[PATCH] arm64: Make kpti command line options x86 compatible

[Alexander Graf]

I've already stumbled over 2 cases where people got confused about how to
disable kpti on AArch64. In both cases, they used existing x86_64 options
and just applied that to an AArch64 system, expecting it to work.

I think it makes a lot of sense to have compatible kernel command line
parameters whenever we can have them be compatible.

So this patch adds the pti= and no_pti kernel command line options, mapping
them into the existing kpti= command line framework. It preserves the old
syntax to maintain compatibility with older command lines.

While at it, the patch also marks the respective options as dual-arch.

Reported-by: Richard Brown <rbr...@suse.de>
Signed-off-by: Alexander Graf <ag...@suse.de>
---
 Documentation/admin-guide/kernel-parameters.txt |  6 +++---
 arch/arm64/kernel/cpufeature.c                  | 15 ++++++++++++++-
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt 
b/Documentation/admin-guide/kernel-parameters.txt
index 81d1d5a74728..4a1c6bcfcdb5 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3522,8 +3522,8 @@
        pt.             [PARIDE]
                        See Documentation/blockdev/paride.txt.
 
-       pti=            [X86_64] Control Page Table Isolation of user and
-                       kernel address spaces.  Disabling this feature
+       pti=            [X86_64,ARM64] Control Page Table Isolation of user
+                       and kernel address spaces.  Disabling this feature
                        removes hardening, but improves performance of
                        system calls and interrupts.
 
@@ -3534,7 +3534,7 @@
 
                        Not specifying this option is equivalent to pti=auto.
 
-       nopti           [X86_64]
+       nopti           [X86_64,ARM64]
                        Equivalent to pti=off
 
        pty.legacy_count=
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index af50064dea51..12bb3b0470dd 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -978,13 +978,26 @@ static int __init parse_kpti(char *str)
        bool enabled;
        int ret = strtobool(str, &enabled);
 
-       if (ret)
+       if (ret) {
+               if (!strncmp(str, "auto", 4)) {
+                       __kpti_forced = 0;
+                       return 0;
+               }
                return ret;
+       }
 
        __kpti_forced = enabled ? 1 : -1;
        return 0;
 }
 early_param("kpti", parse_kpti);
+early_param("pti", parse_kpti);
+
+static int __init handle_no_pti(char *p)
+{
+       __kpti_forced = -1;
+       return 0;
+}
+early_param("nopti", parse_no_pti);
 #endif /* CONFIG_UNMAP_KERNEL_AT_EL0 */
 
 #ifdef CONFIG_ARM64_HW_AFDBM
-- 
2.12.3