[PATCH 4.14 088/164] arm64: SW PAN: Point saved ttbr0 at the zero page when switching to init_mm

[Greg Kroah-Hartman]

4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Will Deacon <will.deacon@xxxxxxx>

commit 0adbdfde8cfc9415aeed2a4955d2d17b3bd9bf13 upstream.

update_saved_ttbr0 mandates that mm->pgd is not swapper, since swapper
contains kernel mappings and should never be installed into ttbr0. However,
this means that callers must avoid passing the init_mm to update_saved_ttbr0
which in turn can cause the saved ttbr0 value to be out-of-date in the context
of the idle thread. For example, EFI runtime services may leave the saved ttbr0
pointing at the EFI page table, and kernel threads may end up with stale
references to freed page tables.

This patch changes update_saved_ttbr0 so that the init_mm points the saved
ttbr0 value to the empty zero page, which always exists and never contains
valid translations. EFI and switch can then call into update_saved_ttbr0
unconditionally.

Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
Cc: Vinayak Menon <vinmenon@xxxxxxxxxxxxxx>
Fixes: 39bc88e5e38e9b21 ("arm64: Disable TTBR0_EL1 during normal kernel execution")
Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
Reviewed-by: Mark Rutland <mark.rutland@xxxxxxx>
Reported-by: Vinayak Menon <vinmenon@xxxxxxxxxxxxxx>
Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 arch/arm64/include/asm/efi.h         |    4 +---
 arch/arm64/include/asm/mmu_context.h |   22 +++++++++++++---------
 2 files changed, 14 insertions(+), 12 deletions(-)

--- a/arch/arm64/include/asm/efi.h
+++ b/arch/arm64/include/asm/efi.h
@@ -132,11 +132,9 @@ static inline void efi_set_pgd(struct mm
 			 * Defer the switch to the current thread's TTBR0_EL1
 			 * until uaccess_enable(). Restore the current
 			 * thread's saved ttbr0 corresponding to its active_mm
-			 * (if different from init_mm).
 			 */
 			cpu_set_reserved_ttbr0();
-			if (current->active_mm != &init_mm)
-				update_saved_ttbr0(current, current->active_mm);
+			update_saved_ttbr0(current, current->active_mm);
 		}
 	}
 }
--- a/arch/arm64/include/asm/mmu_context.h
+++ b/arch/arm64/include/asm/mmu_context.h
@@ -174,11 +174,17 @@ enter_lazy_tlb(struct mm_struct *mm, str
 static inline void update_saved_ttbr0(struct task_struct *tsk,
 				      struct mm_struct *mm)
 {
-	if (system_uses_ttbr0_pan()) {
-		BUG_ON(mm->pgd == swapper_pg_dir);
-		task_thread_info(tsk)->ttbr0 =
-			virt_to_phys(mm->pgd) | ASID(mm) << 48;
-	}
+	u64 ttbr;
+
+	if (!system_uses_ttbr0_pan())
+		return;
+
+	if (mm == &init_mm)
+		ttbr = __pa_symbol(empty_zero_page);
+	else
+		ttbr = virt_to_phys(mm->pgd) | ASID(mm) << 48;
+
+	task_thread_info(tsk)->ttbr0 = ttbr;
 }
 #else
 static inline void update_saved_ttbr0(struct task_struct *tsk,
@@ -214,11 +220,9 @@ switch_mm(struct mm_struct *prev, struct
 	 * Update the saved TTBR0_EL1 of the scheduled-in task as the previous
 	 * value may have not been initialised yet (activate_mm caller) or the
 	 * ASID has changed since the last run (following the context switch
-	 * of another thread of the same process). Avoid setting the reserved
-	 * TTBR0_EL1 to swapper_pg_dir (init_mm; e.g. via idle_task_exit).
+	 * of another thread of the same process).
 	 */
-	if (next != &init_mm)
-		update_saved_ttbr0(tsk, next);
+	update_saved_ttbr0(tsk, next);
 }
 
 #define deactivate_mm(tsk,mm)	do { } while (0)