Re: [PATCH v6 09/14] mips: Properly account for stack randomization and stack guard gap

From: Alexandre Ghiti <alex@ghiti.fr>
To: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>,
	Andrew Morton <akpm@linux-foundation.org>
Cc: Albert Ou <aou@eecs.berkeley.edu>,
	Kees Cook <keescook@chromium.org>,
	James Hogan <jhogan@kernel.org>,
	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>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Luis Chamberlain <mcgrof@kernel.org>,
	Paul Burton <paul.burton@mips.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	linux-fsdevel@vger.kernel.org, linux-riscv@lists.infradead.org,
	linux-mips@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
	linux-arm-kernel@lists.infradead.org,
	Alexander Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH v6 09/14] mips: Properly account for stack randomization and stack guard gap
Date: Fri, 9 Aug 2019 11:44:38 +0200	[thread overview]
Message-ID: <91e31484-b268-2c90-1dd1-98cec349af6c@ghiti.fr> (raw)
In-Reply-To: <bd67507e-8a5b-34b5-1a33-5500bbb724b2@cogentembedded.com>

On 8/8/19 11:16 AM, Sergei Shtylyov wrote:
> Hello!
>
> On 08.08.2019 9:17, Alexandre Ghiti 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://lkml.kernel.org/r/20170622200033.25714-1-riel@redhat.com
>>
>> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
>> Acked-by: Kees Cook <keescook@chromium.org>
>> Acked-by: Paul Burton <paul.burton@mips.com>
>> Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
>> ---
>>   arch/mips/mm/mmap.c | 14 ++++++++++++--
>>   1 file changed, 12 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/mips/mm/mmap.c b/arch/mips/mm/mmap.c
>> index d79f2b432318..f5c778113384 100644
>> --- a/arch/mips/mm/mmap.c
>> +++ b/arch/mips/mm/mmap.c
>> @@ -21,8 +21,9 @@ unsigned long shm_align_mask = PAGE_SIZE - 1;    /* 
>> Sane caches */
>>   EXPORT_SYMBOL(shm_align_mask);
>>     /* gap between mmap and stack */
>> -#define MIN_GAP (128*1024*1024UL)
>> -#define MAX_GAP ((TASK_SIZE)/6*5)
>> +#define MIN_GAP        (128*1024*1024UL)
>> +#define MAX_GAP        ((TASK_SIZE)/6*5)
>
>    Could add spaces around *, while touching this anyway? And parens
> around TASK_SIZE shouldn't be needed...
>

I did not fix checkpatch warnings here since this code gets removed 
afterwards.

>> +#define STACK_RND_MASK    (0x7ff >> (PAGE_SHIFT - 12))
>>     static int mmap_is_legacy(struct rlimit *rlim_stack)
>>   {
>> @@ -38,6 +39,15 @@ 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);
>
>    Parens not needed here.

Belt and braces approach here as I'm never sure about priorities.

Thanks for your time,

Alex

>
>> +
>> +    /* Values close to RLIM_INFINITY can overflow. */
>> +    if (gap + pad > gap)
>> +        gap += pad;
>>         if (gap < MIN_GAP)
>>           gap = MIN_GAP;
>>
>
>
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