linux-kernel - Re: [PATCH -next] x86/mm/pat: fix a data race in cpa_inc_4k

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Message-ID: <CANpmjNPFRCRg9wnFwyJpZVg8Urb9HAdZ++e3xbh1LXPjgAs4kw@mail.gmail.com>
Date:   Tue, 21 Jan 2020 09:19:18 +0100
From:   Marco Elver <elver@...gle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Qian Cai <cai@....pw>, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] x86/mm/pat: fix a data race in cpa_inc_4k_install

On Tue, 21 Jan 2020 at 08:27, Borislav Petkov <bp@...en8.de> wrote:
>
> On Mon, Jan 20, 2020 at 11:12:00PM -0500, Qian Cai wrote:
> > diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> > index 20823392f4f2..31e4a73ae70e 100644
> > --- a/arch/x86/mm/pat/set_memory.c
> > +++ b/arch/x86/mm/pat/set_memory.c
> > @@ -128,7 +128,7 @@ static inline void cpa_inc_2m_checked(void)
> >
> >  static inline void cpa_inc_4k_install(void)
> >  {
> > -     cpa_4k_install++;
> > +     WRITE_ONCE(cpa_4k_install, READ_ONCE(cpa_4k_install) + 1);

As I said in my email that you also copied to the message, this is
just a stats counter. For the general case, I think we reached
consensus that such accesses should intentionally remain data races:
  https://lore.kernel.org/linux-fsdevel/CAHk-=wg5CkOEF8DTez1Qu0XTEFw_oHhxN98bDnFqbY7HL5AB2g@mail.gmail.com/T/#u

Either you can use the data_race() macro, making this
'data_race(cpa_4k_install++)' -- this effectively documents the
intentional data race --

or just blacklist the entire file by putting
  KCSAN_SANITIZE_set_memory.o := n
into the Makefile.

[ Note that there are 2 more ways to blacklist:
  - __no_kcsan function attribute, for blacklisting entire functions.
  - KCSAN_SANITIZE :=n in the Makefile, blacklisting all compilation
units in the Makefile. ]

I leave it to you what makes more sense. I don't know if there are
other data races lurking here, since cpa_4k_install is not the only
stats counter.

> >  }
> >
> >  static inline void cpa_inc_lp_sameprot(int level)
> > --
>
> Fix a data race, says your subject?
>
> If it had to be honest, it probably should say "Make the code ugly
> because the next tool can't handle it".
>
> Frankly, I'm not a fan of all this "change the kernel to fix the tool"
> attitude.

I commented on better ways dealing with this, since this is just stats counting.

Thanks,
-- Marco