Re: [PATCH] mm: Fix overflow check in expand_upwards()

["Jörn Engel" <joern@purestorage.com>]

On Fri, Jun 30, 2017 at 09:34:24AM +0200, Helge Deller wrote:
> 
> I see there are some architectures which define TASK_SIZE not as
> multiple of PAGE_SIZE and as 0xffffffff for which the (address >=
> TASK_SIZE) check will not trigger:
> 
> arch/arm/include/asm/memory.h:#define TASK_SIZE         UL(0xffffffff)
> arch/frv/include/asm/mem-layout.h:#define TASK_SIZE                     __UL(0xFFFFFFFFUL)
> arch/m68k/include/asm/processor.h:#define TASK_SIZE     (0xFFFFFFFFUL)
> arch/blackfin/include/asm/processor.h:#define TASK_SIZE 0xFFFFFFFF
> arch/h8300/include/asm/processor.h:#define TASK_SIZE    (0xFFFFFFFFUL)
> arch/xtensa/include/asm/processor.h:#define TASK_SIZE   __XTENSA_UL_CONST(0xffffffff)
> 
> None of those have an upwards growing stack and thus I believe we don't
> run into issues, but nevertheless the checks could probably be changed
> like this (untested patch):

That would also work.  I have no preference which patch to use.

> diff --git a/mm/mmap.c b/mm/mmap.c
> index a5e3dcd..224bdc2 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2224,15 +2224,17 @@ int expand_upwards(struct vm_area_struct *vma, unsigned long address)
>  {
>  	struct mm_struct *mm = vma->vm_mm;
>  	struct vm_area_struct *next;
> -	unsigned long gap_addr;
> +	unsigned long gap_addr, max_task_size;
>  	int error = 0;
>  
>  	if (!(vma->vm_flags & VM_GROWSUP))
>  		return -EFAULT;
>  
> +	max_task_size = TASK_SIZE & PAGE_MASK;
> +
>  	/* Guard against exceeding limits of the address space. */
>  	address &= PAGE_MASK;
> -	if (address >= TASK_SIZE)
> +	if (address >= max_task_size)
>  		return -ENOMEM;
>  	address += PAGE_SIZE;
>  
> @@ -2240,8 +2242,8 @@ int expand_upwards(struct vm_area_struct *vma, unsigned long address)
>  	gap_addr = address + stack_guard_gap;
>  
>  	/* Guard against overflow */
> -	if (gap_addr < address || gap_addr > TASK_SIZE)
> -		gap_addr = TASK_SIZE;
> +	if (gap_addr < address || gap_addr > max_task_size)
> +		gap_addr = max_task_size;
>  
>  	next = vma->vm_next;
>  	if (next && next->vm_start < gap_addr) {
> 
> Helge

Jörn

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