[PATCH 23/23] mm: Allow slab_nomerge to be set at build time

[Kees Cook]

Some hardened environments want to build kernels with slab_nomerge
already set (so that they do not depend on remembering to set the kernel
command line option). This is desired to reduce the risk of kernel heap
overflows being able to overwrite objects from merged caches, increasing
the difficulty of these attacks. By keeping caches unmerged, these kinds
of exploits can usually only damage objects in the same cache (though the
risk to metadata exploitation is unchanged).

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 mm/slab_common.c |  5 ++---
 security/Kconfig | 13 +++++++++++++
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 6c14d765379f..17a4c4b33283 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -47,13 +47,12 @@ static DECLARE_WORK(slab_caches_to_rcu_destroy_work,
 
 /*
  * Merge control. If this is set then no merging of slab caches will occur.
- * (Could be removed. This was introduced to pacify the merge skeptics.)
  */
-static int slab_nomerge;
+static bool slab_nomerge = !IS_ENABLED(CONFIG_SLAB_MERGE_DEFAULT);
 
 static int __init setup_slab_nomerge(char *str)
 {
-	slab_nomerge = 1;
+	slab_nomerge = true;
 	return 1;
 }
 
diff --git a/security/Kconfig b/security/Kconfig
index 0c181cebdb8a..e40bd2a260f8 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -166,6 +166,19 @@ config HARDENED_USERCOPY_SPLIT_KMALLOC
 	  confined to a separate cache, attackers must find other ways
 	  to prepare heap attacks that will be near their desired target.
 
+config SLAB_MERGE_DEFAULT
+	bool "Allow slab caches to be merged"
+	default y
+	help
+	  For reduced kernel memory fragmentation, slab caches can be
+	  merged when they share the same size and other characteristics.
+	  This carries a small risk of kernel heap overflows being able
+	  to overwrite objects from merged caches, which reduces the
+	  difficulty of such heap attacks. By keeping caches unmerged,
+	  these kinds of exploits can usually only damage objects in the
+	  same cache. To disable merging at runtime, "slab_nomerge" can be
+	  passed on the kernel command line.
+
 config STATIC_USERMODEHELPER
 	bool "Force all usermode helper calls through a single binary"
 	help
-- 
2.7.4