linux-kernel - Re: Dealing with the NMI mess

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Date:	Thu, 30 Jul 2015 17:41:22 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Willy Tarreau <w@....eu>, Steven Rostedt <rostedt@...dmis.org>,
	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: Dealing with the NMI mess



On 24/07/2015 23:08, Andy Lutomirski wrote:
> user_icebp is set if int $0x01 happens, except it isn't because user
> code can't actually do that -- it'll cause #GP instead.
> 
> user_icebp is also set if the user has a bloody in-circuit emulator,
> given the name.  But who on Earth has one of those on a system new
> enough to run Linux and, even if they have one, why on Earth are they
> using it to send SIGTRAP.

You do not need either "int $0x01" or an ICE to set user_icebp = 1.  You
can use the 0xf1 opcode, which is kinda like 0xcc but generates #DB
instead of #BP.

The historical name is ICEBP because in-circuit emulators used it for
software breakpoints, just like your usual debugger used 0xcc aka int3.
 And just like 0xcc it's unprivileged, so you can actually get a SIGTRAP
with asm(".byte 0xf1").

So...

> In any event, user_icebp is only set if user_mode(regs), so it's safe
> locking-wise.  But please let's delete it.

... it's safe, but it has some use (!).

Paolo
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