Re: [PATCH v2 0/3] ubsan: Split out bounds checker

Dmitry Vyukov
2019-11-27T09:34:57Z

On Wed, Nov 27, 2019 at 7:54 AM Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Wed, Nov 27, 2019 at 6:42 AM Kees Cook <keescook@chromium.org> wrote:
> >
> > On Fri, Nov 22, 2019 at 10:07:29AM +0100, Dmitry Vyukov wrote:
> > > On Thu, Nov 21, 2019 at 7:15 PM Kees Cook <keescook@chromium.org> wrote:
> > > >
> > > > v2:
> > > > - clarify Kconfig help text (aryabinin)
> > > > - add reviewed-by
> > > > - aim series at akpm, which seems to be where ubsan goes through?
> > > > v1: https://lore.kernel.org/lkml/20191120010636.27368-1-keescook@chromium.org
> > > >
> > > > This splits out the bounds checker so it can be individually used. This
> > > > is expected to be enabled in Android and hopefully for syzbot. Includes
> > > > LKDTM tests for behavioral corner-cases (beyond just the bounds checker).
> > > >
> > > > -Kees
> > >
> > > +syzkaller mailing list
> > >
> > > This is great!
> >
> > BTW, can I consider this your Acked-by for these patches? :)
> >
> > > I wanted to enable UBSAN on syzbot for a long time. And it's
> > > _probably_ not lots of work. But it was stuck on somebody actually
> > > dedicating some time specifically for it.
> >
> > Do you have a general mechanism to test that syzkaller will actually
> > pick up the kernel log splat of a new check?
>
> Yes. That's one of the most important and critical parts of syzkaller :)
> The tests for different types of bugs are here:
> https://github.com/google/syzkaller/tree/master/pkg/report/testdata/linux/report
>
> But have 3 for UBSAN, but they may be old and it would be useful to
> have 1 example crash per bug type:
>
> syzkaller$ grep UBSAN pkg/report/testdata/linux/report/*
> pkg/report/testdata/linux/report/40:TITLE: UBSAN: Undefined behaviour
> in drivers/usb/core/devio.c:LINE
> pkg/report/testdata/linux/report/40:[ 4.556972] UBSAN: Undefined
> behaviour in drivers/usb/core/devio.c:1517:25
> pkg/report/testdata/linux/report/41:TITLE: UBSAN: Undefined behaviour
> in ./arch/x86/include/asm/atomic.h:LINE
> pkg/report/testdata/linux/report/41:[ 3.805453] UBSAN: Undefined
> behaviour in ./arch/x86/include/asm/atomic.h:156:2
> pkg/report/testdata/linux/report/42:TITLE: UBSAN: Undefined behaviour
> in kernel/time/hrtimer.c:LINE
> pkg/report/testdata/linux/report/42:[ 50.583499] UBSAN: Undefined
> behaviour in kernel/time/hrtimer.c:310:16
>
> One of them is incomplete and is parsed as "corrupted kernel output"
> (won't be reported):
> https://github.com/google/syzkaller/blob/master/pkg/report/testdata/linux/report/42
>
> Also I see that report parsing just takes the first line, which
> includes file name, which is suboptimal (too long, can't report 2 bugs
> in the same file). We seem to converge on "bug-type in function-name"
> format.
> The thing about bug titles is that it's harder to change them later.
> If syzbot already reported 100 bugs and we change titles, it will
> start re-reporting the old one after new names and the old ones will
> look stale, yet they still relevant, just detected under different
> name.
> So we also need to get this part right before enabling.
>
> > I noticed a few things
> > about the ubsan handlers: they don't use any of the common "warn"
> > infrastructure (neither does kasan from what I can see), and was missing
> > a check for panic_on_warn (kasan has this, but does it incorrectly).
>
> Yes, panic_on_warn we also need.
>
> I will look at the patches again for Acked-by.

Acked-by: Dmitry Vyukov <dvyukov@google.com>
for the series.

I see you extended the test module, do you have samples of all UBSAN
report types that are triggered by these functions? Is so, please add
them to:
https://github.com/google/syzkaller/tree/master/pkg/report/testdata/linux/report
with whatever titles they are detected now. Improving titles will then
be the next step, but much simpler with a good collection of tests.

Will you send the panic_on_want patch as well?

> > I think kasan and ubsan should be reworked to use the common warn
> > infrastructure, and at the very least, ubsan needs this:
> >
> > diff --git a/lib/ubsan.c b/lib/ubsan.c
> > index e7d31735950d..a2535a62c9af 100644
> > --- a/lib/ubsan.c
> > +++ b/lib/ubsan.c
> > @@ -160,6 +160,17 @@ static void ubsan_epilogue(unsigned long *flags)
> > "========================================\n");
> > spin_unlock_irqrestore(&report_lock, *flags);
> > current->in_ubsan--;
> > +
> > + if (panic_on_warn) {
> > + /*
> > + * This thread may hit another WARN() in the panic path.
> > + * Resetting this prevents additional WARN() from panicking the
> > + * system on this thread. Other threads are blocked by the
> > + * panic_mutex in panic().
> > + */
> > + panic_on_warn = 0;
> > + panic("panic_on_warn set ...\n");
> > + }
> > }
> >
> > static void handle_overflow(struct overflow_data *data, void *lhs,
> >
> > > Kees, or anybody else interested, could you provide relevant configs
> > > that (1) useful for kernel,
> >
> > As mentioned in the other email (but just to keep the note together with
> > the other thoughts here) after this series, you'd want:
> >
> > CONFIG_UBSAN=y
> > CONFIG_UBSAN_BOUNDS=y
> > # CONFIG_UBSAN_MISC is not set
> >
> > > (2) we want 100% cleanliness,
> >
> > What do you mean here by "cleanliness"? It seems different from (3)
> > about the test tripping a lot?
> >
> > > (3) don't
> > > fire all the time even without fuzzing?
> >
> > I ran with the bounds checker enabled (and the above patch) under
> > syzkaller for the weekend and saw 0 bounds checker reports.
> >
> > > Anything else required to
> > > enable UBSAN? I don't see anything. syzbot uses gcc 8.something, which
> > > I assume should be enough (but we can upgrade if necessary).
> >
> > As mentioned, gcc 8+ should be fine.
> >
> > --
> > Kees Cook
> >
> > --
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