Re: [PATCH -tip v4 10/12] x86/kprobes: Push a fake return address at kretprobe_trampoline

[Masami Hiramatsu]

On Wed, 24 Mar 2021 10:40:58 +0900
Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:

> On Tue, 23 Mar 2021 23:30:07 +0100
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> > On Mon, Mar 22, 2021 at 03:41:40PM +0900, Masami Hiramatsu wrote:
> > >  	".global kretprobe_trampoline\n"
> > >  	".type kretprobe_trampoline, @function\n"
> > >  	"kretprobe_trampoline:\n"
> > >  #ifdef CONFIG_X86_64
> > 
> > So what happens if we get an NMI here? That is, after the RET but before
> > the push? Then our IP points into the trampoline but we've not done that
> > push yet.
> 
> Not only NMI, but also interrupts can happen. There is no cli/sti here.
> 
> Anyway, thanks for pointing!
> I think in UNWIND_HINT_TYPE_REGS and UNWIND_HINT_TYPE_REGS_PARTIAL cases
> ORC unwinder also has to check the state->ip and if it is kretprobe_trampoline,
> it should be recovered.
> What about this?

Hmm, this seems to intoduce another issue on stacktrace from kprobes.

           <...>-137     [003] d.Z.    17.250714: p_full_proxy_read_5: (full_proxy_read+0x5/0x80)
           <...>-137     [003] d.Z.    17.250737: <stack trace>
 => kprobe_trace_func+0x1d0/0x2c0
 => kprobe_dispatcher+0x39/0x60
 => aggr_pre_handler+0x4f/0x90
 => kprobe_int3_handler+0x152/0x1a0
 => exc_int3+0x47/0x140
 => asm_exc_int3+0x31/0x40
 => 0
 => 0
 => 0
 => 0
 => 0
 => 0
 => 0

Let me check...

Thanks,

> 
> diff --git a/arch/x86/include/asm/unwind.h b/arch/x86/include/asm/unwind.h
> index 332aa6174b10..36d3971c0a2c 100644
> --- a/arch/x86/include/asm/unwind.h
> +++ b/arch/x86/include/asm/unwind.h
> @@ -101,6 +101,15 @@ void unwind_module_init(struct module *mod, void *orc_ip, size_t orc_ip_size,
>  			void *orc, size_t orc_size) {}
>  #endif
>  
> +static inline
> +unsigned long unwind_recover_kretprobe(struct unwind_state *state,
> +				       unsigned long addr, unsigned long *addr_p)
> +{
> +	return is_kretprobe_trampoline(addr) ?
> +		kretprobe_find_ret_addr(state->task, addr_p, &state->kr_cur) :
> +		addr;
> +}
> +
>  /* Recover the return address modified by instrumentation (e.g. kretprobe) */
>  static inline
>  unsigned long unwind_recover_ret_addr(struct unwind_state *state,
> @@ -110,10 +119,7 @@ unsigned long unwind_recover_ret_addr(struct unwind_state *state,
>  
>  	ret = ftrace_graph_ret_addr(state->task, &state->graph_idx,
>  				    addr, addr_p);
> -	if (is_kretprobe_trampoline(ret))
> -		ret = kretprobe_find_ret_addr(state->task, addr_p,
> -					      &state->kr_cur);
> -	return ret;
> +	return unwind_recover_kretprobe(state, ret, addr_p);
>  }
>  
>  /*
> diff --git a/arch/x86/kernel/unwind_orc.c b/arch/x86/kernel/unwind_orc.c
> index 839a0698342a..cb59aeca6a4a 100644
> --- a/arch/x86/kernel/unwind_orc.c
> +++ b/arch/x86/kernel/unwind_orc.c
> @@ -549,7 +549,15 @@ bool unwind_next_frame(struct unwind_state *state)
>  					 (void *)orig_ip);
>  			goto err;
>  		}
> -
> +		/*
> +		 * There is a small chance to interrupt at the entry of
> +		 * kretprobe_trampoline where the ORC info doesn't exist.
> +		 * That point is right after the RET to kretprobe_trampoline
> +		 * which was modified return address. So the @addr_p must
> +		 * be right before the regs->sp.
> +		 */
> +		state->ip = unwind_recover_kretprobe(state, state->ip,
> +					state->sp - sizeof(unsigned long));
>  		state->regs = (struct pt_regs *)sp;
>  		state->prev_regs = NULL;
>  		state->full_regs = true;
> @@ -562,6 +570,9 @@ bool unwind_next_frame(struct unwind_state *state)
>  					 (void *)orig_ip);
>  			goto err;
>  		}
> +		/* See UNWIND_HINT_TYPE_REGS case comment. */
> +		state->ip = unwind_recover_kretprobe(state, state->ip,
> +					state->sp - sizeof(unsigned long));
>  
>  		if (state->full_regs)
>  			state->prev_regs = state->regs;
> 
> 
> -- 
> Masami Hiramatsu <mhiramat@xxxxxxxxxx>


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>