linux-kernel - Re: [kernel-hardening] [PATCH 22/23] usercopy: split user-controlled slabs to separate caches

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Date:   Tue, 20 Jun 2017 15:27:44 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        David Windsor <dave@...lcore.net>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [kernel-hardening] [PATCH 22/23] usercopy: split user-controlled
 slabs to separate caches

On Mon, Jun 19, 2017 at 9:47 PM, Eric Biggers <ebiggers3@...il.com> wrote:
> On Mon, Jun 19, 2017 at 04:36:36PM -0700, Kees Cook wrote:
>> From: David Windsor <dave@...lcore.net>
>>
>> Some userspace APIs (e.g. ipc, seq_file) provide precise control over
>> the size of kernel kmallocs, which provides a trivial way to perform
>> heap overflow attacks where the attacker must control neighboring
>> allocations of a specific size. Instead, move these APIs into their own
>> cache so they cannot interfere with standard kmallocs. This is enabled
>> with CONFIG_HARDENED_USERCOPY_SPLIT_KMALLOC.
>>
>> This patch is modified from Brad Spengler/PaX Team's PAX_USERCOPY_SLABS
>> code in the last public patch of grsecurity/PaX based on my understanding
>> of the code. Changes or omissions from the original code are mine and
>> don't reflect the original grsecurity/PaX code.
>>
>> Signed-off-by: David Windsor <dave@...lcore.net>
>> [kees: added SLAB_NO_MERGE flag to allow split of future no-merge Kconfig]
>> Signed-off-by: Kees Cook <keescook@...omium.org>
>> ---
>>  fs/seq_file.c        |  2 +-
>>  include/linux/gfp.h  |  9 ++++++++-
>>  include/linux/slab.h | 12 ++++++++++++
>>  ipc/msgutil.c        |  5 +++--
>>  mm/slab.h            |  3 ++-
>>  mm/slab_common.c     | 29 ++++++++++++++++++++++++++++-
>>  security/Kconfig     | 12 ++++++++++++
>>  7 files changed, 66 insertions(+), 6 deletions(-)
>>
>> diff --git a/fs/seq_file.c b/fs/seq_file.c
>> index dc7c2be963ed..5caa58a19bdc 100644
>> --- a/fs/seq_file.c
>> +++ b/fs/seq_file.c
>> @@ -25,7 +25,7 @@ static void seq_set_overflow(struct seq_file *m)
>>
>>  static void *seq_buf_alloc(unsigned long size)
>>  {
>> -     return kvmalloc(size, GFP_KERNEL);
>> +     return kvmalloc(size, GFP_KERNEL | GFP_USERCOPY);
>>  }
>>
>
> Also forgot to mention the obvious: there are way more places where GFP_USERCOPY
> would need to be (or should be) used.  Helper functions like memdup_user() and
> memdup_user_nul() would be the obvious ones.  And just a random example, some of
> the keyrings syscalls (callable with no privileges) do a kmalloc() with
> user-controlled contents and size.

Looking again at how grsecurity uses it, they have some of those call
sites a couple more (keyctl, char/mem, kcore, memdup_user). Getting
the facility in place at all is a good first step, IMO.

>
> So I think this by itself needs its own patch series.

Sounds reasonable.

-Kees

-- 
Kees Cook
Pixel Security