Global pages stay in the TLB across context switches. Since all contexts share the same kernel mapping, we use global pages to allow kernel entries in the TLB to survive when we context switch.
But, even having these entries in the TLB opens up something that an attacker can use [1]. Disable global pages so that kernel TLB entries are flushed when we run userspace. This way, all accesses to kernel memory result in a TLB miss whether there is good data there or not. Without this, even when KAISER switches pages tables, the kernel entries might remain in the TLB. 1. The double-page-fault attack: http://www.ieee-security.org/TC/SP2013/papers/4977a191.pdf Signed-off-by: Dave Hansen <dave.han...@linux.intel.com> Cc: Moritz Lipp <moritz.l...@iaik.tugraz.at> Cc: Daniel Gruss <daniel.gr...@iaik.tugraz.at> Cc: Michael Schwarz <michael.schw...@iaik.tugraz.at> Cc: Andy Lutomirski <l...@kernel.org> Cc: Linus Torvalds <torva...@linux-foundation.org> Cc: Kees Cook <keesc...@google.com> Cc: Hugh Dickins <hu...@google.com> Cc: x...@kernel.org --- b/arch/x86/Kconfig | 4 ++++ b/arch/x86/include/asm/pgtable_types.h | 5 +++++ 2 files changed, 9 insertions(+) diff -puN arch/x86/include/asm/pgtable_types.h~kaiser-prep-disable-global-pages arch/x86/include/asm/pgtable_types.h --- a/arch/x86/include/asm/pgtable_types.h~kaiser-prep-disable-global-pages 2017-10-31 15:03:49.314064402 -0700 +++ b/arch/x86/include/asm/pgtable_types.h 2017-10-31 15:03:49.323064827 -0700 @@ -47,7 +47,12 @@ #define _PAGE_ACCESSED (_AT(pteval_t, 1) << _PAGE_BIT_ACCESSED) #define _PAGE_DIRTY (_AT(pteval_t, 1) << _PAGE_BIT_DIRTY) #define _PAGE_PSE (_AT(pteval_t, 1) << _PAGE_BIT_PSE) +#ifdef CONFIG_X86_GLOBAL_PAGES #define _PAGE_GLOBAL (_AT(pteval_t, 1) << _PAGE_BIT_GLOBAL) +#else +/* We must ensure that kernel TLBs are unusable while in userspace */ +#define _PAGE_GLOBAL (_AT(pteval_t, 0)) +#endif #define _PAGE_SOFTW1 (_AT(pteval_t, 1) << _PAGE_BIT_SOFTW1) #define _PAGE_SOFTW2 (_AT(pteval_t, 1) << _PAGE_BIT_SOFTW2) #define _PAGE_PAT (_AT(pteval_t, 1) << _PAGE_BIT_PAT) diff -puN arch/x86/Kconfig~kaiser-prep-disable-global-pages arch/x86/Kconfig --- a/arch/x86/Kconfig~kaiser-prep-disable-global-pages 2017-10-31 15:03:49.318064591 -0700 +++ b/arch/x86/Kconfig 2017-10-31 15:03:49.325064922 -0700 @@ -327,6 +327,10 @@ config ARCH_SUPPORTS_UPROBES config FIX_EARLYCON_MEM def_bool y +config X86_GLOBAL_PAGES + def_bool y + depends on ! KAISER + config PGTABLE_LEVELS int default 5 if X86_5LEVEL _
