Messages in this thread | | | Date | Fri, 30 Jun 2017 07:51:51 -0700 | From | Jörn Engel <> | Subject | Re: [PATCH] mm: Fix overflow check in expand_upwards() |
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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@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.
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
-- The Linux community is zillions of people with different cultures and ideas all trying to convince the rest that their vision of the shared culture is right. -- Alan Cox
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