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-rw-r--r--Documentation/admin-guide/kernel-parameters.txt10
-rw-r--r--init/Kconfig14
-rw-r--r--mm/slab_common.c5
3 files changed, 24 insertions, 5 deletions
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index f24ee1c99412..34ae9663aefd 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3760,8 +3760,14 @@
slab_nomerge [MM]
Disable merging of slabs with similar size. May be
necessary if there is some reason to distinguish
- allocs to different slabs. Debug options disable
- merging on their own.
+ allocs to different slabs, especially in hardened
+ environments where the risk of heap overflows and
+ layout control by attackers can usually be
+ frustrated by disabling merging. This will reduce
+ most of the exposure of a heap attack to a single
+ cache (risks via metadata attacks are mostly
+ unchanged). Debug options disable merging on their
+ own.
For more information see Documentation/vm/slub.txt.
slab_max_order= [MM, SLAB]
diff --git a/init/Kconfig b/init/Kconfig
index b0fcbb2c6f56..8514b25db21c 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1548,6 +1548,20 @@ config SLOB
endchoice
+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 risk of kernel heap overflows being able to
+ overwrite objects from merged caches (and more easily control
+ cache layout), which makes such heap attacks easier to exploit
+ by attackers. 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 SLAB_FREELIST_RANDOM
default n
depends on SLAB || SLUB
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 01a0fe2eb332..904a83be82de 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;
}