Re: [PATCH 08/15] x86/alternatives: Teach text_poke_bp() to emulate instructions

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jun 11, 2019 at 05:52:48PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 11, 2019 at 11:14:10AM -0400, Steven Rostedt wrote:
> > On Wed, 05 Jun 2019 15:08:01 +0200
> > Peter Zijlstra <peterz@infradead.org> wrote:
> > 
> > > -void text_poke_bp(void *addr, const void *opcode, size_t len, void *handler)
> > > +void text_poke_bp(void *addr, const void *opcode, size_t len, const void *emulate)
> > >  {
> > >  	unsigned char int3 = 0xcc;
> > >  
> > > -	bp_int3_handler = handler;
> > > +	bp_int3_opcode = emulate ?: opcode;
> > >  	bp_int3_addr = (u8 *)addr + sizeof(int3);
> > >  	bp_patching_in_progress = true;
> > >  
> > >  	lockdep_assert_held(&text_mutex);
> > >  
> > >  	/*
> > > +	 * poke_int3_handler() relies on @opcode being a 5 byte instruction;
> > > +	 * notably a JMP, CALL or NOP5_ATOMIC.
> > > +	 */
> > > +	BUG_ON(len != 5);
> > 
> > If we have a bug on here, why bother with passing in len at all? Just
> > force it to be 5.
> 
> Masami said the same.
> 
> > We could make it a WARN_ON() and return without doing anything.
> > 
> > This also prevents us from ever changing two byte jmps.
> 
> It doesn't; that is, we'd need to add emulation for the 3 byte jump, but
> that'd be pretty trivial.

I can't find a 3 byte jump on x86_64, I could only find a 2 byte one.
But something like so should work I suppose, although at this point I'm
thinking we should just used the instruction decode we have instead of
playing iffy games with packed structures.

diff --git a/arch/x86/include/asm/text-patching.h b/arch/x86/include/asm/text-patching.h
index e1a4bb42eb92..abb9615dcb1d 100644
--- a/arch/x86/include/asm/text-patching.h
+++ b/arch/x86/include/asm/text-patching.h
@@ -57,6 +57,9 @@ static inline void int3_emulate_jmp(struct pt_regs *regs, unsigned long ip)
 #define JMP_INSN_SIZE		5
 #define JMP_INSN_OPCODE		0xE9
 
+#define JMP8_INSN_SIZE		2
+#define JMP8_INSN_OPCODE	0xEB
+
 static inline void int3_emulate_push(struct pt_regs *regs, unsigned long val)
 {
 	/*
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 5d0123a8183b..5df6c74a0b08 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -924,13 +924,18 @@ static void do_sync_core(void *info)
 static bool bp_patching_in_progress;
 static const void *bp_int3_opcode, *bp_int3_addr;
 
+struct poke_insn {
+	u8 opcode;
+	union {
+		s8 rel8;
+		s32 rel32;
+	};
+} __packed;
+
 int poke_int3_handler(struct pt_regs *regs)
 {
 	long ip = regs->ip - INT3_INSN_SIZE + CALL_INSN_SIZE;
-	struct opcode {
-		u8 insn;
-		s32 rel;
-	} __packed opcode;
+	struct poke_insn insn;
 
 	/*
 	 * Having observed our INT3 instruction, we now must observe
@@ -950,15 +955,19 @@ int poke_int3_handler(struct pt_regs *regs)
 	if (user_mode(regs) || regs->ip != (unsigned long)bp_int3_addr)
 		return 0;
 
-	opcode = *(struct opcode *)bp_int3_opcode;
+	insn = *(struct poke_insn *)bp_int3_opcode;
 
-	switch (opcode.insn) {
+	switch (insn.opcode) {
 	case CALL_INSN_OPCODE:
-		int3_emulate_call(regs, ip + opcode.rel);
+		int3_emulate_call(regs, ip + insn.rel32);
 		break;
 
 	case JMP_INSN_OPCODE:
-		int3_emulate_jmp(regs, ip + opcode.rel);
+		int3_emulate_jmp(regs, ip + insn.rel32);
+		break;
+
+	case JMP8_INSN_OPCODE:
+		int3_emulate_jmp(regs, ip + insn.rel8);
 		break;
 
 	default: /* assume NOP */
@@ -992,7 +1001,8 @@ NOKPROBE_SYMBOL(poke_int3_handler);
  */
 void text_poke_bp(void *addr, const void *opcode, size_t len, const void *emulate)
 {
-	unsigned char int3 = 0xcc;
+	unsigned char int3 = INT3_INSN_OPCODE;
+	unsigned char opcode;
 
 	bp_int3_opcode = emulate ?: opcode;
 	bp_int3_addr = (u8 *)addr + sizeof(int3);
@@ -1001,10 +1011,26 @@ void text_poke_bp(void *addr, const void *opcode, size_t len, const void *emulat
 	lockdep_assert_held(&text_mutex);
 
 	/*
-	 * poke_int3_handler() relies on @opcode being a 5 byte instruction;
-	 * notably a JMP, CALL or NOP5_ATOMIC.
+	 * Verify we support the actual instruction in poke_int3_handler().
 	 */
-	BUG_ON(len != 5);
+	opcode = *(unsigned char *)bp_int3_opcode;
+	switch (opcode) {
+	case CALL_INSN_OPCODE:
+		BUG_ON(len != CALL_INSN_SIZE);
+		break;
+
+	case JMP_INSN_OPCODE:
+		BUG_ON(len != JMP_INSN_SIZE);
+		break;
+
+	case JMP8_INSN_OPCODE:
+		BUG_ON(len != JMP8_INSN_SIZE);
+		break;
+
+	default: /* assume NOP5_ATOMIC */
+		BUG_ON(len != 5);
+		break;
+	}
 
 	/*
 	 * Corresponding read barrier in int3 notifier for making sure the