Re: [PATCH v4 00/17] khwasan: kernel hardware assisted address sanitizer

[Will Deacon]

Hi Andrey,

On Tue, Jul 31, 2018 at 03:22:13PM +0200, Andrey Konovalov wrote:
> On Wed, Jul 18, 2018 at 7:16 PM, Andrey Konovalov <andreyknvl@xxxxxxxxxx> wrote:
> > On Tue, Jul 3, 2018 at 7:36 PM, Will Deacon <will.deacon@xxxxxxx> wrote:
> >> Hmm, but elsewhere in this thread, Evgenii is motivating the need for this
> >> patch set precisely because the lower overhead means it's suitable for
> >> "near-production" use. So I don't think writing this off as a debugging
> >> feature is the right approach, and we instead need to put effort into
> >> analysing the impact of address tags on the kernel as a whole. Playing
> >> whack-a-mole with subtle tag issues sounds like the worst possible outcome
> >> for the long-term.
> >
> > I don't see a way to find cases where pointer tags would matter
> > statically, so I've implemented the dynamic approach that I mentioned
> > above. I've instrumented all pointer comparisons/subtractions in an
> > LLVM compiler pass and used a kernel module that would print a bug
> > report whenever two pointers with different tags are being
> > compared/subtracted (ignoring comparisons with NULL pointers and with
> > pointers obtained by casting an error code to a pointer type). Then I
> > tried booting the kernel in QEMU and on an Odroid C2 board and I ran
> > syzkaller overnight.
> >
> > This yielded the following results.
> >
> > ======
> >
> > The two places that look interesting are:
> >
> > is_vmalloc_addr in include/linux/mm.h (already mentioned by Catalin)
> > is_kernel_rodata in mm/util.c
> >
> > Here we compare a pointer with some fixed untagged values to make sure
> > that the pointer lies in a particular part of the kernel address
> > space. Since KWHASAN doesn't add tags to pointers that belong to
> > rodata or vmalloc regions, this should work as is. To make sure I've
> > added debug checks to those two functions that check that the result
> > doesn't change whether we operate on pointers with or without
> > untagging.
> >
> > ======
> >
> > A few other cases that don't look that interesting:
> >
> > Comparing pointers to achieve unique sorting order of pointee objects
> > (e.g. sorting locks addresses before performing a double lock):
> >
> > tty_ldisc_lock_pair_timeout in drivers/tty/tty_ldisc.c
> > pipe_double_lock in fs/pipe.c
> > unix_state_double_lock in net/unix/af_unix.c
> > lock_two_nondirectories in fs/inode.c
> > mutex_lock_double in kernel/events/core.c
> >
> > ep_cmp_ffd in fs/eventpoll.c
> > fsnotify_compare_groups fs/notify/mark.c
> >
> > Nothing needs to be done here, since the tags embedded into pointers
> > don't change, so the sorting order would still be unique.
> >
> > Check that a pointer belongs to some particular allocation:
> >
> > is_sibling_entry lib/radix-tree.c
> > object_is_on_stack in include/linux/sched/task_stack.h
> >
> > Nothing needs to be here either, since two pointers can only belong to
> > the same allocation if they have the same tag.
> >
> > ======
> >
> > Will, Catalin, WDYT?
> 
> ping

Thanks for tracking these cases down and going through each of them. The
obvious follow-up question is: how do we ensure that we keep on top of
this in mainline? Are you going to repeat your experiment at every kernel
release or every -rc or something else? I really can't see how we can
maintain this in the long run, especially given that the coverage we have
is only dynamic -- do you have an idea of how much coverage you're actually
getting for, say, a defconfig+modules build?

I'd really like to enable pointer tagging in the kernel, I'm just still
failing to see how we can do it in a controlled manner where we can reason
about the semantic changes using something other than a best-effort,
case-by-case basis which is likely to be fragile and error-prone.
Unfortunately, if that's all we have, then this gets relegated to a
debug feature, which sort of defeats the point in my opinion.

Will