ void __init alternative_instructions(void)
 
 	restart_nmi();
 	alternatives_patched = 1;
+
+	alt_reloc_selftest();
 }
 
 /**

tools/objtool/arch/x86/special.c

@@ -42,13 +42,7 @@ bool arch_support_alt_relocation(struct special_alt *special_alt,
 				 struct instruction *insn,
 				 struct reloc *reloc)
 {
-	/*
-	 * The x86 alternatives code adjusts the offsets only when it
-	 * encounters a branch instruction at the very beginning of the
-	 * replacement group.
-	 */
-	return insn->offset == special_alt->new_off &&
-	       (insn->type == INSN_CALL || is_jump(insn));
+	return true;
 }
 
 /*