Messages in this thread | | | Subject | Re: [PATCH] mm: Fix overflow check in expand_upwards() | From | Helge Deller <> | Date | Fri, 30 Jun 2017 08:57:27 +0200 |
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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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