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SubjectRe: [PATCH] mm: Fix overflow check in expand_upwards()
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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@gmx.de>
> 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@logfs.org>
> ---
> 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.

Did you faced a real issue?

Helge

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