From: Kees Cook <keescook@xxxxxxxxxxxx> Subject: mm: allow slab_nomerge to be set at build time 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 and changes the requirements for cache layout control, 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). Link: http://lkml.kernel.org/r/20170620230911.GA25238@beast Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> Cc: Daniel Micay <danielmicay@xxxxxxxxx> Cc: David Windsor <dave@xxxxxxxxxxxx> Cc: Eric Biggers <ebiggers3@xxxxxxxxx> Cc: Christoph Lameter <cl@xxxxxxxxx> Cc: Jonathan Corbet <corbet@xxxxxxx> Cc: Daniel Micay <danielmicay@xxxxxxxxx> Cc: David Windsor <dave@xxxxxxxxxxxx> Cc: Eric Biggers <ebiggers3@xxxxxxxxx> Cc: Pekka Enberg <penberg@xxxxxxxxxx> Cc: David Rientjes <rientjes@xxxxxxxxxx> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> Cc: "Rafael J. Wysocki" <rafael.j.wysocki@xxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Mauro Carvalho Chehab <mchehab@xxxxxxxxxx> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> Cc: Arnd Bergmann <arnd@xxxxxxxx> Cc: Andy Lutomirski <luto@xxxxxxxxxx> Cc: Nicolas Pitre <nicolas.pitre@xxxxxxxxxx> Cc: Tejun Heo <tj@xxxxxxxxxx> Cc: Daniel Mack <daniel@xxxxxxxxxx> Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> Cc: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx> Cc: Helge Deller <deller@xxxxxx> Cc: Rik van Riel <riel@xxxxxxxxxx> Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- Documentation/admin-guide/kernel-parameters.txt | 10 ++++++++-- init/Kconfig | 14 ++++++++++++++ mm/slab_common.c | 5 ++--- 3 files changed, 24 insertions(+), 5 deletions(-) diff -puN Documentation/admin-guide/kernel-parameters.txt~mm-allow-slab_nomerge-to-be-set-at-build-time Documentation/admin-guide/kernel-parameters.txt --- a/Documentation/admin-guide/kernel-parameters.txt~mm-allow-slab_nomerge-to-be-set-at-build-time +++ a/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 -puN init/Kconfig~mm-allow-slab_nomerge-to-be-set-at-build-time init/Kconfig --- a/init/Kconfig~mm-allow-slab_nomerge-to-be-set-at-build-time +++ a/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 -puN mm/slab_common.c~mm-allow-slab_nomerge-to-be-set-at-build-time mm/slab_common.c --- a/mm/slab_common.c~mm-allow-slab_nomerge-to-be-set-at-build-time +++ a/mm/slab_common.c @@ -47,13 +47,12 @@ static DECLARE_WORK(slab_caches_to_rcu_d /* * 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; } _ -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
