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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Thu, 13 Sep 2018 10:37:00 +0200
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Subject: Re: [PATCH v6 15/18] khwasan, arm64: add brk handler for inline instrumentation
To:     Jann Horn <jannh@google.com>
Cc:     Andrey Konovalov <andreyknvl@google.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Alexander Potapenko <glider@google.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
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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.