From: Rik van Riel <r...@redhat.com>

When RLIMIT_STACK is, for example, 256MB, the current code results in
a gap between the top of the task and mmap_base of 256MB, failing to
take into account the amount by which the stack address was randomized.
In other words, the stack gets less than RLIMIT_STACK space.

Ensure that the gap between the stack and mmap_base always takes stack
randomization and the stack guard gap into account.

>From Daniel Micay's linux-hardened tree.

Reported-by: Florian Weimer <fwei...@redhat.com>
Signed-off-by: Daniel Micay <danielmi...@gmail.com>
Signed-off-by: Rik van Riel <r...@redhat.com>
---
 arch/x86/mm/mmap.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index 19ad095b41df..7c35dd73dbd4 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -95,13 +95,18 @@ unsigned long arch_mmap_rnd(void)
 static unsigned long mmap_base(unsigned long rnd, unsigned long task_size)
 {
        unsigned long gap = rlimit(RLIMIT_STACK);
+       unsigned long pad = stack_maxrandom_size(task_size) + stack_guard_gap;
        unsigned long gap_min, gap_max;
 
+       /* Values close to RLIM_INFINITY can overflow. */
+       if (gap + pad > gap)
+               gap += pad;
+
        /*
         * Top of mmap area (just below the process stack).
         * Leave an at least ~128 MB hole with possible stack randomization.
         */
-       gap_min = SIZE_128M + stack_maxrandom_size(task_size);
+       gap_min = SIZE_128M;
        gap_max = (task_size / 6) * 5;
 
        if (gap < gap_min)
-- 
2.9.4

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