Subject: Re: [PATCH] Restrict initial stack space expansion to rlimit

From: Helge Deller <[email-masked]>
Date: Tue, 09 Feb 2010 23:27:08 +0100

On 02/09/2010 10:51 PM, Michael Neuling wrote:
>>> I'd still like someone with a CONFIG_STACK_GROWSUP arch to test/ACK it
>>> as well.
>>
>> There's only one CONFIG_GROWSUP arch - parisc.
>> Could someone please test it on parisc?

I did.

> How about doing:
>    'ulimit -s 15; ls'
> before and after the patch is applied.  Before it's applied, 'ls' should
> be killed.  After the patch is applied, 'ls' should no longer be killed.
>
> I'm suggesting a stack limit of 15KB since it's small enough to trigger
> 20*PAGE_SIZE.  Also 15KB not a multiple of PAGE_SIZE, which is a trickier
> case to handle correctly with this code.
>
> 4K pages on parisc should be fine to test with.

Mikey, thanks for the suggested test plan.

I'm not sure if your patch does it correct for parisc/stack-grows-up-case.

I tested your patch on  a 4k pages kernel:
root@c3000:~# uname -a
Linux c3000 2.6.33-rc7-32bit #221 Tue Feb 9 23:17:06 CET 2010 parisc GNU/Linux

Without your patch:
root@c3000:~# ulimit -s 15; ls
Killed
-> correct.

With your patch:
root@c3000:~# ulimit -s 15; ls
Killed
_or_:
root@c3000:~# ulimit -s 15; ls
Segmentation fault
-> ??

Any idea?

Helge

>> From: Michael Neuling<[email-masked]>
>>
>> When reserving stack space for a new process, make sure we're not
>> attempting to expand the stack by more than rlimit allows.
>>
>> This fixes a bug caused by b6a2fea39318e43fee84fa7b0b90d68bed92d2ba ("mm:
>> variable length argument support") and unmasked by
>> fc63cf237078c86214abcb2ee9926d8ad289da9b ("exec: setup_arg_pages() fails
>> to return errors").
>>
>> This bug means that when limiting the stack to less the 20*PAGE_SIZE (eg.
>> 80K on 4K pages or 'ulimit -s 79') all processes will be killed before
>> they start.  This is particularly bad with 64K pages, where a ulimit below
>> 1280K will kill every process.
>>
>> Signed-off-by: Michael Neuling<[email-masked]>
>> Cc: KOSAKI Motohiro<[email-masked]>
>> Cc: Americo Wang<[email-masked]>
>> Cc: Anton Blanchard<[email-masked]>
>> Cc: Oleg Nesterov<[email-masked]>
>> Cc: James Morris<[email-masked]>
>> Cc: Ingo Molnar<[email-masked]>
>> Cc: Serge Hallyn<[email-masked]>
>> Cc: Benjamin Herrenschmidt<[email-masked]>
>> Cc:<[email-masked]>
>>
>>   fs/exec.c |   21 +++++++++++++++++++--
>>   1 file changed, 19 insertions(+), 2 deletions(-)
>>
>> diff -puN fs/exec.c~fs-execc-restrict-initial-stack-space-expansion-to-rlimit
>   fs/exec.c
>> --- a/fs/exec.c~fs-execc-restrict-initial-stack-space-expansion-to-rlimit
>> +++ a/fs/exec.c
>> @@ -571,6 +571,9 @@ int setup_arg_pages(struct linux_binprm
>>   	struct vm_area_struct *prev = NULL;
>>   	unsigned long vm_flags;
>>   	unsigned long stack_base;
>> +	unsigned long stack_size;
>> +	unsigned long stack_expand;
>> +	unsigned long rlim_stack;
>>
>>   #ifdef CONFIG_STACK_GROWSUP
>>   	/* Limit stack size to 1GB */
>> @@ -627,10 +630,24 @@ int setup_arg_pages(struct linux_binprm
>>   			goto out_unlock;
>>   	}
>>
>> +	stack_expand = EXTRA_STACK_VM_PAGES * PAGE_SIZE;
>> +	stack_size = vma->vm_end - vma->vm_start;
>> +	/*
>> +	 * Align this down to a page boundary as expand_stack
>> +	 * will align it up.
>> +	 */
>> +	rlim_stack = rlimit(RLIMIT_STACK)&  PAGE_MASK;
>> +	rlim_stack = min(rlim_stack, stack_size);
>>   #ifdef CONFIG_STACK_GROWSUP
>> -	stack_base = vma->vm_end + EXTRA_STACK_VM_PAGES * PAGE_SIZE;
>> +	if (stack_size + stack_expand>  rlim_stack)
>> +		stack_base = vma->vm_start + rlim_stack;
>> +	else
>> +		stack_base = vma->vm_end + stack_expand;
>>   #else
>> -	stack_base = vma->vm_start - EXTRA_STACK_VM_PAGES * PAGE_SIZE;
>> +	if (stack_size + stack_expand>  rlim_stack)
>> +		stack_base = vma->vm_end - rlim_stack;
>> +	else
>> +		stack_base = vma->vm_start - stack_expand;
>>   #endif
>>   	ret = expand_stack(vma, stack_base);
>>   	if (ret)