Re: [PATCH 0/5] kasan: add workqueue and timer stack for generic KASAN

[Walter Wu]

On Mon, 2020-08-10 at 08:44 -0400, Qian Cai wrote:
> On Mon, Aug 10, 2020 at 07:50:57PM +0800, Walter Wu wrote:
> > On Mon, 2020-08-10 at 07:19 -0400, Qian Cai wrote:
> > > 
> > > > On Aug 10, 2020, at 3:21 AM, Walter Wu <walter-zh.wu@xxxxxxxxxxxx> wrote:
> > > > 
> > > > Syzbot reports many UAF issues for workqueue or timer, see [1] and [2].
> > > > In some of these access/allocation happened in process_one_work(),
> > > > we see the free stack is useless in KASAN report, it doesn't help
> > > > programmers to solve UAF on workqueue. The same may stand for times.
> > > > 
> > > > This patchset improves KASAN reports by making them to have workqueue
> > > > queueing stack and timer queueing stack information. It is useful for
> > > > programmers to solve use-after-free or double-free memory issue.
> > > > 
> > > > Generic KASAN will record the last two workqueue and timer stacks,
> > > > print them in KASAN report. It is only suitable for generic KASAN.
> > > > 
> > > > In order to print the last two workqueue and timer stacks, so that
> > > > we add new members in struct kasan_alloc_meta.
> > > > - two workqueue queueing work stacks, total size is 8 bytes.
> > > > - two timer queueing stacks, total size is 8 bytes.
> > > > 
> > > > Orignial struct kasan_alloc_meta size is 16 bytes. After add new
> > > > members, then the struct kasan_alloc_meta total size is 32 bytes,
> > > > It is a good number of alignment. Let it get better memory consumption.
> > > 
> > > Getting debugging tools complicated surely is the best way to kill it. I would argue that it only make sense to complicate it if it is useful most of the time which I never feel or hear that is the case. This reminds me your recent call_rcu() stacks that most of time just makes parsing the report cumbersome. Thus, I urge this exercise to over-engineer on special cases need to stop entirely.
> > > 
> > 
> > A good debug tool is to have complete information in order to solve
> > issue. We should focus on if KASAN reports always show this debug
> > information or create a option to decide if show it. Because this
> > feature is Dimitry's suggestion. see [1]. So I think it need to be
> > implemented. Maybe we can wait his response. 
> > 
> > [1]https://lkml.org/lkml/2020/6/23/256
> 
> I don't know if it is Dmitry's pipe-dream which every KASAN report would enable
> developers to fix it without reproducing it. It is always an ongoing struggling
> between to make kernel easier to debug and the things less cumbersome.
> 
> On the other hand, Dmitry's suggestion makes sense only if the price we are
> going to pay is fair. With the current diffstat and the recent experience of
> call_rcu() stacks "waste" screen spaces as a heavy KASAN user myself, I can't
> really get that exciting for pushing the limit again at all.
> 

If you are concerned that the report is long, maybe we can create an
option for the user decide whether print them (include call_rcu).
So this should satisfy everyone?

> > 
> > > > 
> > > > [1]https://groups.google.com/g/syzkaller-bugs/search?q=%22use-after-free%22+process_one_work
> > > > [2]https://groups.google.com/g/syzkaller-bugs/search?q=%22use-after-free%22%20expire_timers
> > > > [3]https://bugzilla.kernel.org/show_bug.cgi?id=198437
> > > > 
> > > > Walter Wu (5):
> > > > timer: kasan: record and print timer stack
> > > > workqueue: kasan: record and print workqueue stack
> > > > lib/test_kasan.c: add timer test case
> > > > lib/test_kasan.c: add workqueue test case
> > > > kasan: update documentation for generic kasan
> > > > 
> > > > Documentation/dev-tools/kasan.rst |  4 ++--
> > > > include/linux/kasan.h             |  4 ++++
> > > > kernel/time/timer.c               |  2 ++
> > > > kernel/workqueue.c                |  3 +++
> > > > lib/test_kasan.c                  | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
> > > > mm/kasan/generic.c                | 42 ++++++++++++++++++++++++++++++++++++++++++
> > > > mm/kasan/kasan.h                  |  6 +++++-
> > > > mm/kasan/report.c                 | 22 ++++++++++++++++++++++
> > > > 8 files changed, 134 insertions(+), 3 deletions(-)
> > > > 
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> >