Re: [PATCH v3 05/11] arm: Properly account for stack randomization and stack guard gap

[Alex Ghiti]

On 4/18/19 1:26 AM, Kees Cook wrote:
> On Wed, Apr 17, 2019 at 12:28 AM Alexandre Ghiti <alex@xxxxxxxx> wrote:
> > This commit takes care of stack randomization and stack guard gap when
> > computing mmap base address and checks if the task asked for randomization.
> > This fixes the problem uncovered and not fixed for arm here:
> > https://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg1429066.html
> Please use the official archive instead. This includes headers, linked
> patches, etc:
> https://lkml.kernel.org/r/20170622200033.25714-1-riel@xxxxxxxxxx


Ok, sorry about that, and thanks for the info.


> > Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
> > ---
> >   arch/arm/mm/mmap.c | 14 ++++++++++++--
> >   1 file changed, 12 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm/mm/mmap.c b/arch/arm/mm/mmap.c
> > index f866870db749..bff3d00bda5b 100644
> > --- a/arch/arm/mm/mmap.c
> > +++ b/arch/arm/mm/mmap.c
> > @@ -18,8 +18,9 @@
> >           (((pgoff)<<PAGE_SHIFT) & (SHMLBA-1)))
> >
> >   /* gap between mmap and stack */
> > -#define MIN_GAP (128*1024*1024UL)
> > -#define MAX_GAP ((TASK_SIZE)/6*5)
> > +#define MIN_GAP                (128*1024*1024UL)
> Might as well fix this up as SIZE_128M


I left the code as is because it gets removed in the next commit, I did not
even correct the checkpatch warnings. But I can fix that in v4, since there
will be a v4 :)


> > +#define MAX_GAP                ((TASK_SIZE)/6*5)
> > +#define STACK_RND_MASK (0x7ff >> (PAGE_SHIFT - 12))
> STACK_RND_MASK is already defined so you don't need to add it here, yes?


At this point, I don't think arm has STACK_RND_MASK defined anywhere since
the generic version is in mm/util.c.


> >   static int mmap_is_legacy(struct rlimit *rlim_stack)
> >   {
> > @@ -35,6 +36,15 @@ static int mmap_is_legacy(struct rlimit *rlim_stack)
> >   static unsigned long mmap_base(unsigned long rnd, struct rlimit *rlim_stack)
> >   {
> >          unsigned long gap = rlim_stack->rlim_cur;
> > +       unsigned long pad = stack_guard_gap;
> > +
> > +       /* Account for stack randomization if necessary */
> > +       if (current->flags & PF_RANDOMIZE)
> > +               pad += (STACK_RND_MASK << PAGE_SHIFT);
> > +
> > +       /* Values close to RLIM_INFINITY can overflow. */
> > +       if (gap + pad > gap)
> > +               gap += pad;
> >
> >          if (gap < MIN_GAP)
> >                  gap = MIN_GAP;
> > --
> > 2.20.1
> But otherwise, yes:
>
> Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>


Thanks !


> --
> Kees Cook
>
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