[PATCH 3/8] mm/slub: Support disabling freelist randomisation at boot

[Chris Down <chris@chrisdown.name>]

Add a slab_freelist_random= early parameter, enabled by default on
kernels built with CONFIG_SLAB_FREELIST_RANDOM=y.

Signed-off-by: Chris Down <chris@chrisdown.name>
---
 Documentation/admin-guide/kernel-parameters.txt |  7 +++++++
 mm/Kconfig                                      |  3 +++
 mm/slub.c                                       | 16 ++++++++++++++++
 3 files changed, 26 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index ddf2f6fd60be..5c4d784e7786 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -7021,6 +7021,13 @@ Kernel parameters
 			without that option.
 			Default: on.
 
+	slab_freelist_random= [MM,EARLY]
+			Format: <bool>
+			Enable or disable slab freelist randomisation on
+			kernels built with CONFIG_SLAB_FREELIST_RANDOM=y.
+			Ignored without that option.
+			Default: on.
+
 	slab_max_order= [MM]
 			Determines the maximum allowed order for slabs.
 			A high setting may cause OOMs due to memory
diff --git a/mm/Kconfig b/mm/Kconfig
index 3ae97fec9946..56c422684b44 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -205,6 +205,9 @@ config SLAB_FREELIST_RANDOM
 	  security feature reduces the predictability of the kernel slab
 	  allocator against heap overflows.
 
+	  The randomisation is enabled by default and can be disabled at
+	  boot with "slab_freelist_random=0".
+
 config SLAB_FREELIST_HARDENED
 	bool "Harden slab freelist metadata"
 	depends on !SLUB_TINY
diff --git a/mm/slub.c b/mm/slub.c
index de4937fcb293..7e9dadda3d10 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3424,12 +3424,23 @@ static inline struct slab *alloc_slab_page(gfp_t flags, int node,
 }
 
 #ifdef CONFIG_SLAB_FREELIST_RANDOM
+static bool slab_freelist_random __ro_after_init = true;
+
+static int __init setup_slab_freelist_random(char *str)
+{
+	return kstrtobool(str, &slab_freelist_random);
+}
+early_param("slab_freelist_random", setup_slab_freelist_random);
+
 /* Pre-initialize the random sequence cache */
 static int init_cache_random_seq(struct kmem_cache *s)
 {
 	unsigned int count = oo_objects(s->oo);
 	int err;
 
+	if (!slab_freelist_random)
+		return 0;
+
 	/* Bailout if already initialised */
 	if (s->random_seq)
 		return 0;
@@ -3456,6 +3467,11 @@ static void __init init_freelist_randomization(void)
 {
 	struct kmem_cache *s;
 
+	if (!slab_freelist_random) {
+		pr_info("SLUB: freelist randomisation disabled\n");
+		return;
+	}
+
 	mutex_lock(&slab_mutex);
 
 	list_for_each_entry(s, &slab_caches, list)
-- 
2.54.0