Re: [PATCH v2 4/4] arm64: kprobes: Use arch_populate_kprobe_blacklist()

[Masami Hiramatsu <mhiramat@kernel.org>]

On Mon, 21 Jan 2019 12:20:07 +0000
James Morse <james.morse@arm.com> wrote:

> Hello,
> 
> On 15/01/2019 06:25, Masami Hiramatsu wrote:
> > Use arch_populate_kprobe_blacklist() instead of
> > arch_within_kprobe_blacklist() so that we can see the full
> > blacklisted symbols under the debugfs.
> > diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
> > index b9e9758b6534..6c066c34c8a4 100644
> > --- a/arch/arm64/kernel/probes/kprobes.c
> > +++ b/arch/arm64/kernel/probes/kprobes.c
> > @@ -465,26 +465,30 @@ kprobe_breakpoint_handler(struct pt_regs *regs, unsigned int esr)
> >  	return DBG_HOOK_HANDLED;
> >  }
> >  
> > -bool arch_within_kprobe_blacklist(unsigned long addr)
> > +int __init arch_populate_kprobe_blacklist(void)
> >  {
> > -	if ((addr >= (unsigned long)__kprobes_text_start &&
> > -	    addr < (unsigned long)__kprobes_text_end) ||
> > -	    (addr >= (unsigned long)__entry_text_start &&
> > -	    addr < (unsigned long)__entry_text_end) ||
> > -	    (addr >= (unsigned long)__idmap_text_start &&
> > -	    addr < (unsigned long)__idmap_text_end) ||
> 
> > -	    in_exception_text(addr))
> 
> You added this one in the previous patch, but it disappears here.

Yes, it is easy to explain how we transcribe from 
arch_within_kprobe_blacklist() to arch_populate_kprobe_blacklist().

> 
> 
> > -		return true;
> > -
> > -	if (!is_kernel_in_hyp_mode()) {
> > -		if ((addr >= (unsigned long)__hyp_text_start &&
> > -		    addr < (unsigned long)__hyp_text_end) ||
> > -		    (addr >= (unsigned long)__hyp_idmap_text_start &&
> > -		    addr < (unsigned long)__hyp_idmap_text_end))
> > -			return true;
> > -	}
> > -
> > -	return false;
> > +	int ret;
> 
> 
> > +	ret = kprobe_add_area_blacklist((unsigned long)__kprobes_text_start,
> > +					(unsigned long)__kprobes_text_end);
> > +	if (ret)
> > +		return ret;
> 
> Now that we have arch_populate_kprobe_blacklist(), does the arch-code need to
> blacklist the kprobes section itself?

Ah, good catch! No, we don't need it here. Sorry I worked on older patch.
I'll update it.

> The weak arch_within_kprobe_blacklist() will test it at kprobe-load time, and
> populate_kprobe_blacklist() adds it to the list before it calls
> arch_populate_kprobe_blacklist().
> 
> Won't this result in duplicate entries?

yes, so it should not.

> 
> 
> > +	ret = kprobe_add_area_blacklist((unsigned long)__entry_text_start,
> > +					(unsigned long)__entry_text_end);
> > +	if (ret)
> > +		return ret;
> 
> > +	ret = kprobe_add_area_blacklist((unsigned long)__idmap_text_start,
> > +					(unsigned long)__idmap_text_end);
> 
> > +	if (ret || is_kernel_in_hyp_mode())
> > +		return ret;
> 
> 
> Hmmm, I think we have a bug here today.

OK.

> 
> This is saying we can kprobe KVM when we have VHE, because all of KVMs code runs
> at the same exception-level as the kernel. Which is true...
> But KVM switches VBAR_EL1, so if we run over one of kprobes BRK instructions,
> we're going to hyp-panic, because KVM doesn't handle synchronous exceptions from
> EL2.
> 
> The __hyp_text also contains the guest entry/exit code, which we mustn't probe,
> even on VHE.

Hmm, I'm not sure when the original code decided this. But it sounds reasonable.

> 
> I think we should always blacklist the __hyp_text, and KVM should mark its
> vhe-only functions with __kprobes. I'll post patches for this.

OK, then I should wait for that, because this series is a kind of improvement.
But your's is a bugfix, that should be backported to stable.

Thank you,

> 
> 
> Thanks,
> 
> James
> 
> 
> > +	ret = kprobe_add_area_blacklist((unsigned long)__hyp_text_start,
> > +					(unsigned long)__hyp_text_end);
> > +	if (ret)
> > +		return ret;
> > +	ret = kprobe_add_area_blacklist((unsigned long)__hyp_idmap_text_start,
> > +					(unsigned long)__hyp_idmap_text_end);
> > +	return ret;
> >  }
> >  
> >  void __kprobes __used *trampoline_probe_handler(struct pt_regs *regs)
> > 
> 


-- 
Masami Hiramatsu <mhiramat@kernel.org>