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
