From: Peter Zijlstra <peterz@infradead.org>

Date: Mon, 23 Jan 2023 21:59:17 +0100

Subject: [PATCH] x86/alternatives: Teach text_poke_bp() to patch Jcc.d32

 instructions

References: bsc#1012628

Patch-mainline: 6.2.1

Git-commit: ac0ee0a9560c97fa5fe1409e450c2425d4ebd17a

commit ac0ee0a9560c97fa5fe1409e450c2425d4ebd17a upstream.

In order to re-write Jcc.d32 instructions text_poke_bp() needs to be

taught about them.

The biggest hurdle is that the whole machinery is currently made for 5

byte instructions and extending this would grow struct text_poke_loc

which is currently a nice 16 bytes and used in an array.

However, since text_poke_loc contains a full copy of the (s32)

displacement, it is possible to map the Jcc.d32 2 byte opcodes to

Jcc.d8 1 byte opcode for the int3 emulation.

This then leaves the replacement bytes; fudge that by only storing the

last 5 bytes and adding the rule that 'length == 6' instruction will

be prefixed with a 0x0f byte.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Signed-off-by: Ingo Molnar <mingo@kernel.org>

Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Link: https://lore.kernel.org/r/20230123210607.115718513@infradead.org

Cc: Nathan Chancellor <nathan@kernel.org>

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Signed-off-by: Jiri Slaby <jslaby@suse.cz>

---

 arch/x86/kernel/alternative.c | 62 ++++++++++++++++++++++++++---------

 1 file changed, 47 insertions(+), 15 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c

index 7d8c3cbd..81381a01 100644

--- a/arch/x86/kernel/alternative.c

+++ b/arch/x86/kernel/alternative.c

@@ -340,6 +340,12 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,

 	}

 }

+static inline bool is_jcc32(struct insn *insn)

+{

+	/* Jcc.d32 second opcode byte is in the range: 0x80-0x8f */

+	return insn->opcode.bytes[0] == 0x0f && (insn->opcode.bytes[1] & 0xf0) == 0x80;

+}

+

 #if defined(CONFIG_RETPOLINE) && defined(CONFIG_OBJTOOL)

 /*

@@ -378,12 +384,6 @@ static int emit_indirect(int op, int reg, u8 *bytes)

 	return i;

 }

-static inline bool is_jcc32(struct insn *insn)

-{

-	/* Jcc.d32 second opcode byte is in the range: 0x80-0x8f */

-	return insn->opcode.bytes[0] == 0x0f && (insn->opcode.bytes[1] & 0xf0) == 0x80;

-}

-

 static int emit_call_track_retpoline(void *addr, struct insn *insn, int reg, u8 *bytes)

 {

 	u8 op = insn->opcode.bytes[0];

@@ -1772,6 +1772,11 @@ void text_poke_sync(void)

 	on_each_cpu(do_sync_core, NULL, 1);

 }

+/*

+ * NOTE: crazy scheme to allow patching Jcc.d32 but not increase the size of

+ * this thing. When len == 6 everything is prefixed with 0x0f and we map

+ * opcode to Jcc.d8, using len to distinguish.

+ */

 struct text_poke_loc {

 	/* addr := _stext + rel_addr */

 	s32 rel_addr;

@@ -1893,6 +1898,10 @@ noinstr int poke_int3_handler(struct pt_regs *regs)

 		int3_emulate_jmp(regs, (long)ip + tp->disp);

 		break;

+	case 0x70 ... 0x7f: /* Jcc */

+		int3_emulate_jcc(regs, tp->opcode & 0xf, (long)ip, tp->disp);

+		break;

+

 	default:

 		BUG();

 	}

@@ -1966,16 +1975,26 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries

 	 * Second step: update all but the first byte of the patched range.

 	 */

 	for (do_sync = 0, i = 0; i < nr_entries; i++) {

-		u8 old[POKE_MAX_OPCODE_SIZE] = { tp[i].old, };

+		u8 old[POKE_MAX_OPCODE_SIZE+1] = { tp[i].old, };

+		u8 _new[POKE_MAX_OPCODE_SIZE+1];

+		const u8 *new = tp[i].text;

 		int len = tp[i].len;

 		if (len - INT3_INSN_SIZE > 0) {

 			memcpy(old + INT3_INSN_SIZE,

 			       text_poke_addr(&tp[i]) + INT3_INSN_SIZE,

 			       len - INT3_INSN_SIZE);

+

+			if (len == 6) {

+				_new[0] = 0x0f;

+				memcpy(_new + 1, new, 5);

+				new = _new;

+			}

+

 			text_poke(text_poke_addr(&tp[i]) + INT3_INSN_SIZE,

-				  (const char *)tp[i].text + INT3_INSN_SIZE,

+				  new + INT3_INSN_SIZE,

 				  len - INT3_INSN_SIZE);

+

 			do_sync++;

 		}

@@ -2003,8 +2022,7 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries

 		 * The old instruction is recorded so that the event can be

 		 * processed forwards or backwards.

 		 */

-		perf_event_text_poke(text_poke_addr(&tp[i]), old, len,

-				     tp[i].text, len);

+		perf_event_text_poke(text_poke_addr(&tp[i]), old, len, new, len);

 	}

 	if (do_sync) {

@@ -2021,10 +2039,15 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries

 	 * replacing opcode.

 	 */

 	for (do_sync = 0, i = 0; i < nr_entries; i++) {

-		if (tp[i].text[0] == INT3_INSN_OPCODE)

+		u8 byte = tp[i].text[0];

+

+		if (tp[i].len == 6)

+			byte = 0x0f;

+

+		if (byte == INT3_INSN_OPCODE)

 			continue;

-		text_poke(text_poke_addr(&tp[i]), tp[i].text, INT3_INSN_SIZE);

+		text_poke(text_poke_addr(&tp[i]), &byte, INT3_INSN_SIZE);

 		do_sync++;

 	}

@@ -2042,9 +2065,11 @@ static void text_poke_loc_init(struct text_poke_loc *tp, void *addr,

 			       const void *opcode, size_t len, const void *emulate)

 {

 	struct insn insn;

-	int ret, i;

+	int ret, i = 0;

-	memcpy((void *)tp->text, opcode, len);

+	if (len == 6)

+		i = 1;

+	memcpy((void *)tp->text, opcode+i, len-i);

 	if (!emulate)

 		emulate = opcode;

@@ -2055,6 +2080,13 @@ static void text_poke_loc_init(struct text_poke_loc *tp, void *addr,

 	tp->len = len;

 	tp->opcode = insn.opcode.bytes[0];

+	if (is_jcc32(&insn)) {

+		/*

+		 * Map Jcc.d32 onto Jcc.d8 and use len to distinguish.

+		 */

+		tp->opcode = insn.opcode.bytes[1] - 0x10;

+	}

+

 	switch (tp->opcode) {

 	case RET_INSN_OPCODE:

 	case JMP32_INSN_OPCODE:

@@ -2071,7 +2103,6 @@ static void text_poke_loc_init(struct text_poke_loc *tp, void *addr,

 		BUG_ON(len != insn.length);

 	}

-

 	switch (tp->opcode) {

 	case INT3_INSN_OPCODE:

 	case RET_INSN_OPCODE:

@@ -2080,6 +2111,7 @@ static void text_poke_loc_init(struct text_poke_loc *tp, void *addr,

 	case CALL_INSN_OPCODE:

 	case JMP32_INSN_OPCODE:

 	case JMP8_INSN_OPCODE:

+	case 0x70 ... 0x7f: /* Jcc */

 		tp->disp = insn.immediate.value;

 		break;

-- 

2.35.3