Re: [PATCH] Fix BUG: using smp_processor_id() in preemptible code in print_fatal_signal()

[Ingo Molnar <mingo@elte.hu>]

* Ed Swierk <eswierk@aristanetworks.com> wrote:

> With print-fatal-signals=1 on a kernel with CONFIG_PREEMPT=y, sending an
> unexpected signal to a process causes a BUG: using smp_processor_id() in
> preemptible code.
> 
> get_signal_to_deliver() releases the siglock before calling
> print_fatal_signal(), which calls show_regs(), which calls
> smp_processor_id(), which is not supposed to be called from a
> preemptible thread.
> 
> Calling print_fatal_signal() before releasing the siglock avoids this
> problem.
> 
> Signed-off-by: Ed Swierk <eswierk@aristanetworks.com>
> 
> ---
> Index: linux-2.6.27.4/kernel/signal.c
> ===================================================================
> --- linux-2.6.27.4.orig/kernel/signal.c
> +++ linux-2.6.27.4/kernel/signal.c
> @@ -1848,6 +1848,11 @@ relock:
>  			continue;
>  		}
>  
> +		if (sig_kernel_coredump(signr) && print_fatal_signals) {
> +			/* hold lock for smp_processor_id() */
> +			print_fatal_signal(regs, info->si_signo);
> +		}
> +
>  		spin_unlock_irq(&sighand->siglock);

This trades a (harmless) debug warning against a potential deadlock or 
even a crash, because print_fatal_signal() can do this:

                        __get_user(insn, (unsigned char *)(regs->ip + i));

which will work without a fault most of the time but might also generate a 
pagefault and schedule away from atomic context.

So please add preempt_disable()+preempt_enable() calls around the 
show_regs() call instead.

	Ingo