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/arm64/mm/mmap.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/mm/mmap.c b/arch/arm64/mm/mmap.c
index 7b0d55756eb1..a0cb6b8ccde7 100644
--- a/arch/arm64/mm/mmap.c
+++ b/arch/arm64/mm/mmap.c
@@ -34,7 +34,7 @@
  * Leave enough space between the mmap area and the stack to honour ulimit in
  * the face of randomisation.
  */
-#define MIN_GAP (SZ_128M + ((STACK_RND_MASK << PAGE_SHIFT) + 1))
+#define MIN_GAP (SZ_128M)
 #define MAX_GAP        (STACK_TOP/6*5)
 
 static int mmap_is_legacy(void)
@@ -64,6 +64,11 @@ unsigned long arch_mmap_rnd(void)
 static unsigned long mmap_base(unsigned long rnd)
 {
        unsigned long gap = rlimit(RLIMIT_STACK);
+       unsigned long pad = (STACK_RND_MASK << PAGE_SHIFT) + stack_guard_gap;
+
+       /* Values close to RLIM_INFINITY can overflow. */
+       if (gap + pad > gap)
+               gap += pad;
 
        if (gap < MIN_GAP)
                gap = MIN_GAP;
-- 
2.9.4

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