From: Dave Hansen <[email protected]> Global pages stay in the TLB across context switches. Since all contexts share the same kernel mapping, these mappings are marked as global pages so kernel entries in the TLB are not flushed out on a context switch.
But, even having these entries in the TLB opens up something that an attacker can use [1]. That means that even when KAISER switches page tables on return to user space the global pages would stay in the TLB cache. Disable global pages so that kernel TLB entries can be flushed before returning to user space. This way, all accesses to kernel addresses from userspace result in a TLB miss independent of the existence of a kernel mapping. Replace _PAGE_GLOBAL by __PAGE_KERNEL_GLOBAL and keep _PAGE_GLOBAL available so that it can still be used for a few selected kernel mappings which must be visible to userspace, when KAISER is enabled, like the entry/exit code and data. 1. The double-page-fault attack: http://www.ieee-security.org/TC/SP2013/papers/4977a191.pdf Signed-off-by: Dave Hansen <[email protected]> Reviewed-by: Borislav Petkov <[email protected]> Reviewed-by: Thomas Gleixner <[email protected]> Cc: Moritz Lipp <[email protected]> Cc: Daniel Gruss <[email protected]> Cc: Michael Schwarz <[email protected]> Cc: Richard Fellner <[email protected]> Cc: Andy Lutomirski <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Kees Cook <[email protected]> Cc: Hugh Dickins <[email protected]> Cc: [email protected] --- b/arch/x86/include/asm/pgtable_types.h | 14 +++++++++++++- b/arch/x86/mm/pageattr.c | 16 ++++++++-------- 2 files changed, 21 insertions(+), 9 deletions(-) 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 2017-11-10 11:22:06.621244956 -0800 +++ b/arch/x86/include/asm/pgtable_types.h 2017-11-10 11:22:06.626244956 -0800 @@ -179,8 +179,20 @@ enum page_cache_mode { #define PAGE_READONLY_EXEC __pgprot(_PAGE_PRESENT | _PAGE_USER | \ _PAGE_ACCESSED) +/* + * Disable global pages for anything using the default + * __PAGE_KERNEL* macros. PGE will still be enabled + * and _PAGE_GLOBAL may still be used carefully. + */ +#ifdef CONFIG_KAISER +#define __PAGE_KERNEL_GLOBAL 0 +#else +#define __PAGE_KERNEL_GLOBAL _PAGE_GLOBAL +#endif + #define __PAGE_KERNEL_EXEC \ - (_PAGE_PRESENT | _PAGE_RW | _PAGE_DIRTY | _PAGE_ACCESSED | _PAGE_GLOBAL) + (_PAGE_PRESENT | _PAGE_RW | _PAGE_DIRTY | _PAGE_ACCESSED | \ + __PAGE_KERNEL_GLOBAL) #define __PAGE_KERNEL (__PAGE_KERNEL_EXEC | _PAGE_NX) #define __PAGE_KERNEL_RO (__PAGE_KERNEL & ~_PAGE_RW) diff -puN arch/x86/mm/pageattr.c~kaiser-prep-disable-global-pages arch/x86/mm/pageattr.c --- a/arch/x86/mm/pageattr.c~kaiser-prep-disable-global-pages 2017-11-10 11:22:06.623244956 -0800 +++ b/arch/x86/mm/pageattr.c 2017-11-10 11:22:06.627244956 -0800 @@ -585,9 +585,9 @@ try_preserve_large_page(pte_t *kpte, uns * for the ancient hardware that doesn't support it. */ if (pgprot_val(req_prot) & _PAGE_PRESENT) - pgprot_val(req_prot) |= _PAGE_PSE | _PAGE_GLOBAL; + pgprot_val(req_prot) |= _PAGE_PSE | __PAGE_KERNEL_GLOBAL; else - pgprot_val(req_prot) &= ~(_PAGE_PSE | _PAGE_GLOBAL); + pgprot_val(req_prot) &= ~(_PAGE_PSE | __PAGE_KERNEL_GLOBAL); req_prot = canon_pgprot(req_prot); @@ -705,9 +705,9 @@ __split_large_page(struct cpa_data *cpa, * for the ancient hardware that doesn't support it. */ if (pgprot_val(ref_prot) & _PAGE_PRESENT) - pgprot_val(ref_prot) |= _PAGE_GLOBAL; + pgprot_val(ref_prot) |= __PAGE_KERNEL_GLOBAL; else - pgprot_val(ref_prot) &= ~_PAGE_GLOBAL; + pgprot_val(ref_prot) &= ~__PAGE_KERNEL_GLOBAL; /* * Get the target pfn from the original entry: @@ -938,9 +938,9 @@ static void populate_pte(struct cpa_data * support it. */ if (pgprot_val(pgprot) & _PAGE_PRESENT) - pgprot_val(pgprot) |= _PAGE_GLOBAL; + pgprot_val(pgprot) |= __PAGE_KERNEL_GLOBAL; else - pgprot_val(pgprot) &= ~_PAGE_GLOBAL; + pgprot_val(pgprot) &= ~__PAGE_KERNEL_GLOBAL; pgprot = canon_pgprot(pgprot); @@ -1242,9 +1242,9 @@ repeat: * support it. */ if (pgprot_val(new_prot) & _PAGE_PRESENT) - pgprot_val(new_prot) |= _PAGE_GLOBAL; + pgprot_val(new_prot) |= __PAGE_KERNEL_GLOBAL; else - pgprot_val(new_prot) &= ~_PAGE_GLOBAL; + pgprot_val(new_prot) &= ~__PAGE_KERNEL_GLOBAL; /* * We need to keep the pfn from the existing PTE, _
