Re: [PATCH v2 2/2] x86, refcount: Implement fast refcount overflow protection

[Kees Cook]

On Mon, May 1, 2017 at 8:54 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> On Thu, Apr 27, 2017 at 01:22:05PM -0700, Kees Cook wrote:
>> On Wed, Apr 26, 2017 at 6:31 PM, kbuild test robot <lkp@xxxxxxxxx> wrote:
>> > Hi Kees,
>> >
>> > [auto build test WARNING on next-20170424]
>> > [cannot apply to tip/x86/core linus/master linux/master v4.9-rc8 v4.9-rc7 v4.9-rc6 v4.11-rc8]
>> > [if your patch is applied to the wrong git tree, please drop us a note to help improve the system]
>> >
>> > url:    https://github.com/0day-ci/linux/commits/Kees-Cook/x86-refcount-Implement-fast-refcount-overflow/20170426-210530
>> > config: x86_64-allmodconfig (attached as .config)
>> > compiler: gcc-6 (Debian 6.2.0-3) 6.2.0 20160901
>> > reproduce:
>> >         # save the attached .config to linux build tree
>> >         make ARCH=x86_64
>> >
>> > All warnings (new ones prefixed by >>):
>> >
>> >>> drivers//scsi/scsi_scan.o: warning: objtool: .text.refcount_overflow+0x5: special: can't find orig instruction
>>
>> Hi Josh,
>>
>> I'm seeing this error being generated on areas that are using a
>> cross-section exception handler. I can't quite see why the .o checker
>> is unhappy, so I figured I'd ask you first. :)
>>
>> The code is generated with calls to __REFCOUNT_CHECK() which is
>> defined like this:
>>
>> +#define __REFCOUNT_EXCEPTION(size)                     \
>> +       ".if "__stringify(size)" == 4\n\t"              \
>> +       ".pushsection .text.refcount_overflow\n"        \
>> +       ".elseif "__stringify(size)" == -4\n\t"         \
>> +       ".pushsection .text.refcount_underflow\n"       \
>> +       ".else\n"                                       \
>> +       ".error \"invalid size\"\n"                     \
>> +       ".endif\n"                                      \
>> +       "111:\tlea %[counter],%%"_ASM_CX"\n\t"          \
>> +       "int $"__stringify(X86_REFCOUNT_VECTOR)"\n"     \
>> +       "222:\n\t"                                      \
>> +       ".popsection\n"                                 \
>> +       "333:\n"                                        \
>> +       _ASM_EXTABLE(222b, 333b)
>> +
>> +#define __REFCOUNT_CHECK(size)                         \
>> +       "js 111f\n"                                     \
>> +       __REFCOUNT_EXCEPTION(size)
>> +
>> +#define __REFCOUNT_ERROR(size)                         \
>> +       "jmp 111f\n"                                    \
>> +       __REFCOUNT_EXCEPTION(size)
>>
>> I assume it doesn't like seeing an exception split across .text and
>> .text.refcount_overflow, but I haven't been able to figure out how
>> that distinction would be made by the checker. :P
>
> This code uses the exception table a little differently than normal.
> Usually it's used for catching page faults, where the exception table
> points to the faulting instruction.
>
> But instead of a page fault, here it's doing a software interrupt.  So
> the __ex_table entry doesn't point to the 'int 0x81' instruction, it
> points to the instruction immediately after it.  In this case there
> isn't actually an instruction there, which is why objtool is
> complaining.

What would it take to adjust objtool for this case?

>
> Is it superfluous to use the exception table here, when a simple 'jmp
> 333f' could be used instead after the 'int'?

I thought the exception tables were needed to have the trap handler
notice it correctly, and do the right thing as far as continuing
execution. (This is currently written as a survivable condition: the
kernel can keep running even though it will kill the userspace
process.)

> Also it looks like the handler sends a SIGKILL to the current task.  I
> wonder if something like BUG_ON() could be used instead of implementing
> a custom error interrupt.

It's a rate limited report, but it must always kill. BUG doesn't fit
this usage case (I've got similar problems with other areas; my
intention is go create something that is configurable WARN vs Oops,
respects panic_on_oops, etc, but this doesn't exist yet).

-Kees

-- 
Kees Cook
Pixel Security