Re: [PATCH 3/3] x86/entry/64: Move #BP from IST to the IRQ stack

From: Borislav Petkov <bp@alien8.de>
To: Andy Lutomirski <luto@kernel.org>
Cc: X86 ML <x86@kernel.org>,
	linux-kernel@vger.kernel.org, Brian Gerst <brgerst@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>, Willy Tarreau <w@1wt.eu>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 3/3] x86/entry/64: Move #BP from IST to the IRQ stack
Date: Fri, 24 Jul 2015 13:02:43 +0200	[thread overview]
Message-ID: <20150724110243.GA20411@nazgul.tnic> (raw)
In-Reply-To: <881f70d2156267e8f9cd901ce0f66b6cd029a44c.1437690860.git.luto@kernel.org>

On Thu, Jul 23, 2015 at 03:37:48PM -0700, Andy Lutomirski wrote:
> There's nothing IST-worthy about #BP/int3.  We don't allow kprobes
> in the small handful of places in the kernel that run at CPL0 with
> an invalid stack, and 32-bit kernels have used normal interrupt
> gates for #BP forever.
> 
> Furthermore, we don't allow kprobes in places that have usergs while
> in kernel mode, so "paranoid" is also unnecessary.
> 
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
>  arch/x86/entry/entry_64.S |  2 +-
>  arch/x86/kernel/traps.c   | 26 +++++++++++++-------------
>  2 files changed, 14 insertions(+), 14 deletions(-)

...

> @@ -494,7 +494,15 @@ dotraplinkage void notrace do_int3(struct pt_regs *regs, long error_code)
>  	if (poke_int3_handler(regs))
>  		return;
>  
> +	/*
> +	 * Use ist_enter despite the fact that we don't use an IST stack.
> +	 * We can be called from a kprobe in non-CONTEXT_KERNEL kernel
> +	 * mode or even during context tracking state changes.
> +	 *
> +	 * This means that we can't schedule.  That's okay.
> +	 */
>  	ist_enter(regs);

A good sign that this "ist_enter" name needs to be changed. Otherwise,
this call site will confuse people.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
--