[RFC PATCH 2/4] arm64/sve: KVM: Avoid dereference of dead task during guest entry

[Dave Martin]

When deciding whether to invalidate FPSIMD state cached in the cpu,
the backend function sve_flush_cpu_state() attempts to dereference
__this_cpu_read(fpsimd_last_state).  However, this is not safe:
there is no guarantee that the pointer is still valid, because the
task could have exited in the meantime.  For this reason, this
percpu pointer should only be assigned or compared, never
dereferenced.

This means that we need another means to get the appropriate value
of TIF_SVE for the associated task.

This patch solves this issue by adding a cached copy of the TIF_SVE
flag in fpsimd_last_state, which we can check without dereferencing
the task pointer.

Signed-off-by: Dave Martin <Dave.Martin at arm.com>
---
 arch/arm64/kernel/fpsimd.c | 28 ++++++++++++++++------------
 1 file changed, 16 insertions(+), 12 deletions(-)

diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index 007140b..3dc8058 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -114,7 +114,12 @@
  *   returned from the 2nd syscall yet, TIF_FOREIGN_FPSTATE is still set so
  *   whatever is in the FPSIMD registers is not saved to memory, but discarded.
  */
-static DEFINE_PER_CPU(struct fpsimd_state *, fpsimd_last_state);
+struct fpsimd_last_state_struct {
+	struct fpsimd_state *st;
+	bool sve_in_use;
+};
+
+static DEFINE_PER_CPU(struct fpsimd_last_state_struct, fpsimd_last_state);
 
 /* Default VL for tasks that don't set it explicitly: */
 static int sve_default_vl = -1;
@@ -905,7 +910,7 @@ void fpsimd_thread_switch(struct task_struct *next)
 		 */
 		struct fpsimd_state *st = &next->thread.fpsimd_state;
 
-		if (__this_cpu_read(fpsimd_last_state) == st
+		if (__this_cpu_read(fpsimd_last_state.st) == st
 		    && st->cpu == smp_processor_id())
 			clear_tsk_thread_flag(next, TIF_FOREIGN_FPSTATE);
 		else
@@ -997,9 +1002,12 @@ void fpsimd_signal_preserve_current_state(void)
  */
 static void fpsimd_bind_to_cpu(void)
 {
+	struct fpsimd_last_state_struct *last =
+		this_cpu_ptr(&fpsimd_last_state);
 	struct fpsimd_state *st = &current->thread.fpsimd_state;
 
-	__this_cpu_write(fpsimd_last_state, st);
+	last->st = st;
+	last->sve_in_use = test_thread_flag(TIF_SVE);
 	st->cpu = smp_processor_id();
 }
 
@@ -1057,7 +1065,7 @@ void fpsimd_flush_task_state(struct task_struct *t)
 
 static inline void fpsimd_flush_cpu_state(void)
 {
-	__this_cpu_write(fpsimd_last_state, NULL);
+	__this_cpu_write(fpsimd_last_state.st, NULL);
 }
 
 /*
@@ -1070,14 +1078,10 @@ static inline void fpsimd_flush_cpu_state(void)
 #ifdef CONFIG_ARM64_SVE
 void sve_flush_cpu_state(void)
 {
-	struct fpsimd_state *const fpstate = __this_cpu_read(fpsimd_last_state);
-	struct task_struct *tsk;
-
-	if (!fpstate)
-		return;
+	struct fpsimd_last_state_struct const *last =
+		this_cpu_ptr(&fpsimd_last_state);
 
-	tsk = container_of(fpstate, struct task_struct, thread.fpsimd_state);
-	if (test_tsk_thread_flag(tsk, TIF_SVE))
+	if (last->st && last->sve_in_use)
 		fpsimd_flush_cpu_state();
 }
 #endif /* CONFIG_ARM64_SVE */
@@ -1272,7 +1276,7 @@ static inline void fpsimd_pm_init(void) { }
 #ifdef CONFIG_HOTPLUG_CPU
 static int fpsimd_cpu_dead(unsigned int cpu)
 {
-	per_cpu(fpsimd_last_state, cpu) = NULL;
+	per_cpu(fpsimd_last_state.st, cpu) = NULL;
 	return 0;
 }
 
-- 
2.1.4