From: Dave Hansen <[email protected]> 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. We keep _PAGE_GLOBAL available so that we can use it for things that are global even with KAISER 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]> 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 2017-11-08 10:45:27.525681399 -0800 +++ b/arch/x86/include/asm/pgtable_types.h 2017-11-08 10:45:27.530681399 -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 2017-11-08 10:45:27.527681399 -0800 +++ b/arch/x86/mm/pageattr.c 2017-11-08 10:45:27.531681399 -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, _
