linux-kernel - Re: More hw_breakpoint scariness reduction

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Date:	Sat, 15 Aug 2015 18:55:40 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Subject: Re: More hw_breakpoint scariness reduction

2015-08-15 0:38 GMT+02:00 Andy Lutomirski <luto@...capital.net>:
> Would you all consider it acceptable to disallow watchpoints on per
> cpu data entirely?  I can think of a *lot* of places where hitting #DB
> when accessing per cpu data from entry asm would be bad.
>
> Of course, actually implementing that might be less than entirely fun,
> given that a cpu could be onlined after creating a watchpoint.

Well I think there will always be places where setting a breakpoint is
a bad idea. The same goes for kprobes. We can't fix all of them.
Kernel breakpoints can only be set by root users so it's not a
security issue. Besides, kernel breakpoints should only be used by
kernel hackers (perf, kdb).

Given the wide use of per-cpu data, forbidding all of them will
seriously reduce the usability of kernel breakpoints. Not that I think
they are really used in practice though ;-)
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