[PATCH 3/4] x86/boot: Explicitly pass NX enabling status

From: Dave Hansen
Date: Wed Apr 03 2024 - 11:46:30 EST



From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

The kernel sometimes needs to mask unsupported bits out of page
table entries. It does that with a mask: '__supported_pte_mask'.

That mask can obviously only contain the No-eXecute bit (_PAGE_NX)
on hardware where NX is supported. x86_configure_nx() checks the
boot CPU's NX support and adjusts the mask appropriately.

But it doesn't check support directly. It uses the venerable
'boot_cpu_data' which is a software approximation of the actual CPU
support. Unfortunately, Xen wants to set up '__supported_pte_mask'
before 'boot_cpu_data' has been initialized. It hacks around this
problem by repeating some of the 'boot_cpu_data' setup *just* for
NX.

Have x86_configure_nx() stop consulting 'boot_cpu_data' and move
the NX detection to the caller.

No functional change. That will come later.

Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Reviewed-by: Kai Huang <kai.huang@xxxxxxxxx>
---

b/arch/x86/include/asm/proto.h | 2 +-
b/arch/x86/kernel/setup.c | 6 +++---
b/arch/x86/xen/enlighten_pv.c | 2 +-
3 files changed, 5 insertions(+), 5 deletions(-)

diff -puN arch/x86/include/asm/proto.h~x86_configure_nx-arg arch/x86/include/asm/proto.h
--- a/arch/x86/include/asm/proto.h~x86_configure_nx-arg 2024-04-02 15:22:59.638913056 -0700
+++ b/arch/x86/include/asm/proto.h 2024-04-02 15:22:59.642913062 -0700
@@ -37,7 +37,7 @@ void entry_SYSRETL_compat_end(void);
#define entry_SYSENTER_compat NULL
#endif

-void x86_configure_nx(void);
+void x86_configure_nx(bool nx_supported);

extern int reboot_force;

diff -puN arch/x86/kernel/setup.c~x86_configure_nx-arg arch/x86/kernel/setup.c
--- a/arch/x86/kernel/setup.c~x86_configure_nx-arg 2024-04-02 15:22:59.638913056 -0700
+++ b/arch/x86/kernel/setup.c 2024-04-02 15:22:59.642913062 -0700
@@ -687,9 +687,9 @@ dump_kernel_offset(struct notifier_block
return 0;
}

-void x86_configure_nx(void)
+void x86_configure_nx(bool nx_supported)
{
- if (boot_cpu_has(X86_FEATURE_NX))
+ if (nx_supported)
__supported_pte_mask |= _PAGE_NX;
else
__supported_pte_mask &= ~_PAGE_NX;
@@ -853,7 +853,7 @@ void __init setup_arch(char **cmdline_p)
* whether hardware doesn't support NX (so that the early EHCI debug
* console setup can safely call set_fixmap()).
*/
- x86_configure_nx();
+ x86_configure_nx(boot_cpu_has(X86_FEATURE_NX));

parse_early_param();

diff -puN arch/x86/xen/enlighten_pv.c~x86_configure_nx-arg arch/x86/xen/enlighten_pv.c
--- a/arch/x86/xen/enlighten_pv.c~x86_configure_nx-arg 2024-04-02 15:22:59.638913056 -0700
+++ b/arch/x86/xen/enlighten_pv.c 2024-04-02 15:22:59.642913062 -0700
@@ -1371,7 +1371,7 @@ asmlinkage __visible void __init xen_sta

/* Work out if we support NX */
get_cpu_cap(&boot_cpu_data);
- x86_configure_nx();
+ x86_configure_nx(boot_cpu_has(X86_FEATURE_NX));

/*
* Set up kernel GDT and segment registers, mainly so that
_