linux-kernel - Re: [PATCH v3 3/4] uaccess: Check no rescheduling function is called in unsafe region

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Message-ID: <20190213222146.GC32494@hirez.programming.kicks-ass.net>
Date:   Wed, 13 Feb 2019 23:21:46 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Julien Thierry <julien.thierry@....com>,
        Will Deacon <will.deacon@....com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, mingo@...hat.com,
        catalin.marinas@....com, james.morse@....com, hpa@...or.com,
        valentin.schneider@....com, brgerst@...il.com, jpoimboe@...hat.com,
        luto@...nel.org, bp@...en8.de, dvlasenk@...hat.com,
        torvalds@...ux-foundation.org, tglx@...utronix.de
Subject: Re: [PATCH v3 3/4] uaccess: Check no rescheduling function is called
 in unsafe region

On Wed, Feb 13, 2019 at 10:51:24AM -0800, Andy Lutomirski wrote:
> > On Feb 13, 2019, at 7:45 AM, Peter Zijlstra <peterz@...radead.org> wrote:

> > Which I suppose means that GCC generates the PUSHF/POPF to preserve the
> > EFLAGS, since we mark those explicitly clobbered.
> > 
> 
> Not quite.  A flags clobber doesn’t save the control bits like AC
> except on certain rather buggy llvm compilers. The change you’re
> looking for is:
> 
> http://git.kernel.org/tip/2c7577a7583747c9b71f26dced7f696b739da745

Indeed, failed to find that.

> > For a little bit of context; it turns out that user_access_begin() /
> > user_access_end() sets EFLAGS.AC and scheduling in between there wrecks
> > that because we're apparently not saving that anymore.
> 
> But only explicit scheduling — preemption and sleepy page faults are
> fine because the interrupt frame saves flags.

No, like pointed out elsewhere in this thread, anything that does
preempt_disable() is utterly broken with this.

Because at that point the IRQ return path doesn't reschedule but
preempt_enable() will, and that doesn't preserve EFLAGS again.

> > Now, I'm tempted to add the PUSHF / POPF right back because of this, but
> > first I suppose we need to figure out if that change was on purpose and
> > why that went missing from the Changelog.
> 
> That’s certainly the easy solution. Or we could teach the might_sleep
> checks about this, but that could be a mess.

That's not enough, we'd have to teach preempt_disable(), but worse,
preempt_disable_notrace().

Anything that lands in ftrace, which _will_ use
preempt_disable_notrace(), will screw this thing up.