Re: [PATCH] fs/binfmt_elf_fdpic.c: fix brk area overlap with stack on NOMMU

[Greg Ungerer]

Hi Rich,

On 15/09/15 01:17, Rich Felker wrote:
> On Mon, Sep 14, 2015 at 10:13:03PM +1000, Greg Ungerer wrote:
>> On 26/08/15 11:26, Greg Ungerer wrote:
>>> On 21/08/15 05:11, Rich Felker wrote:
>>>> From: Rich Felker <dalias@xxxxxxxx>
>>>>
>>>> On NOMMU archs, the FDPIC ELF loader sets up the usable brk range to
>>>> overlap with all but the last PAGE_SIZE bytes of the stack. This leads
>>>> to catastrophic memory reuse/corruption if brk is used. Fix by setting
>>>> the brk area to zero size to disable its use.
>>>>
>>>> Signed-off-by: Rich Felker <dalias@xxxxxxxx>
>>>
>>> It would make sense to run this by David Howells <dhowells@xxxxxxxxxx>,
>>> I think he wrote this code (added to CC list).
>>>
>>> I have no problem with it, so:
>>>
>>> Acked-by: Greg Ungerer <gerg@xxxxxxxxxxx>
>>
>> Has anybody picked this up to push to Linus?
>> If not I can take it via the m68knommu tree.
> 
> As far as I know, no. If you can do it that would be great.

Patch applied to m68knommu git tree (for-next branch).
(https://git.kernel.org/cgit/linux/kernel/git/gerg/m68knommu.git/)

Regards
Greg


>>>> ---
>>>>
>>>> There is no reason for the kernel to be providing a brk area at all on
>>>> NOMMU; the bFLT loader does not provide one, uClibc never uses brk on
>>>> NOMMU targets, and musl libc goes out of its way to avoid using brk
>>>> that might run into the stack.
>>>
>>> I recall a long time back someone was playing with the idea of setting
>>> the brk to the unused parts of the last data area page. (Somewhat like
>>> this code seems to be trying). That scheme still allocated the full
>>> requested stack size (IIRC) though. And that would have been on bFLT
>>> executables. Anyway, just some historical reference, not really
>>> relevant now.
>>>
>>> Regards
>>> Greg
>>>
>>>
>>>
>>>> --- fs/binfmt_elf_fdpic.c.orig	2015-08-20 18:05:19.089888654 +0000
>>>> +++ fs/binfmt_elf_fdpic.c	2015-08-20 18:10:01.519871432 +0000
>>>> @@ -374,10 +388,7 @@ static int load_elf_fdpic_binary(struct
>>>>  		PAGE_ALIGN(current->mm->start_brk);
>>>>
>>>>  #else
>>>> -	/* create a stack and brk area big enough for everyone
>>>> -	 * - the brk heap starts at the bottom and works up
>>>> -	 * - the stack starts at the top and works down
>>>> -	 */
>>>> +	/* create a stack area and zero-size brk area */
>>>>  	stack_size = (stack_size + PAGE_SIZE - 1) & PAGE_MASK;
>>>>  	if (stack_size < PAGE_SIZE * 2)
>>>>  		stack_size = PAGE_SIZE * 2;
>>>> @@ -400,8 +411,6 @@ static int load_elf_fdpic_binary(struct
>>>>
>>>>  	current->mm->brk = current->mm->start_brk;
>>>>  	current->mm->context.end_brk = current->mm->start_brk;
>>>> -	current->mm->context.end_brk +=
>>>> -		(stack_size > PAGE_SIZE) ? (stack_size - PAGE_SIZE) : 0;
>>>>  	current->mm->start_stack = current->mm->start_brk + stack_size;
>>>>  #endif
>>>>
>>>>
>>>
> 

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