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

From: Dmitry Vyukov <dvyukov@google.com>
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>,
	Will Deacon <will.deacon@arm.com>,
	Christoph Lameter <cl@linux.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Marc Zyngier <marc.zyngier@arm.com>,
	Dave Martin <dave.martin@arm.com>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Ingo Molnar <mingo@kernel.org>,
	Paul Lawrence <paullawrence@google.com>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Arnd Bergmann <arnd@arndb.de>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Kate Stewart <kstewart@linuxfoundation.org>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>,
	kasan-dev <kasan-dev@googlegroups.com>,
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	kernel list <linux-kernel@vger.kernel.org>,
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	Kostya Serebryany <kcc@google.com>,
	Evgenii Stepanov <eugenis@google.com>,
	Lee Smith <Lee.Smith@arm.com>,
	Ramana Radhakrishnan <Ramana.Radhakrishnan@arm.com>,
	Jacob Bramley <Jacob.Bramley@arm.com>,
	Ruben Ayrapetyan <Ruben.Ayrapetyan@arm.com>,
	Mark Brand <markbrand@google.com>,
	Chintan Pandya <cpandya@codeaurora.org>,
	Vishwath Mohan <vishwath@google.com>
Subject: Re: [PATCH v6 15/18] khwasan, arm64: add brk handler for inline instrumentation
Date: Thu, 13 Sep 2018 10:37:00 +0200	[thread overview]
Message-ID: <CACT4Y+avu_68GoQcc32zpcOpAu-Pw7m71VmuKtEkOw=vKgxi7w@mail.gmail.com> (raw)
In-Reply-To: <CAG48ez2oT1dtDcH8SfPLnoX5F8d6Pd=M-eOKHhYJ83EuL_j6wQ@mail.gmail.com>

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.