x86/mmap: properly account for stack randomization in mmap_base

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.

Obtained from Daniel Micay's linux-hardened tree.

Link: http://lkml.kernel.org/r/20170622200033.25714-2-riel@redhat.com
Signed-off-by: Daniel Micay <danielmicay@gmail.com>
Signed-off-by: Rik van Riel <riel@redhat.com>
Reported-by: Florian Weimer <fweimer@redhat.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Daniel Micay <danielmicay@gmail.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index 797295e792b2fd237beec9f2354972420a83b01d..229d04a83f8561ec08e394f57d8cb5d671bd56ff 100644 (file)
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -92,13 +92,18 @@ 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)