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Date: Mon, 1 May 2017 10:28:53 -0700
From: Kees Cook <keescook@...omium.org>
To: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: LKML <linux-kernel@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org>,
PaX Team <pageexec@...email.hu>, Jann Horn <jannh@...gle.com>,
Eric Biggers <ebiggers3@...il.com>, Christoph Hellwig <hch@...radead.org>,
"axboe@...nel.dk" <axboe@...nel.dk>, James Bottomley <James.Bottomley@...senpartnership.com>,
Elena Reshetova <elena.reshetova@...el.com>, Hans Liljestrand <ishkamiel@...il.com>,
David Windsor <dwindsor@...il.com>, "x86@...nel.org" <x86@...nel.org>, Ingo Molnar <mingo@...nel.org>,
Arnd Bergmann <arnd@...db.de>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
"David S. Miller" <davem@...emloft.net>, Rik van Riel <riel@...hat.com>,
linux-arch <linux-arch@...r.kernel.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v2 2/2] x86, refcount: Implement fast refcount overflow protection
On Mon, May 1, 2017 at 8:54 AM, Josh Poimboeuf <jpoimboe@...hat.com> 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@...el.com> 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
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