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

[Andrey Konovalov <andreyknvl@google.com>]

On Wed, Jul 18, 2018 at 7:16 PM, Andrey Konovalov <andreyknvl@google.com> wrote:
> On Tue, Jul 3, 2018 at 7:36 PM, Will Deacon <will.deacon@arm.com> 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?

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