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SubjectRe: tip -ENOBOOT - bisected to locking/refcounts, x86/asm: Implement fast refcount overflow protection
On Fri, Sep 1, 2017 at 10:52 AM, Mike Galbraith <efault@gmx.de> wrote:
> On Fri, 2017-09-01 at 10:12 -0700, Kees Cook wrote:
>> On Fri, Sep 1, 2017 at 6:09 AM, Mike Galbraith <efault@gmx.de> wrote:
>> > On Fri, 2017-09-01 at 08:57 +0200, Mike Galbraith wrote:
>> >> On Thu, 2017-08-31 at 11:45 -0700, Kees Cook wrote:
>> >> > On Thu, Aug 31, 2017 at 10:19 AM, Mike Galbraith <efault@gmx.de> wrote:
>> >> > > On Thu, 2017-08-31 at 10:00 -0700, Kees Cook wrote:
>> >> > >>
>> >> > >> Oh! So it's gcc-version sensitive? That's alarming. Is this mapping correct:
>> >> > >>
>> >> > >> 4.8.5: WARN, eventual kernel hang
>> >> > >> 6.3.1, 7.0.1: WARN, but continues working
>> >> > >
>> >> > > Yeah, that's correct. I find that troubling, simply because this gcc
>> >> > > version has been through one hell of a lot of kernels with me. Yeah, I
>> >> > > know, that doesn't exempt it from having bugs, but color me suspicious.
>> >> >
>> >> > I still can't hit this with a 4.8.5 build. :(
>> >> >
>> >> > With _RATELIMIT removed, this should, in theory, report whatever goes
>> >> > negative first...
>> >>
>> >> I applied the other patch you posted, and built with gcc-6.3.1 to
>> >> remove the gcc-4.8.5 aspect. Look below the resulting splat.
>> >
>> > Grr, that one has a in6_dev_getx() line missing for the first
>> > increment, where things go pear shaped.
>> >
>> > With that added, looking at counter both before, and after incl, with a
>> > trace_printk() in the exception handler showing it doing its saturate
>> > thing, irqs disabled across the whole damn refcount_inc(), and even
>> > booting box nr_cpus=1 for extra credit...
>> >
>> > HTH can that first refcount_inc() get there?
>> >
>> > # tracer: nop
>> > #
>> > # _-----=> irqs-off
>> > # / _----=> need-resched
>> > # | / _---=> hardirq/softirq
>> > # || / _--=> preempt-depth
>> > # ||| / delay
>> > # TASK-PID CPU# |||| TIMESTAMP FUNCTION
>> > # | | | |||| | |
>> > systemd-1 [000] d..1 1.937284: in6_dev_getx: PRE refs.counter:3
>> > systemd-1 [000] d..1 1.937295: ex_handler_refcount: *(int *)regs->cx = -1073741824
>> > systemd-1 [000] d..1 1.937296: in6_dev_getx: POST refs.counter:-1073741824
>>
>> O_o
>>
>> Can you paste the disassembly of in6_dev_getx? I can't understand how
>> we're landing in the exception handler.
>
> I was hoping you'd say that.
>
> 0xffffffff816b2f72 <+0>: push %rbp
> 0xffffffff816b2f73 <+1>: mov %rsp,%rbp
> 0xffffffff816b2f76 <+4>: push %r12
> 0xffffffff816b2f78 <+6>: push %rbx
> 0xffffffff816b2f79 <+7>: incl %gs:0x7e95a2d0(%rip) # 0xd250 <__preempt_count>
> 0xffffffff816b2f80 <+14>: mov 0x308(%rdi),%rbx
> 0xffffffff816b2f87 <+21>: test %rbx,%rbx
> 0xffffffff816b2f8a <+24>: je 0xffffffff816b2feb <in6_dev_getx+121>
> 0xffffffff816b2f8c <+26>: callq *0xffffffff81c35a00
> 0xffffffff816b2f93 <+33>: mov %rax,%r12
> 0xffffffff816b2f96 <+36>: callq *0xffffffff81c35a10
> 0xffffffff816b2f9d <+43>: mov 0x769ad4(%rip),%rsi # 0xffffffff81e1ca78 <trace_printk_fmt.21733>
> 0xffffffff816b2fa4 <+50>: mov 0xf0(%rbx),%edx
> 0xffffffff816b2faa <+56>: mov $0xffffffff816b2f8c,%rdi
> 0xffffffff816b2fb1 <+63>: callq 0xffffffff81171fc0 <__trace_bprintk>
> 0xffffffff816b2fb6 <+68>: lock incl 0xf0(%rbx)
> 0xffffffff816b2fbd <+75>: js 0xffffffff816b2fbf <in6_dev_getx+77>
> 0xffffffff816b2fbf <+77>: lea 0xf0(%rbx),%rcx
> 0xffffffff816b2fc6 <+84>: (bad)
> 0xffffffff816b2fc8 <+86>: mov 0x769a99(%rip),%rsi # 0xffffffff81e1ca68 <trace_printk_fmt.21744>
> 0xffffffff816b2fcf <+93>: mov 0xf0(%rbx),%edx
> 0xffffffff816b2fd5 <+99>: mov $0xffffffff816b2f8c,%rdi
> 0xffffffff816b2fdc <+106>: callq 0xffffffff81171fc0 <__trace_bprintk>
> 0xffffffff816b2fe1 <+111>: mov %r12,%rdi
> 0xffffffff816b2fe4 <+114>: callq *0xffffffff81c35a08
> 0xffffffff816b2feb <+121>: decl %gs:0x7e95a25e(%rip) # 0xd250 <__preempt_count>
> 0xffffffff816b2ff2 <+128>: mov %rbx,%rax
> 0xffffffff816b2ff5 <+131>: pop %rbx
> 0xffffffff816b2ff6 <+132>: pop %r12
> 0xffffffff816b2ff8 <+134>: pop %rbp
> 0xffffffff816b2ff9 <+135>: retq
>
> I don't get the section business at all, +75 looks to me like we're
> gonna trap no matter what.. as we appear to be doing.

The section stuff is supposed to be a trick to push the error case off
into the .text.unlikely area to avoid needing a jmp over the handler
and with possibly some redundancy removal done by the compiler (though
this appears to be rather limited) if it notices a bunch of error
paths are the same. However, in your disassembly, it's inline (!!) in
the code, as if "pushsection" and "popsection" were entirely ignored.

And when I make my own in6_dev_getx(), I see the same disassembly:

0xffffffff818a757b <+181>: lock incl 0x1e0(%rbx)
0xffffffff818a7582 <+188>: js 0xffffffff818a7584 <in6_dev_getx+190>
0xffffffff818a7584 <+190>: lea 0x1e0(%rbx),%rcx
0xffffffff818a758b <+197>: (bad)

Which is VERY different from how it looks in other places!

e.g. from lkdtm_REFCOUNT_INC_SATURATED:

0xffffffff815657df <+47>: lock incl -0xc(%rbp)
0xffffffff815657e3 <+51>: js 0xffffffff81565cac
...
0xffffffff81565cac: lea -0xc(%rbp),%rcx
0xffffffff81565cb0: (bad)

So, at least I can reproduce this in the build now. I must not be
exercising these paths. FWIW, this is with Ubuntu's 6.3.0 gcc.

I'll try to figure out what's going on here...

-Kees

--
Kees Cook
Pixel Security

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