Re: [PATCH v3 1/6] x86/alternatives: Teach text_poke_bp() to emulate instructions

From: Borislav Petkov <bp@alien8.de>
To: Peter Zijlstra <peterz@infradead.org>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org,
	rostedt@goodmis.org, mhiramat@kernel.org, bristot@redhat.com,
	jbaron@akamai.com, torvalds@linux-foundation.org,
	tglx@linutronix.de, mingo@kernel.org, namit@vmware.com,
	hpa@zytor.com, luto@kernel.org, ard.biesheuvel@linaro.org,
	jpoimboe@redhat.com
Subject: Re: [PATCH v3 1/6] x86/alternatives: Teach text_poke_bp() to emulate instructions
Date: Tue, 8 Oct 2019 16:29:24 +0200	[thread overview]
Message-ID: <20191008142924.GE14765@zn.tnic> (raw)
In-Reply-To: <20191007081944.88332264.2@infradead.org>

On Mon, Oct 07, 2019 at 10:17:17AM +0200, Peter Zijlstra wrote:
> In preparation for static_call and variable size jump_label support,
> teach text_poke_bp() to emulate instructions, namely:
> 
>   JMP32, JMP8, CALL, NOP2, NOP_ATOMIC5, INT3
> 
> The current text_poke_bp() takes a @handler argument which is used as
> a jump target when the temporary INT3 is hit by a different CPU.
> 
> When patching CALL instructions, this doesn't work because we'd miss
> the PUSH of the return address. Instead, teach poke_int3_handler() to
> emulate an instruction, typically the instruction we're patching in.
> 
> This fits almost all text_poke_bp() users, except
> arch_unoptimize_kprobe() which restores random text, and for that site
> we have to build an explicit emulate instruction.

...

> @@ -63,8 +66,17 @@ static inline void int3_emulate_jmp(stru
>  	regs->ip = ip;
>  }
>  
> -#define INT3_INSN_SIZE 1
> -#define CALL_INSN_SIZE 5
> +#define INT3_INSN_SIZE		1
> +#define INT3_INSN_OPCODE	0xCC
> +
> +#define CALL_INSN_SIZE		5
> +#define CALL_INSN_OPCODE	0xE8
> +
> +#define JMP32_INSN_SIZE		5
> +#define JMP32_INSN_OPCODE	0xE9
> +
> +#define JMP8_INSN_SIZE		2
> +#define JMP8_INSN_OPCODE	0xEB

You probably should switch those to have the name prefix come first and
make them even shorter:

OPCODE_CALL
INSN_SIZE_CALL
OPCODE_JMP32
INSN_SIZE_JMP32
OPCODE_JMP8
...

This way you have the opcodes prefixed with OPCODE_ and the insn sizes
with INSN_SIZE_. I.e., what they actually are.

> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c

...

> @@ -1027,9 +1046,9 @@ NOKPROBE_SYMBOL(poke_int3_handler);
>   */
>  void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries)
>  {
> -	int patched_all_but_first = 0;
> -	unsigned char int3 = 0xcc;
> +	unsigned char int3 = INT3_INSN_OPCODE;
>  	unsigned int i;
> +	int do_sync;
>  
>  	lockdep_assert_held(&text_mutex);
>  
> @@ -1053,16 +1072,16 @@ void text_poke_bp_batch(struct text_poke
>  	/*
>  	 * Second step: update all but the first byte of the patched range.
>  	 */
> -	for (i = 0; i < nr_entries; i++) {
> +	for (do_sync = 0, i = 0; i < nr_entries; i++) {
>  		if (tp[i].len - sizeof(int3) > 0) {
>  			text_poke((char *)tp[i].addr + sizeof(int3),
> -				  (const char *)tp[i].opcode + sizeof(int3),
> +				  (const char *)tp[i].text + sizeof(int3),
>  				  tp[i].len - sizeof(int3));
> -			patched_all_but_first++;
> +			do_sync++;
>  		}
>  	}
>  
> -	if (patched_all_but_first) {
> +	if (do_sync) {
>  		/*
>  		 * According to Intel, this core syncing is very likely
>  		 * not necessary and we'd be safe even without it. But
> @@ -1075,10 +1094,17 @@ void text_poke_bp_batch(struct text_poke
>  	 * Third step: replace the first byte (int3) by the first byte of
>  	 * replacing opcode.
>  	 */
> -	for (i = 0; i < nr_entries; i++)
> -		text_poke(tp[i].addr, tp[i].opcode, sizeof(int3));
> +	for (do_sync = 0, i = 0; i < nr_entries; i++) {

Can we have the do_sync reset outside of the loop?

> +		if (tp[i].text[0] == INT3_INSN_OPCODE)
> +			continue;

I'm guessing we preset the 0th byte to 0xcc somewhere.... I just can't
seem to find it...

-- 
Regards/Gruss,
    Boris.

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