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From: Masami Hiramatsu <mhiramat@kernel.org>
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, jeyu@kernel.org, paulmck@kernel.org,
	mathieu.desnoyers@efficios.com
Subject: Re: [PATCH v4 12/16] x86/kprobes: Fix ordering
Date: Tue, 22 Oct 2019 10:35:21 +0900	[thread overview]
Message-ID: <20191022103521.3015bc5e128cd68fa645013c@kernel.org> (raw)
In-Reply-To: <20191018074634.629386219@infradead.org>

On Fri, 18 Oct 2019 09:35:37 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> Kprobes does something like:
> 
> register:
> 	arch_arm_kprobe()
> 	  text_poke(INT3)
>           /* guarantees nothing, INT3 will become visible at some point, maybe */
> 
>         kprobe_optimizer()
> 	  /* guarantees the bytes after INT3 are unused */
> 	  syncrhonize_rcu_tasks();
> 	  text_poke_bp(JMP32);
> 	  /* implies IPI-sync, kprobe really is enabled */
> 
> 
> unregister:
> 	__disarm_kprobe()
> 	  unoptimize_kprobe()
> 	    text_poke_bp(INT3 + tail);
> 	    /* implies IPI-sync, so tail is guaranteed visible */
>           arch_disarm_kprobe()
>             text_poke(old);
> 	    /* guarantees nothing, old will maybe become visible */
> 
> 	synchronize_rcu()
> 
>         free-stuff

Note that this is only for the case of optimized kprobe.
(On some probe points we can not optimize it)

> 
> Now the problem is that on register, the synchronize_rcu_tasks() does
> not imply sufficient to guarantee all CPUs have already observed INT3
> (although in practise this is exceedingly unlikely not to have
> happened) (similar to how MEMBARRIER_CMD_PRIVATE_EXPEDITED does not
> imply MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE).

OK, so the sync_core() after int3 is needed to guarantee the probe
is enabled on each core.

> 
> Worse, even if it did, we'd have to do 2 synchronize calls to provide
> the guarantee we're looking for, the first to ensure INT3 is visible,
> the second to guarantee nobody is then still using the instruction
> bytes after INT3.

I think this 2nd guarantee is done by syncrhonize_rcu() if we
put sync_core() after int3. syncrhonize_rcu() ensures that
all cores once scheduled and all interrupts have done.

> 
> Similar on unregister; the synchronize_rcu() between
> __unregister_kprobe_top() and __unregister_kprobe_bottom() does not
> guarantee all CPUs are free of the INT3 (and observe the old text).

I agree with putting sync_core() after putting/removing INT3.

> 
> Therefore, sprinkle some IPI-sync love around. This guarantees that
> all CPUs agree on the text and RCU once again provides the required
> guaranteed.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Cc: hpa@zytor.com
> Cc: paulmck@kernel.org
> Cc: mathieu.desnoyers@efficios.com
> ---
>  arch/x86/include/asm/text-patching.h |    1 +
>  arch/x86/kernel/alternative.c        |   11 ++++++++---
>  arch/x86/kernel/kprobes/core.c       |    2 ++
>  arch/x86/kernel/kprobes/opt.c        |   12 ++++--------
>  4 files changed, 15 insertions(+), 11 deletions(-)
> 
> --- a/arch/x86/include/asm/text-patching.h
> +++ b/arch/x86/include/asm/text-patching.h
> @@ -42,6 +42,7 @@ extern void text_poke_early(void *addr,
>   * an inconsistent instruction while you patch.
>   */
>  extern void *text_poke(void *addr, const void *opcode, size_t len);
> +extern void text_poke_sync(void);
>  extern void *text_poke_kgdb(void *addr, const void *opcode, size_t len);
>  extern int poke_int3_handler(struct pt_regs *regs);
>  extern void text_poke_bp(void *addr, const void *opcode, size_t len, const void *emulate);
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -936,6 +936,11 @@ static void do_sync_core(void *info)
>  	sync_core();
>  }
>  
> +void text_poke_sync(void)
> +{
> +	on_each_cpu(do_sync_core, NULL, 1);
> +}
> +
>  struct text_poke_loc {
>  	s32 rel_addr; /* addr := _stext + rel_addr */
>  	s32 rel32;
> @@ -1085,7 +1090,7 @@ static void text_poke_bp_batch(struct te
>  	for (i = 0; i < nr_entries; i++)
>  		text_poke(text_poke_addr(&tp[i]), &int3, sizeof(int3));
>  
> -	on_each_cpu(do_sync_core, NULL, 1);
> +	text_poke_sync();
>  
>  	/*
>  	 * Second step: update all but the first byte of the patched range.
> @@ -1107,7 +1112,7 @@ static void text_poke_bp_batch(struct te
>  		 * not necessary and we'd be safe even without it. But
>  		 * better safe than sorry (plus there's not only Intel).
>  		 */
> -		on_each_cpu(do_sync_core, NULL, 1);
> +		text_poke_sync();
>  	}
>  
>  	/*
> @@ -1123,7 +1128,7 @@ static void text_poke_bp_batch(struct te
>  	}
>  
>  	if (do_sync)
> -		on_each_cpu(do_sync_core, NULL, 1);
> +		text_poke_sync();
>  
>  	/*
>  	 * sync_core() implies an smp_mb() and orders this store against
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -502,11 +502,13 @@ int arch_prepare_kprobe(struct kprobe *p
>  void arch_arm_kprobe(struct kprobe *p)
>  {
>  	text_poke(p->addr, ((unsigned char []){INT3_INSN_OPCODE}), 1);
> +	text_poke_sync();
>  }
>  
>  void arch_disarm_kprobe(struct kprobe *p)
>  {
>  	text_poke(p->addr, &p->opcode, 1);
> +	text_poke_sync();
>  }

This looks good to me.

>  
>  void arch_remove_kprobe(struct kprobe *p)
> --- a/arch/x86/kernel/kprobes/opt.c
> +++ b/arch/x86/kernel/kprobes/opt.c
> @@ -444,14 +444,10 @@ void arch_optimize_kprobes(struct list_h
>  /* Replace a relative jump with a breakpoint (int3).  */
>  void arch_unoptimize_kprobe(struct optimized_kprobe *op)
>  {
> -	u8 insn_buff[JMP32_INSN_SIZE];
> -
> -	/* Set int3 to first byte for kprobes */
> -	insn_buff[0] = INT3_INSN_OPCODE;
> -	memcpy(insn_buff + 1, op->optinsn.copied_insn, DISP32_SIZE);
> -
> -	text_poke_bp(op->kp.addr, insn_buff, JMP32_INSN_SIZE,
> -		     text_gen_insn(JMP32_INSN_OPCODE, op->kp.addr, op->optinsn.insn));
> +	arch_arm_kprobe(&op->kp);
> +	text_poke(op->kp.addr + INT3_INSN_SIZE,
> +		  op->optinsn.copied_insn, DISP32_SIZE);
> +	text_poke_sync();
>  }

For this part, I thought it was same as what text_poke_bp() does.
But, indeed, this looks better (simpler & lighter) than using
text_poke_bp()...

So, in total, this looks good to me.

Acked-by: Masami Hiramatsu <mhiramat@kernel.org>


Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>

  reply	other threads:[~2019-10-22  1:35 UTC|newest]

Thread overview: 70+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-18  7:35 [PATCH v4 00/16] Rewrite x86/ftrace to use text_poke (and more) Peter Zijlstra
2019-10-18  7:35 ` [PATCH v4 01/16] x86/alternatives: Teach text_poke_bp() to emulate instructions Peter Zijlstra
2019-10-18  7:35 ` [PATCH v4 02/16] x86/alternatives: Update int3_emulate_push() comment Peter Zijlstra
2019-10-18  7:35 ` [PATCH v4 03/16] x86/alternatives,jump_label: Provide better text_poke() batching interface Peter Zijlstra
2019-10-21  8:48   ` Ingo Molnar
2019-10-21  9:21     ` Peter Zijlstra
2019-10-18  7:35 ` [PATCH v4 04/16] x86/alternatives: Add and use text_gen_insn() helper Peter Zijlstra
2019-10-18  7:35 ` [PATCH v4 05/16] x86/ftrace: Use text_poke() Peter Zijlstra
2019-10-18  7:35 ` [PATCH v4 06/16] x86/mm: Remove set_kernel_text_r[ow]() Peter Zijlstra
2019-10-18  7:35 ` [PATCH v4 07/16] x86/alternative: Add text_opcode_size() Peter Zijlstra
2019-10-18  7:35 ` [PATCH v4 08/16] x86/ftrace: Use text_gen_insn() Peter Zijlstra
2019-10-18  7:35 ` [PATCH v4 09/16] x86/alternative: Remove text_poke_loc::len Peter Zijlstra
2019-10-21  8:58   ` Ingo Molnar
2019-10-21  9:02     ` Ingo Molnar
2019-10-18  7:35 ` [PATCH v4 10/16] x86/alternative: Shrink text_poke_loc Peter Zijlstra
2019-10-21  9:01   ` Ingo Molnar
2019-10-21  9:25     ` Peter Zijlstra
2019-10-21  9:33       ` Ingo Molnar
2019-10-18  7:35 ` [PATCH v4 11/16] x86/kprobes: Convert to text-patching.h Peter Zijlstra
2019-10-21 14:57   ` Masami Hiramatsu
2019-10-18  7:35 ` [PATCH v4 12/16] x86/kprobes: Fix ordering Peter Zijlstra
2019-10-22  1:35   ` Masami Hiramatsu [this message]
2019-10-22 10:31     ` Peter Zijlstra
2019-10-18  7:35 ` [PATCH v4 13/16] arm/ftrace: Use __patch_text_real() Peter Zijlstra
2019-10-28 16:25   ` Will Deacon
2019-10-28 16:34     ` Peter Zijlstra
2019-10-28 16:35       ` Peter Zijlstra
2019-10-28 16:47       ` Will Deacon
2019-10-28 16:55         ` Peter Zijlstra
2019-10-18  7:35 ` [PATCH v4 14/16] module: Remove set_all_modules_text_*() Peter Zijlstra
2019-10-18  7:35 ` [PATCH v4 15/16] module: Move where we mark modules RO,X Peter Zijlstra
2019-10-21 13:53   ` Josh Poimboeuf
2019-10-21 14:14     ` Peter Zijlstra
2019-10-21 15:34       ` Peter Zijlstra
2019-10-21 15:44         ` Peter Zijlstra
2019-10-21 16:11         ` Peter Zijlstra
2019-10-22 11:31           ` Heiko Carstens
2019-10-22 12:31             ` Peter Zijlstra
2019-10-23 11:48       ` Peter Zijlstra
2019-10-23 15:16         ` Peter Zijlstra
2019-10-23 17:15           ` Josh Poimboeuf
2019-10-24 10:59             ` Peter Zijlstra
2019-10-24 18:31               ` Josh Poimboeuf
2019-10-24 20:33                 ` Peter Zijlstra
2019-10-23 17:00         ` Josh Poimboeuf
2019-10-24 13:16           ` Peter Zijlstra
2019-10-25  6:44             ` Petr Mladek
2019-10-25  8:43               ` Peter Zijlstra
2019-10-25 10:06                 ` Peter Zijlstra
2019-10-25 13:50                   ` Josh Poimboeuf
2019-10-26  1:17                   ` Josh Poimboeuf
2019-10-28 10:07                     ` Peter Zijlstra
2019-10-28 10:43                     ` Peter Zijlstra
2019-10-25  9:16               ` Peter Zijlstra
2019-10-22  2:21   ` Steven Rostedt
2019-10-22 20:24     ` Peter Zijlstra
2019-10-22 20:40       ` Steven Rostedt
2019-10-23  9:07         ` Peter Zijlstra
2019-10-23 18:52       ` Steven Rostedt
2019-10-24 10:16         ` Peter Zijlstra
2019-10-24 10:18           ` Peter Zijlstra
2019-10-24 15:00           ` Steven Rostedt
2019-10-24 16:43             ` Peter Zijlstra
2019-10-24 18:17               ` Steven Rostedt
2019-10-24 20:24                 ` Peter Zijlstra
2019-10-24 20:28                   ` Steven Rostedt
2019-10-18  7:35 ` [PATCH v4 16/16] ftrace: Merge ftrace_module_{init,enable}() Peter Zijlstra
2019-10-18  8:20   ` Peter Zijlstra
2019-10-21  9:09 ` [PATCH v4 00/16] Rewrite x86/ftrace to use text_poke (and more) Ingo Molnar
2019-10-21 13:38   ` Steven Rostedt

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