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

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Date:   Wed, 13 Feb 2019 16:45:32 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Julien Thierry <julien.thierry@....com>
Cc:     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 03:41:45PM +0100, Peter Zijlstra wrote:
> On Wed, Feb 13, 2019 at 02:39:22PM +0000, Julien Thierry wrote:
> > Hi Peter,
> > 
> > On 13/02/2019 14:25, Peter Zijlstra wrote:
> > > On Wed, Feb 13, 2019 at 02:00:26PM +0000, Will Deacon wrote:
> > >> The difference is because getting preempted in the sequence above is
> > >> triggered off the back of an interrupt. On arm64, and I think also on x86,
> > >> the user access state (SMAP or PAN) is saved and restored across exceptions
> > >> but not across context switch.
> > > 
> > > A quick reading of the SDM seems to suggest the SMAP state is part of
> > > EFLAGS, which is context switched just fine AFAIK.
> > > 
> > I fail to see where this is happening when looking at the switch_to()
> > logic in x86_64.
> 
> Yeah, me too.. we obviously preserve EFLAGS for user context, but for
> kernel-kernel switches we do not seem to preserve it :-(

So I dug around the context switch code a little, and I think we lost it
here:

  0100301bfdf5 ("sched/x86: Rewrite the switch_to() code")

Before that, x86_64 switch_to() read like (much simplified):

	asm volatile ( /* do RSP twiddle */
	  : /* output */
	  : /* input */
	  : "memory", "cc", .... "flags");

(see __EXTRA_CLOBBER)

Which I suppose means that GCC generates the PUSHF/POPF to preserve the
EFLAGS, since we mark those explicitly clobbered.

Before that:

  f05e798ad4c0 ("Disintegrate asm/system.h for X86")

We had explicit PUSHF / POPF in SAVE_CONTEXT / RESTORE_CONTEXT resp.

Now I cannot see how the current code preserves EFLAGS (if indeed it
does), and the changelog doesn't mention this change _AT_ALL_.

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.

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.