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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [PATCH 1/7] x86/entry: Fix #UD vs WARN more
Date:   Thu, 18 Jun 2020 12:29:50 -0700
Message-Id: <257B4193-08FB-4B3E-85E9-6C512B52C2C2@amacapital.net>
References: <20200618190207.GO576905@hirez.programming.kicks-ass.net>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Dmitry Vyukov <dvyukov@google.com>,
        Marco Elver <elver@google.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        Mark Rutland <Mark.Rutland@arm.com>,
        Matthew Helsley <mhelsley@vmware.com>,
        Steven Rostedt <rostedt@goodmis.org>, jthierry@redhat.com,
        Miroslav Benes <mbenes@suse.cz>
In-Reply-To: <20200618190207.GO576905@hirez.programming.kicks-ass.net>
To:     Peter Zijlstra <peterz@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Jun 18, 2020, at 12:02 PM, Peter Zijlstra <peterz@infradead.org> wrote:=

>=20
> =EF=BB=BFOn Thu, Jun 18, 2020 at 11:36:53AM -0700, Andy Lutomirski wrote:
>=20
>> I wasn't imagining going far down the rabbit hole at all -- I think
>> that, at most, we should cover the path for when the fault wasn't a
>> BUG/WARN in the first place.  I admit that, for #UD in particular,
>> this isn't a big deal, but if it were a different vector, this could
>> matter.
>=20
> Right, so there's 3 cases for ud2:
>=20
> - WARN;  ud2,  bug_entry, recovers
> - BUG;   ud2,  bug_entry, dies
> - UBSAN; ud2, !bug_entry, dies

4. The #UD matches an extable entry. I don=E2=80=99t know whether this ever h=
appens for real.

The failure is still a bit farfetched: we=E2=80=99d need an extable to hit i=
n an inconsistent state where we blow up due to a lack of entry handling.

>=20
> Nothing else should be generating ud2 instructions, any other #UD goes
> into handle_invalid_op() -> do_error_trap() -> ... -> die().
>=20
> [ while there, we should probably restructure do_trap() to have
>  cond_local_irq_enable() _after_ do_trap_no_signal(). ]
>=20
> We could probably change is_valid_bugaddr() to not use
> probe_kernel_address(), because if it couldn't read the instruction,
> we'd not be getting #UD in the first place.
>=20
> If we've gotten rid of probe_kernel_address() we can noinstr/inline that
> and then we can only call into report_bug() IFF ud2.
>=20
> Does that make things 'better' ? This can only go realy bad if there's a
> 1 byte instruction that triggers #UD, but I think that was ruled out.
>=20
>=20
> ---
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index c26751e303f1..275a621f1aff 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -91,10 +91,7 @@ int is_valid_bugaddr(unsigned long addr)
>    if (addr < TASK_SIZE_MAX)
>        return 0;
>=20
> -    if (probe_kernel_address((unsigned short *)addr, ud))
> -        return 0;
> -
> -    return ud =3D=3D INSN_UD0 || ud =3D=3D INSN_UD2;
> +    return *(unsigned short *)addr =3D=3D INSN_UD2;
> }

I=E2=80=99m okay with this, at least until we get PKRS or kernel XO memory. B=
ut probe_kernel_addr() would be wrong then, too.  We need probe_kernel_text(=
).

But I think you might need some IRQ fiddling. With your patch, a WARN with I=
RQs on will execute the printk code with IRQs off without lockstep handling,=
 and an appropriately configured debugging kernel may get a recursive splat.=
  Or if irq tracing somehow notices that IRQs got turned off, the warning re=
covery might return back to an IF=3D1 context with IRQs traced as off.

So maybe also do an untraced cond_local_irq_enable()?  After all, if we=E2=80=
=99re trying to report a bug from IRQs on, it should be okay to have IRQs on=
 while reporting it. It might even work better than having IRQs off.

>=20
> static nokprobe_inline int
> @@ -220,15 +217,17 @@ static noinstr bool handle_bug(struct pt_regs *regs)=

> {
>    bool handled =3D false;
>=20
> -    /*
> -     * All lies, just get the WARN/BUG out.
> -     */
> -    instrumentation_begin();
> -    if (report_bug(regs->ip, regs) =3D=3D BUG_TRAP_TYPE_WARN) {
> -        regs->ip +=3D LEN_UD2;
> -        handled =3D true;
> +    if (is_valid_bugaddr(regs->ip)) {
> +        /*
> +         * All lies, just get the WARN/BUG out.
> +         */
> +        instrumentation_begin();
> +        if (report_bug(regs->ip, regs) =3D=3D BUG_TRAP_TYPE_WARN) {
> +            regs->ip +=3D LEN_UD2;
> +            handled =3D true;
> +        }
> +        instrumentation_end();
>    }
> -    instrumentation_end();
>=20
>    return handled;
> }