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    SubjectRe: [PATCH v4] x86: increase MIN_GAP to include randomized stack
    On Tue, 8 Sep 2009, Michal Hocko wrote:

    > Currently we are not including randomized stack size when calculating
    > mmap_base address in arch_pick_mmap_layout for topdown case. This might
    > cause that mmap_base starts in the stack reserved area because stack is
    > randomized by 1GB for 64b (8MB for 32b) and the minimum gap is 128MB.
    >
    > If the stack really grows down to mmap_base then we can get silent mmap
    > region overwrite by the stack values.
    >
    > Let's include maximum stack randomization size into MIN_GAP which is
    > used as the low bound for the gap in mmap.
    >
    > Signed-off-by: Michal Hocko <mhocko@suse.cz>

    Acked-by: Jiri Kosina <jkosina@suse.cz>

    Ingo, could you please consider applying this?

    > ---
    > arch/x86/include/asm/elf.h | 2 ++
    > arch/x86/mm/mmap.c | 17 +++++++++++++++--
    > 2 files changed, 17 insertions(+), 2 deletions(-)
    >
    > diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
    > index 83c1bc8..456a304 100644
    > --- a/arch/x86/include/asm/elf.h
    > +++ b/arch/x86/include/asm/elf.h
    > @@ -299,6 +299,8 @@ do { \
    >
    > #ifdef CONFIG_X86_32
    >
    > +#define STACK_RND_MASK (0x7ff)
    > +
    > #define VDSO_HIGH_BASE (__fix_to_virt(FIX_VDSO))
    >
    > #define ARCH_DLINFO ARCH_DLINFO_IA32(vdso_enabled)
    > diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
    > index 1658296..c8191de 100644
    > --- a/arch/x86/mm/mmap.c
    > +++ b/arch/x86/mm/mmap.c
    > @@ -29,13 +29,26 @@
    > #include <linux/random.h>
    > #include <linux/limits.h>
    > #include <linux/sched.h>
    > +#include <asm/elf.h>
    > +
    > +static unsigned int stack_maxrandom_size(void)
    > +{
    > + unsigned int max = 0;
    > + if ((current->flags & PF_RANDOMIZE) &&
    > + !(current->personality & ADDR_NO_RANDOMIZE)) {
    > + max = ((-1U) & STACK_RND_MASK) << PAGE_SHIFT;
    > + }
    > +
    > + return max;
    > +}
    > +
    >
    > /*
    > * Top of mmap area (just below the process stack).
    > *
    > - * Leave an at least ~128 MB hole.
    > + * Leave an at least ~128 MB hole with possible stack randomization.
    > */
    > -#define MIN_GAP (128*1024*1024)
    > +#define MIN_GAP (128*1024*1024UL + stack_maxrandom_size())
    > #define MAX_GAP (TASK_SIZE/6*5)
    >
    > /*
    > --
    > 1.6.3.3
    >

    --
    Jiri Kosina
    SUSE Labs


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