x86, smep, smap: Make the switching functions one-way

There is no fundamental reason why we should switch SMEP and SMAP on
during early cpu initialization just to switch them off again. Now
with %eflags and %cr4 forced to be initialized to a clean state, we
only need the one-way enable. Also, make the functions inline to make
them (somewhat) harder to abuse.

This does mean that SMEP and SMAP do not get initialized anywhere near
as early. Even using early_param() instead of __setup() doesn't give
us control early enough to do this during the early cpu initialization
phase. This seems reasonable to me, because SMEP and SMAP should not
matter until we have userspace to protect ourselves from, but it does
potentially make it possible for a bug involving a "leak of
permissions to userspace" to get uncaught.

Signed-off-by: H. Peter Anvin <[e-mail]>

arch/x86/kernel/cpu/common.c

···
 }
 #endif
 
-static int disable_smep __cpuinitdata;
 static __init int setup_disable_smep(char *arg)
 {
-	disable_smep = 1;
+	setup_clear_cpu_cap(X86_FEATURE_SMEP);
 	return 1;
 }
 __setup("nosmep", setup_disable_smep);
 
-static __cpuinit void setup_smep(struct cpuinfo_x86 *c)
+static __always_inline void setup_smep(struct cpuinfo_x86 *c)
 {
-	if (cpu_has(c, X86_FEATURE_SMEP)) {
-		if (unlikely(disable_smep)) {
-			setup_clear_cpu_cap(X86_FEATURE_SMEP);
-			clear_in_cr4(X86_CR4_SMEP);
-		} else
-			set_in_cr4(X86_CR4_SMEP);
-	}
+	if (cpu_has(c, X86_FEATURE_SMEP))
+		set_in_cr4(X86_CR4_SMEP);
 }
 
-static int disable_smap __cpuinitdata;
 static __init int setup_disable_smap(char *arg)
 {
-	disable_smap = 1;
+	setup_clear_cpu_cap(X86_FEATURE_SMAP);
 	return 1;
 }
 __setup("nosmap", setup_disable_smap);
 
-static __cpuinit void setup_smap(struct cpuinfo_x86 *c)
+static __always_inline void setup_smap(struct cpuinfo_x86 *c)
 {
-	if (cpu_has(c, X86_FEATURE_SMAP)) {
-		if (unlikely(disable_smap)) {
-			setup_clear_cpu_cap(X86_FEATURE_SMAP);
-			clear_in_cr4(X86_CR4_SMAP);
-		} else {
-			set_in_cr4(X86_CR4_SMAP);
-			/*
-			 * Don't use clac() here since alternatives
-			 * haven't run yet...
-			 */
-			asm volatile(__stringify(__ASM_CLAC) ::: "memory");
-		}
-	}
+	unsigned long eflags;
+
+	/* This should have been cleared long ago */
+	raw_local_save_flags(eflags);
+	BUG_ON(eflags & X86_EFLAGS_AC);
+
+	if (cpu_has(c, X86_FEATURE_SMAP))
+		set_in_cr4(X86_CR4_SMAP);
 }
 
 /*
···
 	c->cpu_index = 0;
 	filter_cpuid_features(c, false);
 
-	setup_smep(c);
-	setup_smap(c);
-
 	if (this_cpu->c_bsp_init)
 		this_cpu->c_bsp_init(c);
 }
···
 		c->phys_proc_id = c->initial_apicid;
 	}
 
-	setup_smep(c);
-
 	get_model_name(c); /* Default name */
 
 	detect_nopl(c);
···
 
 	/* Disable the PN if appropriate */
 	squash_the_stupid_serial_number(c);
+
+	/* Set up SMEP/SMAP */
+	setup_smep(c);
+	setup_smap(c);
 
 	/*
 	 * The vendor-specific functions might have changed features.