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Re: [kernel-hardening] Re: [PATCH 4/4] refcount: Report failures through CHECK_DATA_CORRUPTION

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On Tue, Feb 07, 2017 at 11:10:12AM +0000, Mark Rutland wrote:
> On Tue, Feb 07, 2017 at 09:34:05AM +0100, Peter Zijlstra wrote:
> > On Mon, Feb 06, 2017 at 08:54:38AM -0800, Kees Cook wrote:
> > > >
> > > > Like I wrote, ideally we'd end up using something like the x86 exception
> > > > table with a custom handler. Just no idea how to pull that off without
> > > > doing a full blown arch specific implementation, so I didn't go there
> > > > quite yet.
> > > 
> > > I haven't spent much time looking at the extable stuff. (Though
> > > coincidentally, I was poking at it for x86's test_nx stuff...) I
> > > thought there was a way to build arch-agnostic extables already?
> > > kernel/extable.c is unconditionally built-in, for example.
> > 
> > That doesn't seem to be of much use. It only contains section sort and
> > search functions.
> > 
> > Another problem for generic code would be to figure out what register
> > the relevant variable would live in at the time of exception. Here its
> > 'obviously' EAX because that's what cmpxchg requires, but in generic
> > you'd need a means of querying GCC's register allocator at the exception
> > point and somehow using that information for the generation of the
> > exception handler.
> 
> I think we only need two arch-specific primitives:
> (a) mangle a GCC assigned register into an idx stored in the extable
> (b) take said index, and grab the relevant register from pt_regs
> 
> Then you can have a BUG_VALUE(v, ...), where we use an input "r" (val),
> and mangle that into the idx in the extable. In the common case, I'd
> hope GCC would leave the register in-place from the cmpxchg.
> 
> ... or have I misundertood? :)

Right something along those lines. (a) will need GCC help, and (b) would
be kernel-arch specific. So this isn't something we can quickly do.

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[PATCH 0/4] refcount_t followups... Kees Cook <keescook@chromium.org> - 2017-02-04 00:30 +0100
  [PATCH 2/4] lkdtm: convert to refcount_t testing Kees Cook <keescook@chromium.org> - 2017-02-04 00:30 +0100
    [tip:locking/core] lkdtm: Convert to refcount_t testing tip-bot for Kees Cook <tipbot@zytor.com> - 2017-02-10 09:50 +0100
  [PATCH 3/4] bug: Switch data corruption check to __must_check Kees Cook <keescook@chromium.org> - 2017-02-04 00:30 +0100
  [PATCH 1/4] refcount_t: fix Kconfig help Kees Cook <keescook@chromium.org> - 2017-02-04 00:30 +0100
  [PATCH 4/4] refcount: Report failures through CHECK_DATA_CORRUPTION Kees Cook <keescook@chromium.org> - 2017-02-04 00:30 +0100
    Re: [PATCH 4/4] refcount: Report failures through  CHECK_DATA_CORRUPTION Peter Zijlstra <peterz@infradead.org> - 2017-02-05 16:50 +0100
      Re: [PATCH 4/4] refcount: Report failures through CHECK_DATA_CORRUPTION Kees Cook <keescook@chromium.org> - 2017-02-06 00:40 +0100
        Re: [PATCH 4/4] refcount: Report failures through  CHECK_DATA_CORRUPTION Peter Zijlstra <peterz@infradead.org> - 2017-02-06 10:00 +0100
          Re: [PATCH 4/4] refcount: Report failures through CHECK_DATA_CORRUPTION Kees Cook <keescook@chromium.org> - 2017-02-06 18:00 +0100
            Re: [PATCH 4/4] refcount: Report failures through  CHECK_DATA_CORRUPTION Peter Zijlstra <peterz@infradead.org> - 2017-02-07 09:40 +0100
              Re: [kernel-hardening] Re: [PATCH 4/4] refcount: Report failures  through CHECK_DATA_CORRUPTION Mark Rutland <mark.rutland@arm.com> - 2017-02-07 12:20 +0100
                Re: [kernel-hardening] Re: [PATCH 4/4] refcount: Report failures  through CHECK_DATA_CORRUPTION Peter Zijlstra <peterz@infradead.org> - 2017-02-07 13:40 +0100
                Re: [kernel-hardening] Re: [PATCH 4/4] refcount: Report failures  through CHECK_DATA_CORRUPTION Mark Rutland <mark.rutland@arm.com> - 2017-02-07 15:00 +0100
                Re: [kernel-hardening] Re: [PATCH 4/4] refcount: Report failures  through CHECK_DATA_CORRUPTION Peter Zijlstra <peterz@infradead.org> - 2017-02-07 16:10 +0100
                Re: [kernel-hardening] Re: [PATCH 4/4] refcount: Report failures  through CHECK_DATA_CORRUPTION Mark Rutland <mark.rutland@arm.com> - 2017-02-07 17:10 +0100
                Re: [kernel-hardening] Re: [PATCH 4/4] refcount: Report failures  through CHECK_DATA_CORRUPTION Peter Zijlstra <peterz@infradead.org> - 2017-02-07 18:40 +0100
                Re: [kernel-hardening] Re: [PATCH 4/4] refcount: Report failures  through CHECK_DATA_CORRUPTION Mark Rutland <mark.rutland@arm.com> - 2017-02-07 19:00 +0100
                Re: [kernel-hardening] Re: [PATCH 4/4] refcount: Report failures  through CHECK_DATA_CORRUPTION Peter Zijlstra <peterz@infradead.org> - 2017-02-08 10:20 +0100
                Re: [kernel-hardening] Re: [PATCH 4/4] refcount: Report failures  through CHECK_DATA_CORRUPTION Peter Zijlstra <peterz@infradead.org> - 2017-02-08 10:50 +0100
                Re: [kernel-hardening] Re: [PATCH 4/4] refcount: Report failures  through CHECK_DATA_CORRUPTION Mark Rutland <mark.rutland@arm.com> - 2017-02-08 16:00 +0100
                Re: [kernel-hardening] Re: [PATCH 4/4] refcount: Report failures  through CHECK_DATA_CORRUPTION Kees Cook <keescook@chromium.org> - 2017-02-08 22:30 +0100
                Re: [kernel-hardening] Re: [PATCH 4/4] refcount: Report failures  through CHECK_DATA_CORRUPTION Peter Zijlstra <peterz@infradead.org> - 2017-02-09 11:30 +0100
                Re: [kernel-hardening] Re: [PATCH 4/4] refcount: Report failures  through CHECK_DATA_CORRUPTION Kees Cook <keescook@chromium.org> - 2017-02-11 00:40 +0100

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