Re: [PATCH v4 12/16] x86/kprobes: Fix ordering

[Masami Hiramatsu]

On Fri, 18 Oct 2019 09:35:37 +0200
Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> Cc: hpa@xxxxxxxxx
> Cc: paulmck@xxxxxxxxxx
> Cc: mathieu.desnoyers@xxxxxxxxxxxx
> ---
>  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@xxxxxxxxxx>


Thank you,

-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>