Messages in this thread | | | From | Dmitry Vyukov <> | Date | Fri, 17 Jan 2020 11:03:07 +0100 | Subject | Re: [RFC PATCH] UML: add support for KASAN under x86_64 |
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On Fri, Jan 17, 2020 at 10:59 AM Dmitry Vyukov <dvyukov@google.com> wrote: > > On Thu, Jan 16, 2020 at 10:39 PM Patricia Alfonso > <trishalfonso@google.com> wrote: > > > > On Thu, Jan 16, 2020 at 1:23 AM Dmitry Vyukov <dvyukov@google.com> wrote: > > > > > > On Thu, Jan 16, 2020 at 10:20 AM Johannes Berg > > > <johannes@sipsolutions.net> wrote: > > > > > > > > On Thu, 2020-01-16 at 10:18 +0100, Dmitry Vyukov wrote: > > > > > > > > > > Looking at this problem and at the number of KASAN_SANITIZE := n in > > > > > Makefiles (some of which are pretty sad, e.g. ignoring string.c, > > > > > kstrtox.c, vsprintf.c -- that's where the bugs are!), I think we > > > > > initialize KASAN too late. I think we need to do roughly what we do in > > > > > user-space asan (because it is user-space asan!). Constructors run > > > > > before main and it's really good, we need to initialize KASAN from > > > > > these constructors. Or if that's not enough in all cases, also add own > > > > > constructor/.preinit array entry to initialize as early as possible. > > > > > > > > I am not too happy with the number of KASAN_SANITIZE := n's either. > > This sounds like a good idea. Let me look into it; I am not familiar > > with constructors or .preint array. > > > > > > We even control the linker in this case, so we can put something into > > > > the .preinit array *first*. > > > > > > Even better! If we can reliably put something before constructors, we > > > don't even need lazy init in constructors. > > > > > > > > All we need to do is to call mmap syscall, there is really no > > > > > dependencies on anything kernel-related. > > > > > > > > OK. I wasn't really familiar with those details. > > > > > > > > > This should resolve the problem with constructors (after they > > > > > initialize KASAN, they can proceed to do anything they need) and it > > > > > should get rid of most KASAN_SANITIZE (in particular, all of > > > > > lib/Makefile and kernel/Makefile) and should fix stack instrumentation > > > > > (in case it does not work now). The only tiny bit we should not > > > > > instrument is the path from constructor up to mmap call. > > > > This sounds like a great solution. I am getting this KASAN report: > > "BUG: KASAN: stack-out-of-bounds in syscall_stub_data+0x2a5/0x2c7", > > which is probably because of this stack instrumentation problem you > > point out. > > [reposting to the list] > > If that part of the code I mentioned is instrumented, manifestation > would be different -- stack instrumentation will try to access shadow, > shadow is not mapped yet, so it would crash on the shadow access. > > What you are seeing looks like, well, a kernel bug where it does a bad > stack access. Maybe it's KASAN actually _working_? :)
Though, stack instrumentation may have issues with longjmp-like things. I would suggest first turning off stack instrumentation and getting that work. Solving problems one-by-one is always easier. If you need help debugging this, please post more info: patch, what you are doing, full kernel output (preferably from start, if it's not too lengthy).
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