[PATCH v8 3/5] arm64: Change the tagged_addr sysctl control semantics to only prevent the opt-in

[Catalin Marinas]

First rename the sysctl control to abi.tagged_addr_disabled and make it
default off (zero). When abi.tagged_addr_disabled == 1, only block the
enabling of the TBI ABI via prctl(PR_SET_TAGGED_ADDR_CTRL, 
PR_TAGGED_ADDR_ENABLE).
Getting the status of the ABI or disabling it is still allowed.

Signed-off-by: Catalin Marinas <catalin.mari...@arm.com>
---
 arch/arm64/kernel/process.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 76b7c55026aa..03689c0beb34 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -579,17 +579,22 @@ void arch_setup_new_exec(void)
 /*
  * Control the relaxed ABI allowing tagged user addresses into the kernel.
  */
-static unsigned int tagged_addr_prctl_allowed = 1;
+static unsigned int tagged_addr_disabled;
 
 long set_tagged_addr_ctrl(unsigned long arg)
 {
-       if (!tagged_addr_prctl_allowed)
-               return -EINVAL;
        if (is_compat_task())
                return -EINVAL;
        if (arg & ~PR_TAGGED_ADDR_ENABLE)
                return -EINVAL;
 
+       /*
+        * Do not allow the enabling of the tagged address ABI if globally
+        * disabled via sysctl abi.tagged_addr_disabled.
+        */
+       if (arg & PR_TAGGED_ADDR_ENABLE && tagged_addr_disabled)
+               return -EINVAL;
+
        update_thread_flag(TIF_TAGGED_ADDR, arg & PR_TAGGED_ADDR_ENABLE);
 
        return 0;
@@ -597,8 +602,6 @@ long set_tagged_addr_ctrl(unsigned long arg)
 
 long get_tagged_addr_ctrl(void)
 {
-       if (!tagged_addr_prctl_allowed)
-               return -EINVAL;
        if (is_compat_task())
                return -EINVAL;
 
@@ -618,9 +621,9 @@ static int one = 1;
 
 static struct ctl_table tagged_addr_sysctl_table[] = {
        {
-               .procname       = "tagged_addr",
+               .procname       = "tagged_addr_disabled",
                .mode           = 0644,
-               .data           = &tagged_addr_prctl_allowed,
+               .data           = &tagged_addr_disabled,
                .maxlen         = sizeof(int),
                .proc_handler   = proc_dointvec_minmax,
                .extra1         = &zero,