s390: limit brk randomization to 32MB

[ Upstream commit cd479eccd2e057116d504852814402a1e68ead80 ]

For a 64-bit process the randomization of the program break is quite
large with 1GB. That is as big as the randomization of the anonymous
mapping base, for a test case started with '/lib/ld64.so.1 <exec>'
it can happen that the heap is placed after the stack. To avoid
this limit the program break randomization to 32MB for 64-bit and
keep 8MB for 31-bit.
Reported-by: NStefan Liebler <stli@linux.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: NSasha Levin (Microsoft) <sashal@kernel.org>

arch/s390/include/asm/elf.h

@@ -252,11 +252,14 @@ do {								\
 
 /*
  * Cache aliasing on the latest machines calls for a mapping granularity
- * of 512KB. For 64-bit processes use a 512KB alignment and a randomization
- * of up to 1GB. For 31-bit processes the virtual address space is limited,
- * use no alignment and limit the randomization to 8MB.
+ * of 512KB for the anonymous mapping base. For 64-bit processes use a
+ * 512KB alignment and a randomization of up to 1GB. For 31-bit processes
+ * the virtual address space is limited, use no alignment and limit the
+ * randomization to 8MB.
+ * For the additional randomization of the program break use 32MB for
+ * 64-bit and 8MB for 31-bit.
  */
-#define BRK_RND_MASK	(is_compat_task() ? 0x7ffUL : 0x3ffffUL)
+#define BRK_RND_MASK	(is_compat_task() ? 0x7ffUL : 0x1fffUL)
 #define MMAP_RND_MASK	(is_compat_task() ? 0x7ffUL : 0x3ff80UL)
 #define MMAP_ALIGN_MASK	(is_compat_task() ? 0 : 0x7fUL)
 #define STACK_RND_MASK	MMAP_RND_MASK