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Date: Fri, 26 Feb 2016 14:19:16 -0800
From: Laura Abbott <>
To: Kees Cook <>
Cc: Laura Abbott <>,
        Greg Kroah-Hartman <>,
        Arnd Bergmann <>,
        "" <>,
        LKML <>
Subject: Re: [PATCHv2] lkdtm: Add READ_AFTER_FREE test

On 02/26/2016 08:03 AM, Kees Cook wrote:
> On Thu, Feb 25, 2016 at 3:15 PM, Laura Abbott <> wrote:
>> On 02/25/2016 09:35 AM, Kees Cook wrote:
>>> Ah-ha, yes, that was one of the missing pieces:
>>> [   10.790970] lkdtm: Performing direct entry READ_AFTER_FREE
>>> [   10.790992] lkdtm: Value in memory before free: 12345678
>>> [   10.790996] lkdtm: Attempting bad read from freed memory
>>> [   10.790998] lkdtm: Memory correctly poisoned, calling BUG
>>> [   10.791067] ------------[ cut here ]------------
>>> [   10.792037] kernel BUG at drivers/misc/lkdtm.c:465!
>>> I see that "F" is also needed to do the sanity checks, but the poison
>>> ends up being different again from what I was expected:
>>> [    8.643902] lkdtm: Performing direct entry WRITE_AFTER_FREE
>>> [    8.645215] lkdtm: Allocated memory ffff88007b446850-ffff88007b446c50
>>> [    8.646700] lkdtm: Attempting bad write to freed memory at
>>> ffff88007b446a50
>>> [    8.648295]
>>> =============================================================================
>>> [    8.649275] BUG kmalloc-1024 (Tainted: G      D        ): Poison
>>> overwritten
>>> [    8.649275]
>>> -----------------------------------------------------------------------------
>>> [    8.649275]
>>> [    8.649275] INFO: 0xffff88007b446a50-0xffff88007b446a53. First byte
>>> 0xf0 instead of 0x6b
>>> 0x6b is POISON_FREE:
>>> #define POISON_INUSE    0x5a    /* for use-uninitialised poisoning */
>>> #define POISON_FREE     0x6b    /* for use-after-free poisoning */
>>> #define POISON_END      0xa5    /* end-byte of poisoning */
>> Yep, 0x6b is a magic number I've seen all too frequently before ;)
>> The current poisoning with slub_debug=P covers multiple cases. On
>> alloc, the memory is set with POISON_INUSE to catch uninitailized
>> usage. on free, the memory is set to POISON_FREE To catch use after
>> free bugs. The last bit POISON_END is set at the end of the block
>> to catch users who might run off the end of the buffer. Having the
>> different values makes it easier to determine which bug it is.
>>> So it seems like there are separate poisonings going on? Modifying
>>> READ_AFTER_FREE a bit more, I see that it looks like only the buddy
>>> allocator is getting the zero poisoning?
>> Yes. The buddy allocator and SL*B allocators are two separate pieces
>> of code which need independent poisoning mechanisms. Currently, only
>> the buddy allocator has the zero poisoning. The same functionality
>> can be added to SL*B allocator as well if it seems beneficial.
> My concerns are with the performance characteristics, mostly. To match
> PAX_MEMORY_SANITIZE, zero poisoning almost everything should get us
> into the 3% range, I'm hoping.

I did a quick hack of zero poisoning for the slab allocator and I
didn't see any improvement in hackbench performance which is fairly
sensitive to slab performance. This doesn't surprise me when I
actually think about it.

Before I sent out my last set of performance optimizations for
SLUB debug path, I did a profile with ftrace to see if there was
anything else quick I could do. My profiling showed that the
poisoning itself was not where most of the allocation time was
spent. 25-50% of the time was being spent in removing the CPU slab.
Considering poisoning means that the CPU slab is never really used,
this can probably be improved. It's worth noting that the
PAX_MEMORY_SANITIZE implementation still uses the fast path so it
isn't affected here. (The trade off is a minor penalty on the
fast path even when poisoning is disabled which isn't acceptable
to the maintainers currently.)

Basically, until we've optimized other things I don't think the
zero poisoning will have a significant effect on performance.
The next set of optimizations will involve changing some of the
guts of the SLUB allocator. I have some ideas how to approach this
but we'll see if they pan out.


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