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

When we parse the ZA 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-v3-6-4545c94b20ff@kernel.orgSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/signal.c

@@ -435,17 +435,18 @@ static int preserve_za_context(struct za_context __user *ctx)
 
 static int restore_za_context(struct user_ctxs *user)
 {
-	int err;
+	int err = 0;
 	unsigned int vq;
-	struct za_context za;
+	u16 user_vl;
 
 	if (user->za_size < sizeof(*user->za))
 		return -EINVAL;
 
-	if (__copy_from_user(&za, user->za, sizeof(za)))
-		return -EFAULT;
+	__get_user_error(user_vl, &(user->za->vl), err);
+	if (err)
+		return err;
 
-	if (za.vl != task_get_sme_vl(current))
+	if (user_vl != task_get_sme_vl(current))
 		return -EINVAL;
 
 	if (user->za_size == sizeof(*user->za)) {
@@ -453,7 +454,7 @@ static int restore_za_context(struct user_ctxs *user)
 		return 0;
 	}
 
-	vq = sve_vq_from_vl(za.vl);
+	vq = sve_vq_from_vl(user_vl);
 
 	if (user->za_size < ZA_SIG_CONTEXT_SIZE(vq))
 		return -EINVAL;