On Mon, May 1, 2017 at 8:54 AM, Josh Poimboeuf [off-list ref] wrote:

On Thu, Apr 27, 2017 at 01:22:05PM -0700, Kees Cook wrote:

quoted

On Wed, Apr 26, 2017 at 6:31 PM, kbuild test robot [off-list ref] wrote:

quoted

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 >>):

quoted

quoted

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