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
