[PATCH v7 2/5] powerpc/64s: Introduce temporary mm for Radix MMU

[Jordan Niethe <jniethe5@gmail.com>]

From: "Christopher M. Riedl" <cmr@bluescreens.de>

x86 supports the notion of a temporary mm which restricts access to
temporary PTEs to a single CPU. A temporary mm is useful for situations
where a CPU needs to perform sensitive operations (such as patching a
STRICT_KERNEL_RWX kernel) requiring temporary mappings without exposing
said mappings to other CPUs. Another benefit is that other CPU TLBs do
not need to be flushed when the temporary mm is torn down.

Mappings in the temporary mm can be set in the userspace portion of the
address-space.

Interrupts must be disabled while the temporary mm is in use. HW
breakpoints, which may have been set by userspace as watchpoints on
addresses now within the temporary mm, are saved and disabled when
loading the temporary mm. The HW breakpoints are restored when unloading
the temporary mm. All HW breakpoints are indiscriminately disabled while
the temporary mm is in use - this may include breakpoints set by perf.

Based on x86 implementation:

commit cefa929c034e
("x86/mm: Introduce temporary mm structs")

Signed-off-by: Christopher M. Riedl <cmr@bluescreens.de>
Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
---
v7: - use breakpoint_pause()/breakpoint_unpause()
    - simplify the temp mm struct, don't need init_temp_mm()
---
 arch/powerpc/lib/code-patching.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
index c5ed98823835..29a30c3068ff 100644
--- a/arch/powerpc/lib/code-patching.c
+++ b/arch/powerpc/lib/code-patching.c
@@ -17,6 +17,9 @@
 #include <asm/code-patching.h>
 #include <asm/setup.h>
 #include <asm/inst.h>
+#include <asm/mmu_context.h>
+#include <asm/debug.h>
+#include <asm/tlb.h>
 
 static int __patch_instruction(u32 *exec_addr, struct ppc_inst instr, u32 *patch_addr)
 {
@@ -45,6 +48,32 @@ int raw_patch_instruction(u32 *addr, struct ppc_inst instr)
 }
 
 #ifdef CONFIG_STRICT_KERNEL_RWX
+
+struct temp_mm_state {
+	struct mm_struct *mm;
+};
+
+static inline struct temp_mm_state start_using_temp_mm(struct mm_struct *mm)
+{
+	struct temp_mm_state temp_state;
+
+	lockdep_assert_irqs_disabled();
+	temp_state.mm = current->active_mm;
+	switch_mm_irqs_off(current->active_mm, mm, current);
+
+	WARN_ON(!mm_is_thread_local(mm));
+
+	pause_breakpoints();
+	return temp_state;
+}
+
+static inline void stop_using_temp_mm(struct temp_mm_state prev_state)
+{
+	lockdep_assert_irqs_disabled();
+	switch_mm_irqs_off(current->active_mm, prev_state.mm, current);
+	unpause_breakpoints();
+}
+
 static DEFINE_PER_CPU(struct vm_struct *, text_poke_area);
 
 static int text_area_cpu_up(unsigned int cpu)
-- 
2.25.1