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Subject[PATCH 04/30] x86, kaiser: disable global pages by default with KAISER
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From: Dave Hansen <dave.hansen@linux.intel.com>

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 <dave.hansen@linux.intel.com>
Reviewed-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Moritz Lipp <moritz.lipp@iaik.tugraz.at>
Cc: Daniel Gruss <daniel.gruss@iaik.tugraz.at>
Cc: Michael Schwarz <michael.schwarz@iaik.tugraz.at>
Cc: Richard Fellner <richard.fellner@student.tugraz.at>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Kees Cook <keescook@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: x86@kernel.org
---

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,
_
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