arm64: reject prctl(PR_PAC_RESET_KEYS) on compat tasks

It doesn't make sense to issue prctl(PR_PAC_RESET_KEYS) on a
compat task because the 32-bit instruction set does not offer PAuth
instructions. For consistency with other 64-bit only prctls such as
{SET,GET}_TAGGED_ADDR_CTRL, reject the prctl on compat tasks.

Although this is a userspace-visible change, maybe it isn't too late
to make this change given that the hardware isn't available yet and
it's very unlikely that anyone has 32-bit software that actually
depends on this succeeding.
Signed-off-by: Peter Collingbourne <pcc@google.com>
Link: https://linux-review.googlesource.com/id/Ie885a1ff84ab498cc9f62d6451e9f2cfd4b1d06a
Link: https://lore.kernel.org/r/20201014052430.11630-1-pcc@google.com
[will: Do the same for the SVE prctl()s]
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/fpsimd.c

@@ -678,7 +678,7 @@ int sve_set_current_vl(unsigned long arg)
 	vl = arg & PR_SVE_VL_LEN_MASK;
 	flags = arg & ~vl;
 
-	if (!system_supports_sve())
+	if (!system_supports_sve() || is_compat_task())
 		return -EINVAL;
 
 	ret = sve_set_vector_length(current, vl, flags);
@@ -691,7 +691,7 @@ int sve_set_current_vl(unsigned long arg)
 /* PR_SVE_GET_VL */
 int sve_get_current_vl(void)
 {
-	if (!system_supports_sve())
+	if (!system_supports_sve() || is_compat_task())
 		return -EINVAL;
 
 	return sve_prctl_status(0);

arch/arm64/kernel/pointer_auth.c

 // SPDX-License-Identifier: GPL-2.0
 
+#include <linux/compat.h>
 #include <linux/errno.h>
 #include <linux/prctl.h>
 #include <linux/random.h>
@@ -17,6 +18,9 @@ int ptrauth_prctl_reset_keys(struct task_struct *tsk, unsigned long arg)
 	if (!system_supports_address_auth() && !system_supports_generic_auth())
 		return -EINVAL;
 
+	if (is_compat_thread(task_thread_info(tsk)))
+		return -EINVAL;
+
 	if (!arg) {
 		ptrauth_keys_init_user(keys);
 		return 0;