brk randomization: introduce CONFIG_COMPAT_BRK

based on similar patch from: Pavel Machek <pavel@ucw.cz>

Introduce CONFIG_COMPAT_BRK. If disabled then the kernel is free
(but not obliged to) randomize the brk area.

Heap randomization breaks ancient binaries, so we keep COMPAT_BRK
enabled by default.

Signed-off-by: Ingo Molnar <mingo@elte.hu>

3 files changed, 25 insertions, 2 deletions

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 4628c42ca892..111771d38e6e 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -1077,7 +1077,7 @@ static int load_elf_binary(struct linux_binprm *bprm, struct pt_regs *regs)
         current->mm->start_stack = bprm->p;
 
 #ifdef arch_randomize_brk
-        if (current->flags & PF_RANDOMIZE)
+        if ((current->flags & PF_RANDOMIZE) && (randomize_va_space > 1))
                 current->mm->brk = current->mm->start_brk =
                         arch_randomize_brk(current->mm);
 #endif
diff --git a/init/Kconfig b/init/Kconfig
index 87f50df58893..92b23e256614 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -541,6 +541,18 @@ config ELF_CORE
         help
           Enable support for generating core dumps. Disabling saves about 4k.
 
+config COMPAT_BRK
+        bool "Disable heap randomization"
+        default y
+        help
+          Randomizing heap placement makes heap exploits harder, but it
+          also breaks ancient binaries (including anything libc5 based).
+          This option changes the bootup default to heap randomization
+          disabled, and can be overriden runtime by setting
+          /proc/sys/kernel/randomize_va_space to 2.
+
+          On non-ancient distros (post-2000 ones) Y is usually a safe choice.
+
 config BASE_FULL
         default y
         bool "Enable full-sized data structures for core" if EMBEDDED
diff --git a/mm/memory.c b/mm/memory.c
index 7bb70728bb52..9d073fa0a2d0 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -82,7 +82,18 @@ void * high_memory;
 EXPORT_SYMBOL(num_physpages);
 EXPORT_SYMBOL(high_memory);
 
-int randomize_va_space __read_mostly = 1;
+/*
+ * Randomize the address space (stacks, mmaps, brk, etc.).
+ *
+ * ( When CONFIG_COMPAT_BRK=y we exclude brk from randomization,
+ *   as ancient (libc5 based) binaries can segfault. )
+ */
+int randomize_va_space __read_mostly =
+#ifdef CONFIG_COMPAT_BRK
+                                        1;
+#else
+                                        2;
+#endif
 
 static int __init disable_randmaps(char *s)
 {