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From:   Andy Lutomirski <luto@amacapital.net>
Date:   Wed, 10 Mar 2021 17:28:12 -0800
Message-ID: <CALCETrVqkK29n=6wtVhd7qgTWf83x3SUk6+bkD30asHyWSqppw@mail.gmail.com>
Subject: Re: [PATCH v3] x86/fault: Send a SIGBUS to user process always for
 hwpoison page access.
To:     Aili Yao <yaoaili@kingsoft.com>
Cc:     "Luck, Tony" <tony.luck@intel.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Linux API <linux-api@vger.kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        HORIGUCHI NAOYA <naoya.horiguchi@nec.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>,
        yangfeng1@kingsoft.com, Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>, sunhao2@kingsoft.com
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On Wed, Mar 10, 2021 at 5:19 PM Aili Yao <yaoaili@kingsoft.com> wrote:
>
> On Mon, 8 Mar 2021 11:00:28 -0800
> Andy Lutomirski <luto@amacapital.net> wrote:
>
> > > On Mar 8, 2021, at 10:31 AM, Luck, Tony <tony.luck@intel.com> wrote:
> > >
> > > =EF=BB=BF
> > >>
> > >> Can you point me at that SIGBUS code in a current kernel?
> > >
> > > It is in kill_me_maybe().  mce_vaddr is setup when we disassemble wha=
tever get_user()
> > > or copy from user variant was in use in the kernel when the poison me=
mory was consumed.
> > >
> > >        if (p->mce_vaddr !=3D (void __user *)-1l) {
> > >                force_sig_mceerr(BUS_MCEERR_AR, p->mce_vaddr, PAGE_SHI=
FT);
> >
> > Hmm. On the one hand, no one has complained yet. On the other hand, har=
dware that supports this isn=E2=80=99t exactly common.
> >
> > We may need some actual ABI design here. We also need to make sure that=
 things like io_uring accesses or, more generally, anything using the use_m=
m / use_temporary_mm ends up either sending no signal or sending a signal t=
o the right target.
> >
> > >
> > > Would it be any better if we used the BUS_MCEERR_AO code that goes in=
to siginfo?
> >
> > Dunno.
>
> I have one thought here but don't know if it's proper:
>
> Previous patch use force_sig_mceerr to the user process for such a scenar=
io; with this method
> The SIGBUS can't be ignored as force_sig_mceerr() was designed to.
>
> If the user process don't want this signal, will it set signal config to =
ignore?
> Maybe we can use a send_sig_mceerr() instead of force_sig_mceerr(), if pr=
ocess want to
> ignore the SIGBUS, then it will ignore that, or it can also process the S=
IGBUS?

I don't think the signal blocking mechanism makes sense for this.
Blocking a signal is for saying that, if another process sends the
signal (or an async event like ctrl-C), then the process doesn't want
it.  Blocking doesn't block synchronous things like faults.

I think we need to at least fix the existing bug before we add more
signals.  AFAICS the MCE_IN_KERNEL_COPYIN code is busted for kernel
threads.

--Andy