powerpc/64s: Fix copy-paste data exposure into newly created tasks

copy-paste contains implicit "copy buffer" state that can contain
arbitrary user data (if the user process executes a copy instruction).
This could be snooped by another process if a context switch hits while
the state is live. So cp_abort is executed on context switch to clear
out possible sensitive data and prevent the leak.

cp_abort is done after the low level _switch(), which means it is never
reached by newly created tasks, so they could snoop on this buffer
between their first and second context switch.

Fix this by doing the cp_abort before calling _switch. Add some
comments which should make the issue harder to miss.

Fixes: 07d2a628 ("powerpc/64s: Avoid cpabort in context switch when possible")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210622053036.474678-1-npiggin@gmail.com

arch/powerpc/kernel/process.c

@@ -1223,6 +1223,19 @@ struct task_struct *__switch_to(struct task_struct *prev,
 			__flush_tlb_pending(batch);
 		batch->active = 0;
 	}
+
+	/*
+	 * On POWER9 the copy-paste buffer can only paste into
+	 * foreign real addresses, so unprivileged processes can not
+	 * see the data or use it in any way unless they have
+	 * foreign real mappings. If the new process has the foreign
+	 * real address mappings, we must issue a cp_abort to clear
+	 * any state and prevent snooping, corruption or a covert
+	 * channel. ISA v3.1 supports paste into local memory.
+	 */
+	if (new->mm && (cpu_has_feature(CPU_FTR_ARCH_31) ||
+			atomic_read(&new->mm->context.vas_windows)))
+		asm volatile(PPC_CP_ABORT);
 #endif /* CONFIG_PPC_BOOK3S_64 */
 
 #ifdef CONFIG_PPC_ADV_DEBUG_REGS
@@ -1273,30 +1286,33 @@ struct task_struct *__switch_to(struct task_struct *prev,
 #endif
 	last = _switch(old_thread, new_thread);
 
+	/*
+	 * Nothing after _switch will be run for newly created tasks,
+	 * because they switch directly to ret_from_fork/ret_from_kernel_thread
+	 * etc. Code added here should have a comment explaining why that is
+	 * okay.
+	 */
+
 #ifdef CONFIG_PPC_BOOK3S_64
+	/*
+	 * This applies to a process that was context switched while inside
+	 * arch_enter_lazy_mmu_mode(), to re-activate the batch that was
+	 * deactivated above, before _switch(). This will never be the case
+	 * for new tasks.
+	 */
 	if (current_thread_info()->local_flags & _TLF_LAZY_MMU) {
 		current_thread_info()->local_flags &= ~_TLF_LAZY_MMU;
 		batch = this_cpu_ptr(&ppc64_tlb_batch);
 		batch->active = 1;
 	}
 
-	if (current->thread.regs) {
+	/*
+	 * Math facilities are masked out of the child MSR in copy_thread.
+	 * A new task does not need to restore_math because it will
+	 * demand fault them.
+	 */
+	if (current->thread.regs)
 		restore_math(current->thread.regs);
-
-		/*
-		 * On POWER9 the copy-paste buffer can only paste into
-		 * foreign real addresses, so unprivileged processes can not
-		 * see the data or use it in any way unless they have
-		 * foreign real mappings. If the new process has the foreign
-		 * real address mappings, we must issue a cp_abort to clear
-		 * any state and prevent snooping, corruption or a covert
-		 * channel. ISA v3.1 supports paste into local memory.
-		 */
-		if (current->mm &&
-			(cpu_has_feature(CPU_FTR_ARCH_31) ||
-			atomic_read(&current->mm->context.vas_windows)))
-			asm volatile(PPC_CP_ABORT);
-	}
 #endif /* CONFIG_PPC_BOOK3S_64 */
 
 	return last;