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From: Peter Zijlstra <peterz@infradead.org>
To: Masami Hiramatsu <mhiramat@kernel.org>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
	Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH] x86/optprobe: Fix OPTPROBE vs UACCESS
Date: Thu, 5 Mar 2020 12:59:16 +0100	[thread overview]
Message-ID: <20200305115916.GE2579@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200305194006.e3c447159aa866ac95fd02de@kernel.org>

On Thu, Mar 05, 2020 at 07:40:06PM +0900, Masami Hiramatsu wrote:
> Hi Peter,
> 
> On Thu, 5 Mar 2020 10:21:30 +0100
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > 
> > While looking at an objtool UACCESS warning, it suddenly occurred to me
> > that it is entirely possible to have an OPTPROBE right in the middle of
> > an UACCESS region.
> > 
> > In this case we must of course clear FLAGS.AC while running the KPROBE.
> > Luckily the trampoline already saves/restores [ER]FLAGS, so all we need
> > to do is inject a CLAC. Unfortunately we cannot use ALTERNATIVE() in the
> > trampoline text, so we have to frob that manually.
> 
> Good catch! so this prevents optprobe handler to access user space
> avoiding SMAP feature.

Yes that, but also worse, since the patch referenced by Fixes, x86_64 no
longer saves/restores FLAGS on context switch, and if the OPTPROBE were
to (accidentally) call into schedule() (say through preempt_enable()),
the next task could also run without SMAP for a while.

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

Thanks!

  reply	other threads:[~2020-03-05 11:59 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-05  9:21 [PATCH] x86/optprobe: Fix OPTPROBE vs UACCESS Peter Zijlstra
2020-03-05 10:40 ` Masami Hiramatsu
2020-03-05 11:59   ` Peter Zijlstra [this message]
2020-03-06  0:56 ` kbuild test robot
2020-03-06  0:56   ` kbuild test robot
2020-03-09 16:42   ` Peter Zijlstra
2020-03-09 16:42     ` Peter Zijlstra
2020-03-20 12:58 ` [tip: perf/core] " tip-bot2 for Peter Zijlstra

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