From: Andy Lutomirski <luto@xxxxxxxxxx>
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: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Reviewed-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
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>
---
arch/x86/include/asm/mmu_context.h | 32 ++++++++++++++++++++++++++++++
1 file changed, 32 insertions(+)
diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 0ca50611e8ce..0141b7fa6d01 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -338,4 +338,36 @@ static inline unsigned long __get_current_cr3_fast(void)
return cr3;
}
+typedef struct {
+ struct mm_struct *prev;
+} temporary_mm_state_t;
+
+/*
+ * Using a temporary mm allows to set temporary mappings that are not accessible
+ * by other cores. 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 cores.
+ *
+ * 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 override the
+ * kernel memory protection.
+ */
+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 */
--
2.17.1
