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Date:   Thu, 30 Aug 2018 10:32:14 -0700
From:   Nadav Amit <>
To:     Thomas Gleixner <>
CC:     <>, Ingo Molnar <>,
        <>, Arnd Bergmann <>,
        Dave Hansen <>,
        Andy Lutomirski <>,
        Masami Hiramatsu <>,
        Kees Cook <>,
        Peter Zijlstra <>,
        Dave Hansen <>,
        Nadav Amit <>
Subject: [PATCH 2/6] x86/mm: temporary mm struct

From: Andy Lutomirski <>

Sometimes we want to set a temporary page-table entries (PTEs) in one of
the cores, without allowing other cores to use - even speculatively -
these mappings. There are two benefits for doing so:

(1) Security: if sensitive PTEs are set, temporary mm prevents their use
in other cores. This hardens the security as it prevents exploding a
dangling pointer to overwrite sensitive data using the sensitive PTE.

(2) Avoiding TLB shootdowns: the PTEs do not need to be flushed in
remote page-tables.

To do so a temporary mm_struct can be used. Mappings which are private
for this mm can be set in the userspace part of the address-space.
During the whole time in which the temporary mm is loaded, interrupts
must be disabled.

The first use-case for temporary PTEs, which will follow, is for poking
the kernel text.

[ Commit message was written by Nadav ]

Cc: Masami Hiramatsu <>
Cc: Kees Cook <>
Cc: Peter Zijlstra <>
Cc: Dave Hansen <>
Signed-off-by: Andy Lutomirski <>
Signed-off-by: Nadav Amit <>
 arch/x86/include/asm/mmu_context.h | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index eeeb9289c764..96afc8c0cf15 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -338,4 +338,24 @@ static inline unsigned long __get_current_cr3_fast(void)
 	return cr3;
+typedef struct {
+	struct mm_struct *prev;
+} temporary_mm_state_t;
+static inline temporary_mm_state_t use_temporary_mm(struct mm_struct *mm)
+	temporary_mm_state_t state;
+	lockdep_assert_irqs_disabled();
+	state.prev = this_cpu_read(cpu_tlbstate.loaded_mm);
+	switch_mm_irqs_off(NULL, mm, current);
+	return state;
+static inline void unuse_temporary_mm(temporary_mm_state_t prev)
+	lockdep_assert_irqs_disabled();
+	switch_mm_irqs_off(NULL, prev.prev, current);
 #endif /* _ASM_X86_MMU_CONTEXT_H */

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