On 06/20/2017 04:29 PM, Kees Cook wrote: > On Tue, Jun 20, 2017 at 4:16 PM, Randy Dunlap <rdunlap@xxxxxxxxxxxxx> wrote: >> On 06/20/2017 04:09 PM, Kees Cook wrote: >>> 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). >>> >>> Cc: Daniel Micay <danielmicay@xxxxxxxxx> >>> Cc: David Windsor <dave@xxxxxxxxxxxx> >>> Cc: Eric Biggers <ebiggers3@xxxxxxxxx> >>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> >>> --- >>> v2: split out of slab whitelisting series >>> --- >>> Documentation/admin-guide/kernel-parameters.txt | 10 ++++++++-- >>> init/Kconfig | 14 ++++++++++++++ >>> mm/slab_common.c | 5 ++--- >>> 3 files changed, 24 insertions(+), 5 deletions(-) >> >>> diff --git a/init/Kconfig b/init/Kconfig >>> index 1d3475fc9496..ce813acf2f4f 100644 >>> --- a/init/Kconfig >>> +++ b/init/Kconfig >>> @@ -1891,6 +1891,20 @@ 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. >> >> command line or this option can be disabled in the kernel config. > > Isn't that implicit in that it is Kconfig help text? Happy to add it, > but seems redundant to me. > Just trying for completeness instead of being implicit. > >> >>> + >>> config SLAB_FREELIST_RANDOM >>> default n >>> depends on SLAB || SLUB >> >> -- >> ~Randy > > > -- ~Randy -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
