[PATCH] arm64: reject prctl(PR_PAC_RESET_KEYS) on compat tasks

[Peter Collingbourne]

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.

Link: https://linux-review.googlesource.com/id/Ie885a1ff84ab498cc9f62d6451e9f2cfd4b1d06a
Signed-off-by: Peter Collingbourne <pcc at google.com>
---
 arch/arm64/kernel/pointer_auth.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm64/kernel/pointer_auth.c b/arch/arm64/kernel/pointer_auth.c
index 1e77736a4f66..245c3ee97ed8 100644
--- a/arch/arm64/kernel/pointer_auth.c
+++ b/arch/arm64/kernel/pointer_auth.c
@@ -1,5 +1,6 @@
 // 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_task())
+		return -EINVAL;
+
 	if (!arg) {
 		ptrauth_keys_init_user(keys);
 		return 0;
-- 
2.28.0.1011.ga647a8990f-goog