[tip:x86/mm] x86/mm: Introduce temporary mm structs

[tip-bot for Andy Lutomirski]

Commit-ID:  cefa929c034eb5d9c15c50088235a0093a219687
Gitweb:     https://git.kernel.org/tip/cefa929c034eb5d9c15c50088235a0093a219687
Author:     Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate: Thu, 25 Apr 2019 17:11:23 -0700
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 30 Apr 2019 12:37:50 +0200

x86/mm: Introduce temporary mm structs

Using a dedicated page-table for temporary PTEs prevents other cores
from using - even speculatively - these PTEs, thereby providing two
benefits:

(1) Security hardening: an attacker that gains kernel memory writing
    abilities cannot easily overwrite sensitive data.

(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 mm struct, which will follow, is for
poking the kernel text.

[ Commit message was written by Nadav Amit ]

Tested-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: <ard.biesheuvel@xxxxxxxxxx>
Cc: <deneen.t.dock@xxxxxxxxx>
Cc: <kernel-hardening@xxxxxxxxxxxxxxxxxx>
Cc: <kristen@xxxxxxxxxxxxxxx>
Cc: <linux_dti@xxxxxxxxxx>
Cc: <will.deacon@xxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20190426001143.4983-4-namit@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/include/asm/mmu_context.h | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 19d18fae6ec6..24dc3b810970 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -356,4 +356,37 @@ static inline unsigned long __get_current_cr3_fast(void)
 	return cr3;
 }
 
+typedef struct {
+	struct mm_struct *mm;
+} temp_mm_state_t;
+
+/*
+ * Using a temporary mm allows to set temporary mappings that are not accessible
+ * by other CPUs. Such mappings are needed to perform sensitive memory writes
+ * that override the kernel memory protections (e.g., W^X), without exposing the
+ * temporary page-table mappings that are required for these write operations to
+ * other CPUs. Using a temporary mm also allows to avoid TLB shootdowns when the
+ * mapping is torn down.
+ *
+ * Context: The temporary mm needs to be used exclusively by a single core. To
+ *          harden security IRQs must be disabled while the temporary mm is
+ *          loaded, thereby preventing interrupt handler bugs from overriding
+ *          the kernel memory protection.
+ */
+static inline temp_mm_state_t use_temporary_mm(struct mm_struct *mm)
+{
+	temp_mm_state_t temp_state;
+
+	lockdep_assert_irqs_disabled();
+	temp_state.mm = this_cpu_read(cpu_tlbstate.loaded_mm);
+	switch_mm_irqs_off(NULL, mm, current);
+	return temp_state;
+}
+
+static inline void unuse_temporary_mm(temp_mm_state_t prev_state)
+{
+	lockdep_assert_irqs_disabled();
+	switch_mm_irqs_off(NULL, prev_state.mm, current);
+}
+
 #endif /* _ASM_X86_MMU_CONTEXT_H */