From: Dave Hansen <[email protected]> We poison kernel PGDs that map userspace with the NX bit. This ensures that if we miss a kernel->user CR3 switch, userspace crashes instead of running in an unhardened state.
We will need this code in a moment when we turn kaiser on and off at runtime. Note that we now need an __ASSEMBLY__ #ifdef since we are now indirectly including kaiser.h into assembly. 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_64.h | 16 ++++++++++++++- b/arch/x86/mm/kaiser.c | 38 ++++++++++++++++++++++++++++++++++++ b/include/linux/kaiser.h | 3 +- 3 files changed, 55 insertions(+), 2 deletions(-) diff -puN arch/x86/include/asm/pgtable_64.h~kaiser-dynamic-unpoison-pgd arch/x86/include/asm/pgtable_64.h --- a/arch/x86/include/asm/pgtable_64.h~kaiser-dynamic-unpoison-pgd 2017-11-08 10:45:40.781681366 -0800 +++ b/arch/x86/include/asm/pgtable_64.h 2017-11-08 10:45:40.788681366 -0800 @@ -2,6 +2,7 @@ #define _ASM_X86_PGTABLE_64_H #include <linux/const.h> +#include <linux/kaiser.h> #include <asm/pgtable_64_types.h> #ifndef __ASSEMBLY__ @@ -196,6 +197,18 @@ static inline bool pgd_userspace_access( return (pgd.pgd & _PAGE_USER); } +static inline void kaiser_poison_pgd(pgd_t *pgd) +{ + if (pgd->pgd & _PAGE_PRESENT) + pgd->pgd |= _PAGE_NX; +} + +static inline void kaiser_unpoison_pgd(pgd_t *pgd) +{ + if (pgd->pgd & _PAGE_PRESENT) + pgd->pgd &= ~_PAGE_NX; +} + /* * Returns the pgd_t that the kernel should use in its page tables. */ @@ -216,7 +229,8 @@ static inline pgd_t kaiser_set_shadow_pg * wrong CR3 value, userspace will crash * instead of running. */ - pgd.pgd |= _PAGE_NX; + if (kaiser_active()) + kaiser_poison_pgd(&pgd); } } else if (!pgd.pgd) { /* diff -puN arch/x86/mm/kaiser.c~kaiser-dynamic-unpoison-pgd arch/x86/mm/kaiser.c --- a/arch/x86/mm/kaiser.c~kaiser-dynamic-unpoison-pgd 2017-11-08 10:45:40.783681366 -0800 +++ b/arch/x86/mm/kaiser.c 2017-11-08 10:45:40.788681366 -0800 @@ -477,6 +477,9 @@ static ssize_t kaiser_enabled_write_file if (enable > 1) return -EINVAL; + if (kaiser_enabled == enable) + return count; + WRITE_ONCE(kaiser_enabled, enable); return count; } @@ -494,3 +497,38 @@ static int __init create_kaiser_enabled( return 0; } late_initcall(create_kaiser_enabled); + +enum poison { + KAISER_POISON, + KAISER_UNPOISON +}; +void kaiser_poison_pgd_page(pgd_t *pgd_page, enum poison do_poison) +{ + int i = 0; + + for (i = 0; i < PTRS_PER_PGD; i++) { + pgd_t *pgd = &pgd_page[i]; + + /* Stop once we hit kernel addresses: */ + if (!pgdp_maps_userspace(pgd)) + break; + + if (do_poison == KAISER_POISON) + kaiser_poison_pgd(pgd); + else + kaiser_unpoison_pgd(pgd); + } + +} + +void kaiser_poison_pgds(enum poison do_poison) +{ + struct page *page; + + spin_lock(&pgd_lock); + list_for_each_entry(page, &pgd_list, lru) { + pgd_t *pgd = (pgd_t *)page_address(page); + kaiser_poison_pgd_page(pgd, do_poison); + } + spin_unlock(&pgd_lock); +} diff -puN include/linux/kaiser.h~kaiser-dynamic-unpoison-pgd include/linux/kaiser.h --- a/include/linux/kaiser.h~kaiser-dynamic-unpoison-pgd 2017-11-08 10:45:40.784681366 -0800 +++ b/include/linux/kaiser.h 2017-11-08 10:45:40.788681366 -0800 @@ -4,7 +4,7 @@ #ifdef CONFIG_KAISER #include <asm/kaiser.h> #else - +#ifndef __ASSEMBLY__ /* * These stubs are used whenever CONFIG_KAISER is off, which * includes architectures that support KAISER, but have it @@ -29,5 +29,6 @@ static inline bool kaiser_active(void) { return 0; } +#endif /* __ASSEMBLY__ */ #endif /* !CONFIG_KAISER */ #endif /* _INCLUDE_KAISER_H */ _
