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

[Jörn Engel]

On Fri, Jun 30, 2017 at 08:57:27AM +0200, Helge Deller wrote:
> On 30.06.2017 01:02, Jörn Engel wrote:
> > I believe the overflow check was correct, then got subtly broken by
> > 	commit bd726c90b6b8
> > 	Author: Helge Deller <deller@xxxxxx>
> > 	Date:   Mon Jun 19 17:34:05 2017 +0200
> > 
> > 	    Allow stack to grow up to address space limit
> > 
> > The old overflow check may have been a bit subtle and I suppose Helge
> > missed its importance.
> > 
> > 	if (!address)
> > 		return -ENOMEM;
> > 
> > Functionally the my check is identical to the old one.  I just hope the
> > alternative form (and comment!) make it harder to miss and break things
> > in a future patch.
> > 
> > Signed-off-by: Joern Engel <joern@xxxxxxxxx>
> > ---
> >  mm/mmap.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index a5e3dcd75e79..7366f62c31f4 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -2232,7 +2232,8 @@ int expand_upwards(struct vm_area_struct *vma, unsigned long address)
> >  
> >  	/* Guard against exceeding limits of the address space. */
> >  	address &= PAGE_MASK;
> > -	if (address >= TASK_SIZE)
> > +	/* second check is for integer overflow */
> > +	if (address >= TASK_SIZE || address + PAGE_SIZE < address)
> >  		return -ENOMEM;
> >  	address += PAGE_SIZE;
> 
> That overflow check is still there.
> Look at the next few lines in mmap.c:
> 
>        /* Enforce stack_guard_gap */
>         gap_addr = address + stack_guard_gap;
> 
>         /* Guard against overflow */
>         if (gap_addr < address || gap_addr > TASK_SIZE)
>                 gap_addr = TASK_SIZE;
> 
> If the requested page plus the gap (=gap_addr) wraps around, then the
> code will limit it to TASK_SIZE.
> So, the code should already take care of all possible areas >=TASK_SIZE,
> including wrap-arounds.

Does it cover the case where address is (unsigned long)-PAGE_SIZE?

I believe you catch every other case, but not that one.

> Did you faced a real issue?

No.  I don't even own a computer with stacks growing up.  Just spotted
this while reviewing some patches going by.

Jörn

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