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Subject: Re: [PATCH v3 05/11] arm: Properly account for stack randomization
 and stack guard gap
To:     Kees Cook <keescook@chromium.org>
Cc:     Albert Ou <aou@eecs.berkeley.edu>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Palmer Dabbelt <palmer@sifive.com>,
        Will Deacon <will.deacon@arm.com>,
        Russell King <linux@armlinux.org.uk>,
        Ralf Baechle <ralf@linux-mips.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        Paul Burton <paul.burton@mips.com>,
        linux-riscv@lists.infradead.org,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        James Hogan <jhogan@kernel.org>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-mips@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        Luis Chamberlain <mcgrof@kernel.org>
References: <20190417052247.17809-1-alex@ghiti.fr>
 <20190417052247.17809-6-alex@ghiti.fr>
 <CAGXu5j+V_kJk-Lu=u82CrA291EPpgJtX951EKigprozXt7=ORA@mail.gmail.com>
From:   Alex Ghiti <alex@ghiti.fr>
Message-ID: <dabdc658-62b0-4854-f84f-9c4672fce842@ghiti.fr>
Date:   Thu, 18 Apr 2019 02:01:20 -0400
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On 4/18/19 1:26 AM, Kees Cook wrote:
> On Wed, Apr 17, 2019 at 12:28 AM Alexandre Ghiti <alex@ghiti.fr> wrote:
>> This commit takes care of stack randomization and stack guard gap when
>> computing mmap base address and checks if the task asked for randomization.
>> This fixes the problem uncovered and not fixed for arm here:
>> https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1429066.html
> Please use the official archive instead. This includes headers, linked
> patches, etc:
> https://lkml.kernel.org/r/20170622200033.25714-1-riel@redhat.com


Ok, sorry about that, and thanks for the info.


>
>> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
>> ---
>>   arch/arm/mm/mmap.c | 14 ++++++++++++--
>>   1 file changed, 12 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm/mm/mmap.c b/arch/arm/mm/mmap.c
>> index f866870db749..bff3d00bda5b 100644
>> --- a/arch/arm/mm/mmap.c
>> +++ b/arch/arm/mm/mmap.c
>> @@ -18,8 +18,9 @@
>>           (((pgoff)<<PAGE_SHIFT) & (SHMLBA-1)))
>>
>>   /* gap between mmap and stack */
>> -#define MIN_GAP (128*1024*1024UL)
>> -#define MAX_GAP ((TASK_SIZE)/6*5)
>> +#define MIN_GAP                (128*1024*1024UL)
> Might as well fix this up as SIZE_128M


I left the code as is because it gets removed in the next commit, I did not
even correct the checkpatch warnings. But I can fix that in v4, since there
will be a v4 :)


>
>> +#define MAX_GAP                ((TASK_SIZE)/6*5)
>> +#define STACK_RND_MASK (0x7ff >> (PAGE_SHIFT - 12))
> STACK_RND_MASK is already defined so you don't need to add it here, yes?


At this point, I don't think arm has STACK_RND_MASK defined anywhere since
the generic version is in mm/util.c.


>
>>   static int mmap_is_legacy(struct rlimit *rlim_stack)
>>   {
>> @@ -35,6 +36,15 @@ static int mmap_is_legacy(struct rlimit *rlim_stack)
>>   static unsigned long mmap_base(unsigned long rnd, struct rlimit *rlim_stack)
>>   {
>>          unsigned long gap = rlim_stack->rlim_cur;
>> +       unsigned long pad = stack_guard_gap;
>> +
>> +       /* Account for stack randomization if necessary */
>> +       if (current->flags & PF_RANDOMIZE)
>> +               pad += (STACK_RND_MASK << PAGE_SHIFT);
>> +
>> +       /* Values close to RLIM_INFINITY can overflow. */
>> +       if (gap + pad > gap)
>> +               gap += pad;
>>
>>          if (gap < MIN_GAP)
>>                  gap = MIN_GAP;
>> --
>> 2.20.1
>>
> But otherwise, yes:
>
> Acked-by: Kees Cook <keescook@chromium.org>


Thanks !


>
> --
> Kees Cook
>
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