powerpc: Use mm_context vas_windows counter to issue CP_ABORT

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.

Moved set_thread_uses_vas() code to vas_tx_win_open() as this
functionality is needed only for userspace open windows. We are adding
VAS userspace support along with this fix. So no need to include this
fix in stable releases.

Fixes: 9d2a4d71332c ("powerpc: Define set_thread_uses_vas()")
Signed-off-by: Haren Myneni <haren@linux.ibm.com>
Reported-by: Nicholas Piggin <npiggin@gmail.com>
Suggested-by: Milton Miller <miltonm@us.ibm.com>
Suggested-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1587017291.2275.1077.camel@hbabu-laptop

4 files changed, 33 insertions, 3 deletions

diff --git a/arch/powerpc/include/asm/book3s/64/mmu.h b/arch/powerpc/include/asm/book3s/64/mmu.h
index bb3deb76c951..f0a9ff690881 100644
--- a/arch/powerpc/include/asm/book3s/64/mmu.h
+++ b/arch/powerpc/include/asm/book3s/64/mmu.h
@@ -116,6 +116,9 @@ typedef struct {
 	/* 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 360367c579de..1a474f6b1992 100644
--- a/arch/powerpc/include/asm/mmu_context.h
+++ b/arch/powerpc/include/asm/mmu_context.h
@@ -185,11 +185,41 @@ 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.
+ *
+ * For user context, register a copro so that TLBIs are seen by the
+ * nest MMU. mm_context_add/remove_vas_window() are used only for user
+ * space windows.
+ */
+static inline void mm_context_add_vas_window(struct mm_struct *mm)
+{
+	atomic_inc(&mm->context.vas_windows);
+	mm_context_add_copro(mm);
+}
+
+static inline void mm_context_remove_vas_window(struct mm_struct *mm)
+{
+	int v;
+
+	mm_context_remove_copro(mm);
+	v = atomic_dec_if_positive(&mm->context.vas_windows);
+
+	/* Detect imbalance between add and remove */
+	WARN_ON(v < 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 eedcbfb9a6ff..bfa336fbcfeb 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/include/asm/switch_to.h b/arch/powerpc/include/asm/switch_to.h
index b867b58b1093..fdab93428372 100644
--- a/arch/powerpc/include/asm/switch_to.h
+++ b/arch/powerpc/include/asm/switch_to.h
@@ -102,8 +102,6 @@ static inline void clear_task_ebb(struct task_struct *t)
 #endif
 }
 
-extern int set_thread_uses_vas(void);
-
 extern int set_thread_tidr(struct task_struct *t);
 
 #endif /* _ASM_POWERPC_SWITCH_TO_H */