linux-kernel - RE: [PATCH 1/3] x86/perf/hw_breakpoint: Disallow kernel breakpoints unless kprobe-safe

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Date:	Tue, 4 Aug 2015 23:29:14 +0000
From:	平松雅巳 / HIRAMATU，MASAMI 
	<masami.hiramatsu.pt@...achi.com>
To:	"'Frederic Weisbecker'" <fweisbec@...il.com>,
	Andy Lutomirski <luto@...nel.org>
CC:	Peter Zijlstra <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Brian Gerst <brgerst@...il.com>,
	"Steven Rostedt" <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	X86 ML <x86@...nel.org>
Subject: RE: [PATCH 1/3] x86/perf/hw_breakpoint: Disallow kernel breakpoints
 unless kprobe-safe

Hi,

> From: Frederic Weisbecker [mailto:fweisbec@...il.com]
> 
> On Thu, Jul 30, 2015 at 08:32:40PM -0700, Andy Lutomirski wrote:
> > Code on the kprobe blacklist doesn't want unexpected int3
> > exceptions.  It probably doesn't want unexpected debug exceptions
> > either.  Be safe: disallow breakpoints in nokprobes code.
> >
> > On non-CONFIG_KPROBES kernels, there is no kprobe blacklist.  In
> > that case, disallow kernel breakpoints entirely.
> >
> > It will be particularly important to keep hw breakpoints out of the
> > entry and NMI code once we move debug exceptions off the IST stack.
> >
> > Signed-off-by: Andy Lutomirski <luto@...nel.org>
> > ---
> >  arch/x86/kernel/hw_breakpoint.c | 15 +++++++++++++++
> >  include/linux/kprobes.h         |  2 ++
> >  kernel/kprobes.c                |  2 +-
> >  3 files changed, 18 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
> > index 7114ba220fd4..78f3e90c5659 100644
> > --- a/arch/x86/kernel/hw_breakpoint.c
> > +++ b/arch/x86/kernel/hw_breakpoint.c
> > @@ -32,6 +32,7 @@
> >  #include <linux/irqflags.h>
> >  #include <linux/notifier.h>
> >  #include <linux/kallsyms.h>
> > +#include <linux/kprobes.h>
> >  #include <linux/percpu.h>
> >  #include <linux/kdebug.h>
> >  #include <linux/kernel.h>
> > @@ -243,6 +244,20 @@ static int arch_build_bp_info(struct perf_event *bp)
> >  		info->type = X86_BREAKPOINT_RW;
> >  		break;
> >  	case HW_BREAKPOINT_X:
> > +		/*
> > +		 * We don't allow kernel breakpoints in places that are not
> > +		 * acceptable for kprobes.  On non-kprobes kernels, we don't
> > +		 * allow kernel breakpoints at all.
> > +		 */
> > +		if (bp->attr.bp_addr >= TASK_SIZE_MAX) {
> > +#ifdef CONFIG_KPROBES
> > +			if (within_kprobe_blacklist(bp->attr.bp_addr))
> > +				return -EINVAL;
> > +#else
> > +			return -EINVAL;
> > +#endif
> > +		}
> > +
> 
> It should be done on generic code I think. In validate_hw_breakpoint()
> under the arch_check_bp_in_kernelspace() check.

Agreed, kprobes also does it in generic code.

> 
> >  		info->type = X86_BREAKPOINT_EXECUTE;
> >  		/*
> >  		 * x86 inst breakpoints need to have a specific undefined len.
> > diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
> > index 1ab54754a86d..8f6849084248 100644
> > --- a/include/linux/kprobes.h
> > +++ b/include/linux/kprobes.h
> > @@ -267,6 +267,8 @@ extern void show_registers(struct pt_regs *regs);
> >  extern void kprobes_inc_nmissed_count(struct kprobe *p);
> >  extern bool arch_within_kprobe_blacklist(unsigned long addr);
> >
> > +extern bool within_kprobe_blacklist(unsigned long addr);
> 
> The name was fine for a kprobe's private function. But if you make
> it public, maybe standardize the prefix like kprobes_within_blacklist().

No, there is the "kprobe_blacklist", that function means 
"whether the address is within kprobe_blacklist or not?" like within_module_core.

Thank you,

> 

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