[PATCH v10 14/14] powerpc: Use mm_context vas_windows counter to issue CP_ABORT

[Haren Myneni]

set_thread_uses_vas() sets used_vas flag for a process that opened VAS
window and issue CP_ABORT during context switch for only that process.
In multi-thread application, windows can be shared. For example Thread A
can open a window and Thread B can run COPY/PASTE instructions to send
NX request which may cause corruption or snooping or a covert channel.
Also once this flag is set, continue to run CP_ABORT even the VAS window
is closed.

So define vas-windows counter in process mm_context, increment this
counter for each window open and decrement it for window close. If
vas-windows is set, issue CP_ABORT during context switch. It means
clear the foreign real address mapping only if the process / thread uses
COPY/PASTE. Then disable it for that process if windows are not open.

Signed-off-by: Haren Myneni <haren@xxxxxxxxxxxxx>
Reported-by: Nicholas Piggin <npiggin@xxxxxxxxx>
Suggested-by: Milton Miller <miltonm@xxxxxxxxxx>
Suggested-by: Nicholas Piggin <npiggin@xxxxxxxxx>
---
 arch/powerpc/include/asm/book3s/64/mmu.h    |  3 +++
 arch/powerpc/include/asm/mmu_context.h      | 22 ++++++++++++++++++++++
 arch/powerpc/include/asm/processor.h        |  1 -
 arch/powerpc/kernel/process.c               |  8 ++++++--
 arch/powerpc/platforms/powernv/vas-window.c |  1 +
 5 files changed, 32 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/include/asm/book3s/64/mmu.h b/arch/powerpc/include/asm/book3s/64/mmu.h
index bb3deb7..f0a9ff6 100644
--- a/arch/powerpc/include/asm/book3s/64/mmu.h
+++ b/arch/powerpc/include/asm/book3s/64/mmu.h
@@ -116,6 +116,9 @@ struct patb_entry {
 	/* Number of users of the external (Nest) MMU */
 	atomic_t copros;
 
+	/* Number of user space windows opened in process mm_context */
+	atomic_t vas_windows;
+
 	struct hash_mm_context *hash_context;
 
 	unsigned long vdso_base;
diff --git a/arch/powerpc/include/asm/mmu_context.h b/arch/powerpc/include/asm/mmu_context.h
index 360367c..7fd249498 100644
--- a/arch/powerpc/include/asm/mmu_context.h
+++ b/arch/powerpc/include/asm/mmu_context.h
@@ -185,11 +185,33 @@ static inline void mm_context_remove_copro(struct mm_struct *mm)
 			dec_mm_active_cpus(mm);
 	}
 }
+
+/*
+ * vas_windows counter shows number of open windows in the mm
+ * context. During context switch, use this counter to clear the
+ * foreign real address mapping (CP_ABORT) for the thread / process
+ * that intend to use COPY/PASTE. When a process closes all windows,
+ * disable CP_ABORT which is expensive to run.
+ */
+static inline void mm_context_add_vas_windows(struct mm_struct *mm)
+{
+	atomic_inc(&mm->context.vas_windows);
+}
+
+static inline void mm_context_remove_vas_windows(struct mm_struct *mm)
+{
+	int c = atomic_dec_if_positive(&mm->context.vas_windows);
+
+	/* Detect imbalance between add and remove */
+	WARN_ON(c < 0);
+}
 #else
 static inline void inc_mm_active_cpus(struct mm_struct *mm) { }
 static inline void dec_mm_active_cpus(struct mm_struct *mm) { }
 static inline void mm_context_add_copro(struct mm_struct *mm) { }
 static inline void mm_context_remove_copro(struct mm_struct *mm) { }
+static inline void mm_context_add_vas_windows(struct mm_struct *mm) { }
+static inline void mm_context_remove_vas_windows(struct mm_struct *mm) { }
 #endif
 
 
diff --git a/arch/powerpc/include/asm/processor.h b/arch/powerpc/include/asm/processor.h
index eedcbfb..bfa336f 100644
--- a/arch/powerpc/include/asm/processor.h
+++ b/arch/powerpc/include/asm/processor.h
@@ -272,7 +272,6 @@ struct thread_struct {
 	unsigned 	mmcr0;
 
 	unsigned 	used_ebb;
-	unsigned int	used_vas;
 #endif
 };
 
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index fad50db..a3ecaf9 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -1221,7 +1221,8 @@ struct task_struct *__switch_to(struct task_struct *prev,
 		 * mappings, we must issue a cp_abort to clear any state and
 		 * prevent snooping, corruption or a covert channel.
 		 */
-		if (current->thread.used_vas)
+		if (current->mm &&
+			atomic_read(&current->mm->context.vas_windows))
 			asm volatile(PPC_CP_ABORT);
 	}
 #endif /* CONFIG_PPC_BOOK3S_64 */
@@ -1466,7 +1467,10 @@ int set_thread_uses_vas(void)
 	if (!cpu_has_feature(CPU_FTR_ARCH_300))
 		return -EINVAL;
 
-	current->thread.used_vas = 1;
+	if (!current->mm)
+		return -EINVAL;
+
+	mm_context_add_vas_windows(current->mm);
 
 	/*
 	 * Even a process that has no foreign real address mapping can use
diff --git a/arch/powerpc/platforms/powernv/vas-window.c b/arch/powerpc/platforms/powernv/vas-window.c
index 3ffad5a..33dfbbf 100644
--- a/arch/powerpc/platforms/powernv/vas-window.c
+++ b/arch/powerpc/platforms/powernv/vas-window.c
@@ -1333,6 +1333,7 @@ int vas_win_close(struct vas_window *window)
 			put_pid(window->pid);
 			if (window->mm) {
 				mm_context_remove_copro(window->mm);
+				mm_context_remove_vas_windows(window->mm);
 				mmdrop(window->mm);
 			}
 		}
-- 
1.8.3.1