Re: [PATCH v6 15/18] khwasan, arm64: add brk handler for inline instrumentation

[Dmitry Vyukov <dvyukov@google.com>]

On Thu, Sep 13, 2018 at 8:09 PM, 'Nick Desaulniers' via kasan-dev
<kasan-dev@googlegroups.com> wrote:
> On Thu, Sep 13, 2018 at 1:37 AM Dmitry Vyukov <dvyukov@google.com> wrote:
>>
>> On Wed, Sep 12, 2018 at 7:39 PM, Jann Horn <jannh@google.com> wrote:
>> > On Wed, Sep 12, 2018 at 7:16 PM Dmitry Vyukov <dvyukov@google.com> wrote:
>> >> On Wed, Aug 29, 2018 at 1:35 PM, Andrey Konovalov <andreyknvl@google.com> wrote:
>> > [...]
>> >> > +static int khwasan_handler(struct pt_regs *regs, unsigned int esr)
>> >> > +{
>> >> > +       bool recover = esr & KHWASAN_ESR_RECOVER;
>> >> > +       bool write = esr & KHWASAN_ESR_WRITE;
>> >> > +       size_t size = KHWASAN_ESR_SIZE(esr);
>> >> > +       u64 addr = regs->regs[0];
>> >> > +       u64 pc = regs->pc;
>> >> > +
>> >> > +       if (user_mode(regs))
>> >> > +               return DBG_HOOK_ERROR;
>> >> > +
>> >> > +       kasan_report(addr, size, write, pc);
>> >> > +
>> >> > +       /*
>> >> > +        * The instrumentation allows to control whether we can proceed after
>> >> > +        * a crash was detected. This is done by passing the -recover flag to
>> >> > +        * the compiler. Disabling recovery allows to generate more compact
>> >> > +        * code.
>> >> > +        *
>> >> > +        * Unfortunately disabling recovery doesn't work for the kernel right
>> >> > +        * now. KHWASAN reporting is disabled in some contexts (for example when
>> >> > +        * the allocator accesses slab object metadata; same is true for KASAN;
>> >> > +        * this is controlled by current->kasan_depth). All these accesses are
>> >> > +        * detected by the tool, even though the reports for them are not
>> >> > +        * printed.
>> >> > +        *
>> >> > +        * This is something that might be fixed at some point in the future.
>> >> > +        */
>> >> > +       if (!recover)
>> >> > +               die("Oops - KHWASAN", regs, 0);
>> >>
>> >> Why die and not panic? Die seems to be much less used function, and it
>> >> calls panic anyway, and we call panic in kasan_report if panic_on_warn
>> >> is set.
>> >
>> > die() is vaguely equivalent to BUG(); die() and BUG() normally only
>> > terminate the current process, which may or may not leave the system
>> > somewhat usable, while panic() always brings down the whole system.
>> > AFAIK panic() shouldn't be used unless you're in some very low-level
>> > code where you know that trying to just kill the current process can't
>> > work and the entire system is broken beyond repair.
>> >
>> > If KASAN traps on some random memory access, there's a good chance
>> > that just killing the current process will allow at least parts of the
>> > system to continue. I'm not sure whether BUG() or die() is more
>> > appropriate here, but I think it definitely should not be a panic().
>>
>>
>> Nick, do you know if die() will be enough to catch problems on Android
>> phones? panic_on_warn would turn this into panic, but I guess one does
>> not want panic_on_warn on a canary phone.
>
> die() has arch specific implementations, so looking at:
>
> arch/arm64/kernel/traps.c:196#die
>
> it looks like panic is invoked if in_interrupt() or panic_on_oops(),
> which is a configure option.  So maybe the config for KHWASAN should
> also enable that? Otherwise seems easy to forget.  But maybe that
> should remain configurable separately?
>
> Looking at the kernel configs for the Pixel 2, it does seem like
> CONFIG_PANIC_ON_OOPS=y is already enabled.
> https://android.googlesource.com/kernel/msm/+/android-msm-wahoo-4.4-pie/arch/arm64/configs/wahoo_defconfig#746

Then I think we are good here.

> Specifically to catch problems on Android, our internal debug builds
> can report on panics, but not oops, IIUC.