arm64/signal: Only read new data when parsing the SVE context

When we parse the SVE signal context we read the entire context from
userspace, including the generic signal context header which was already
read by parse_user_sigframe() and padding bytes that we ignore. Avoid the
possibility of relying on the second read of the data read twice by only
reading the data which we are actually going to use.
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221212-arm64-signal-cleanup-v3-5-4545c94b20ff@kernel.orgSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/signal.c

@@ -271,18 +271,20 @@ static int preserve_sve_context(struct sve_context __user *ctx)
 
 static int restore_sve_fpsimd_context(struct user_ctxs *user)
 {
-	int err;
+	int err = 0;
 	unsigned int vl, vq;
 	struct user_fpsimd_state fpsimd;
-	struct sve_context sve;
+	u16 user_vl, flags;
 
 	if (user->sve_size < sizeof(*user->sve))
 		return -EINVAL;
 
-	if (__copy_from_user(&sve, user->sve, sizeof(sve)))
-		return -EFAULT;
+	__get_user_error(user_vl, &(user->sve->vl), err);
+	__get_user_error(flags, &(user->sve->flags), err);
+	if (err)
+		return err;
 
-	if (sve.flags & SVE_SIG_FLAG_SM) {
+	if (flags & SVE_SIG_FLAG_SM) {
 		if (!system_supports_sme())
 			return -EINVAL;
 
@@ -294,7 +296,7 @@ static int restore_sve_fpsimd_context(struct user_ctxs *user)
 		vl = task_get_sve_vl(current);
 	}
 
-	if (sve.vl != vl)
+	if (user_vl != vl)
 		return -EINVAL;
 
 	if (user->sve_size == sizeof(*user->sve)) {
@@ -304,7 +306,7 @@ static int restore_sve_fpsimd_context(struct user_ctxs *user)
 		goto fpsimd_only;
 	}
 
-	vq = sve_vq_from_vl(sve.vl);
+	vq = sve_vq_from_vl(vl);
 
 	if (user->sve_size < SVE_SIG_CONTEXT_SIZE(vq))
 		return -EINVAL;
@@ -332,7 +334,7 @@ static int restore_sve_fpsimd_context(struct user_ctxs *user)
 	if (err)
 		return -EFAULT;
 
-	if (sve.flags & SVE_SIG_FLAG_SM)
+	if (flags & SVE_SIG_FLAG_SM)
 		current->thread.svcr |= SVCR_SM_MASK;
 	else
 		set_thread_flag(TIF_SVE);