linux-kernel - Re: [PATCH] x86/optprobe: Fix OPTPROBE vs UACCESS

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Message-ID: <20200305115916.GE2579@hirez.programming.kicks-ass.net>
Date:   Thu, 5 Mar 2020 12:59:16 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] x86/optprobe: Fix OPTPROBE vs UACCESS

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@...radead.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@...nel.org>

Thanks!