x86/fpu: Make xfeatures_mask_all __ro_after_init

Nothing has to modify this after init.

But of course there is code which unconditionally masks
xfeatures_mask_all on CPU hotplug. This goes unnoticed during boot
hotplug because at that point the variable is still RW mapped.

This is broken in several ways:

  1) Masking this in post init CPU hotplug means that any
     modification of this state goes unnoticed until actual hotplug
     happens.

  2) If that ever happens then these bogus feature bits are already
     populated all over the place and the system is in inconsistent state
     vs. the compacted XSTATE offsets. If at all then this has to panic the
     machine because the inconsistency cannot be undone anymore.

Make this a one-time paranoia check in xstate init code and disable
xsave when this happens.

Reported-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210623121451.712803952@linutronix.de

arch/x86/kernel/fpu/xstate.c

@@ -59,7 +59,7 @@ static short xsave_cpuid_features[] __initdata = {
  * This represents the full set of bits that should ever be set in a kernel
  * XSAVE buffer, both supervisor and user xstates.
  */
-u64 xfeatures_mask_all __read_mostly;
+u64 xfeatures_mask_all __ro_after_init;
 
 static unsigned int xstate_offsets[XFEATURE_MAX] __ro_after_init =
 	{ [ 0 ... XFEATURE_MAX - 1] = -1};
@@ -213,19 +213,8 @@ void fpstate_sanitize_xstate(struct fpu *fpu)
  */
 void fpu__init_cpu_xstate(void)
 {
-	u64 unsup_bits;
-
 	if (!boot_cpu_has(X86_FEATURE_XSAVE) || !xfeatures_mask_all)
 		return;
-	/*
-	 * Unsupported supervisor xstates should not be found in
-	 * the xfeatures mask.
-	 */
-	unsup_bits = xfeatures_mask_all & XFEATURE_MASK_SUPERVISOR_UNSUPPORTED;
-	WARN_ONCE(unsup_bits, "x86/fpu: Found unsupported supervisor xstates: 0x%llx\n",
-		  unsup_bits);
-
-	xfeatures_mask_all &= ~XFEATURE_MASK_SUPERVISOR_UNSUPPORTED;
 
 	cr4_set_bits(X86_CR4_OSXSAVE);
 
@@ -825,6 +814,7 @@ void __init fpu__init_system_xstate(void)
 {
 	unsigned int eax, ebx, ecx, edx;
 	static int on_boot_cpu __initdata = 1;
+	u64 xfeatures;
 	int err;
 	int i;
 
@@ -879,6 +869,8 @@ void __init fpu__init_system_xstate(void)
 	}
 
 	xfeatures_mask_all &= fpu__get_supported_xfeatures_mask();
+	/* Store it for paranoia check at the end */
+	xfeatures = xfeatures_mask_all;
 
 	/* Enable xstate instructions to be able to continue with initialization: */
 	fpu__init_cpu_xstate();
@@ -896,8 +888,18 @@ void __init fpu__init_system_xstate(void)
 	setup_init_fpu_buf();
 	setup_xstate_comp_offsets();
 	setup_supervisor_only_offsets();
-	print_xstate_offset_size();
 
+	/*
+	 * Paranoia check whether something in the setup modified the
+	 * xfeatures mask.
+	 */
+	if (xfeatures != xfeatures_mask_all) {
+		pr_err("x86/fpu: xfeatures modified from 0x%016llx to 0x%016llx during init, disabling XSAVE\n",
+		       xfeatures, xfeatures_mask_all);
+		goto out_disable;
+	}
+
+	print_xstate_offset_size();
 	pr_info("x86/fpu: Enabled xstate features 0x%llx, context size is %d bytes, using '%s' format.\n",
 		xfeatures_mask_all,
 		fpu_kernel_xstate_size,