Re: [PATCH v3 2/7] x86: inline kprobe_exceptions_notify() into do_general_protection()

From: Masami Hiramatsu <mhiramat@kernel.org>
To: Jann Horn <jannh@google.com>
Cc: Kees Cook <keescook@chromium.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	x86@kernel.org, Andy Lutomirski <luto@kernel.org>,
	kernel-hardening@lists.openwall.com,
	linux-kernel@vger.kernel.org, dvyukov@google.com,
	Masami Hiramatsu <mhiramat@kernel.org>,
	"Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>,
	Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>,
	"David S. Miller" <davem@davemloft.net>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	linux-fsdevel@vger.kernel.org, Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH v3 2/7] x86: inline kprobe_exceptions_notify() into do_general_protection()
Date: Wed, 29 Aug 2018 09:08:32 +0900	[thread overview]
Message-ID: <20180829090832.337553da18089643a107cf56@kernel.org> (raw)
In-Reply-To: <20180828201421.157735-3-jannh@google.com>

On Tue, 28 Aug 2018 22:14:16 +0200
Jann Horn <jannh@google.com> wrote:

> The opaque plumbing of #GP from do_general_protection() through
> notify_die() into kprobe_exceptions_notify() makes it hard to understand
> what's going on.

OK, this seems reasonable optimization, since kprobe_exceptions_notify
only handles DIE_GPF now.

Acked-by: Masami Hiramatsu <mhiramat@kernel.org>

Hmm, I think I should introduce ARCH_KPROBE_HANDLE_EXCEPTION and if it
is enabled, kernel/kprobes.c stops using exception notifier. It is
no more needed on x86.

Thank you!

> 
> Suggested-by: Andy Lutomirski <luto@kernel.org>
> Signed-off-by: Jann Horn <jannh@google.com>
> ---
>  arch/x86/kernel/kprobes/core.c | 31 +------------------------------
>  arch/x86/kernel/traps.c        | 10 ++++++++++
>  2 files changed, 11 insertions(+), 30 deletions(-)
> 
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index b0d1e81c96bb..467ac22691b0 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -1028,42 +1028,13 @@ int kprobe_fault_handler(struct pt_regs *regs, int trapnr)
>  		if (fixup_exception(regs, trapnr))
>  			return 1;
>  
> -		/*
> -		 * fixup routine could not handle it,
> -		 * Let do_page_fault() fix it.
> -		 */
> +		/* fixup routine could not handle it. */
>  	}
>  
>  	return 0;
>  }
>  NOKPROBE_SYMBOL(kprobe_fault_handler);
>  
> -/*
> - * Wrapper routine for handling exceptions.
> - */
> -int kprobe_exceptions_notify(struct notifier_block *self, unsigned long val,
> -			     void *data)
> -{
> -	struct die_args *args = data;
> -	int ret = NOTIFY_DONE;
> -
> -	if (args->regs && user_mode(args->regs))
> -		return ret;
> -
> -	if (val == DIE_GPF) {
> -		/*
> -		 * To be potentially processing a kprobe fault and to
> -		 * trust the result from kprobe_running(), we have
> -		 * be non-preemptible.
> -		 */
> -		if (!preemptible() && kprobe_running() &&
> -		    kprobe_fault_handler(args->regs, args->trapnr))
> -			ret = NOTIFY_STOP;
> -	}
> -	return ret;
> -}
> -NOKPROBE_SYMBOL(kprobe_exceptions_notify);
> -
>  bool arch_within_kprobe_blacklist(unsigned long addr)
>  {
>  	bool is_in_entry_trampoline_section = false;
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index e6db475164ed..bf9ab1aaa175 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -556,6 +556,16 @@ do_general_protection(struct pt_regs *regs, long error_code)
>  
>  		tsk->thread.error_code = error_code;
>  		tsk->thread.trap_nr = X86_TRAP_GP;
> +
> +		/*
> +		 * To be potentially processing a kprobe fault and to
> +		 * trust the result from kprobe_running(), we have to
> +		 * be non-preemptible.
> +		 */
> +		if (!preemptible() && kprobe_running() &&
> +		    kprobe_fault_handler(regs, X86_TRAP_GP))
> +			return;
> +
>  		if (notify_die(DIE_GPF, "general protection fault", regs, error_code,
>  			       X86_TRAP_GP, SIGSEGV) != NOTIFY_STOP)
>  			die("general protection fault", regs, error_code);
> -- 
> 2.19.0.rc0.228.g281dcd1b4d0-goog
> 

-- 
Masami Hiramatsu <mhiramat@kernel.org>