MIPS: init upper 64b of vector registers when MSA is first used

When a task first makes use of MSA we need to ensure that the upper
64b of the vector registers are set to some value such that no
information can be leaked to it from the previous task to use MSA
context on the CPU. The architecture formerly specified that these
bits would be cleared to 0 when a scalar FP instructions wrote to the
aliased FP registers, which would have implicitly handled this as the
kernel restored scalar FP context. However more recent versions of the
specification now state that the value of the bits in such cases is
unpredictable. Initialise them explictly to be sure, and set all the
bits to 1 rather than 0 for consistency with the least significant
64b.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/7497/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

2 files changed, 35 insertions, 9 deletions

diff --git a/arch/mips/kernel/r4k_switch.S b/arch/mips/kernel/r4k_switch.S
index 1a1aef0..4c4ec18 100644
--- a/arch/mips/kernel/r4k_switch.S
+++ b/arch/mips/kernel/r4k_switch.S
@@ -144,6 +144,11 @@ LEAF(_restore_msa)
 	jr	ra
 	END(_restore_msa)
 
+LEAF(_init_msa_upper)
+	msa_init_all_upper
+	jr	ra
+	END(_init_msa_upper)
+
 #endif
 
 /*
diff --git a/arch/mips/kernel/traps.c b/arch/mips/kernel/traps.c
index 649c151..1ed8457 100644
--- a/arch/mips/kernel/traps.c
+++ b/arch/mips/kernel/traps.c
@@ -1089,13 +1089,15 @@ static int default_cu2_call(struct notifier_block *nfb, unsigned long action,
 
 static int enable_restore_fp_context(int msa)
 {
-	int err, was_fpu_owner;
+	int err, was_fpu_owner, prior_msa;
 
 	if (!used_math()) {
 		/* First time FP context user. */
 		err = init_fpu();
-		if (msa && !err)
+		if (msa && !err) {
 			enable_msa();
+			_init_msa_upper();
+		}
 		if (!err)
 			set_used_math();
 		return err;
@@ -1147,18 +1149,37 @@ static int enable_restore_fp_context(int msa)
 	/*
 	 * If this is the first time that the task is using MSA and it has
 	 * previously used scalar FP in this time slice then we already nave
-	 * FP context which we shouldn't clobber.
+	 * FP context which we shouldn't clobber. We do however need to clear
+	 * the upper 64b of each vector register so that this task has no
+	 * opportunity to see data left behind by another.
 	 */
-	if (!test_and_set_thread_flag(TIF_MSA_CTX_LIVE) && was_fpu_owner)
+	prior_msa = test_and_set_thread_flag(TIF_MSA_CTX_LIVE);
+	if (!prior_msa && was_fpu_owner) {
+		_init_msa_upper();
 		return 0;
+	}
 
-	/* We need to restore the vector context. */
-	restore_msa(current);
+	if (!prior_msa) {
+		/*
+		 * Restore the least significant 64b of each vector register
+		 * from the existing scalar FP context.
+		 */
+		_restore_fp(current);
 
-	/* Restore the scalar FP control & status register */
-	if (!was_fpu_owner)
-		asm volatile("ctc1 %0, $31" : : "r"(current->thread.fpu.fcr31));
+		/*
+		 * The task has not formerly used MSA, so clear the upper 64b
+		 * of each vector register such that it cannot see data left
+		 * behind by another task.
+		 */
+		_init_msa_upper();
+	} else {
+		/* We need to restore the vector context. */
+		restore_msa(current);
 
+		/* Restore the scalar FP control & status register */
+		if (!was_fpu_owner)
+			asm volatile("ctc1 %0, $31" : : "r"(current->thread.fpu.fcr31));
+	}
 	return 0;
 }