Re: [PATCH] x86/kprobes: Prohibit probing on rcu_nmi_exit() and ist_exit()

[Joel Fernandes]

On Fri, Mar 06, 2020 at 07:28:31PM -0800, Paul E. McKenney wrote:
> On Sat, Mar 07, 2020 at 12:02:12PM +0900, Masami Hiramatsu wrote:
> > Prohibit probing on rcu_nmi_exit() and ist_exit() which
> > are called from do_int3()'s kprobe path after kprobe_int3_handler().
> > 
> > The commit c13324a505c7 ("x86/kprobes: Prohibit probing on
> > functions before kprobe_int3_handler()") tried to fix similar
> > issue, but it only marks the functions before kprobe_int3_handler()
> > in do_int3().
> > 
> > If we put a kprobe on rcu_nmi_exit() or ist_exit(), the kprobes
> > will detect reentrance. However, it only skips the kprobe handler,
> > exits from do_int3() and hits ist_exit() and rcu_nmi_exit() again.
> > Thus, it causes another int3 exception and finally we will get
> > the kernel panic with "Unrecoverable kprobe detected." error message.
> > 
> > This is reproducible by the following commands.
> > 
> > / # echo 0 > /proc/sys/debug/kprobes-optimization
> > / # echo p vfs_read > /sys/kernel/debug/tracing/kprobe_events
> > / # echo p rcu_nmi_exit >> /sys/kernel/debug/tracing/kprobe_events
> > / # echo 1 > /sys/kernel/debug/tracing/events/kprobes/enable
> > 
> > Fixes: c13324a505c7 ("x86/kprobes: Prohibit probing on functions before kprobe_int3_handler()")
> > Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> 
> From an RCU perspective:
> 
> Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> 

Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

thanks,

 - Joel

> > ---
> >  arch/x86/kernel/traps.c |    1 +
> >  kernel/rcu/tree.c       |    1 +
> >  2 files changed, 2 insertions(+)
> > 
> > diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> > index 6ef00eb6fbb9..c63fb7697794 100644
> > --- a/arch/x86/kernel/traps.c
> > +++ b/arch/x86/kernel/traps.c
> > @@ -115,6 +115,7 @@ void ist_exit(struct pt_regs *regs)
> >  	if (!user_mode(regs))
> >  		rcu_nmi_exit();
> >  }
> > +NOKPROBE_SYMBOL(ist_exit);
> >  
> >  /**
> >   * ist_begin_non_atomic() - begin a non-atomic section in an IST exception
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index d91c9156fab2..c49ea0e919f9 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -670,6 +670,7 @@ void rcu_nmi_exit(void)
> >  {
> >  	rcu_nmi_exit_common(false);
> >  }
> > +NOKPROBE_SYMBOL(rcu_nmi_exit);
> >  
> >  /**
> >   * rcu_irq_exit - inform RCU that current CPU is exiting irq towards idle
> >