[Xen-devel] [PATCH] x86: introduce and use setup_force_cpu_cap()

["Jan Beulich" <JBeulich@xxxxxxxx>]

For XEN_SMEP and XEN_SMAP to not be cleared while bringing up APs we'd
need to clone the respective hack used for CPUID_FAULTING. Introduce an
inverse of setup_clear_cpu_cap() instead, but let clearing of features
overrule forced setting of them.

XEN_SMAP being wrong post-boot is a problem specifically for live
patching, as a live patch may need alternative instruction patching
keyed off of that feature flag.

Reported-by: Sarah Newman <security@xxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/apic.c
+++ b/xen/arch/x86/apic.c
@@ -853,7 +853,7 @@ static int __init detect_init_APIC (void
         return -1;
     }
 
-    __set_bit(X86_FEATURE_APIC, boot_cpu_data.x86_capability);
+    setup_force_cpu_cap(X86_FEATURE_APIC);
     mp_lapic_addr = APIC_DEFAULT_PHYS_BASE;
 
     /* The BIOS may have set up the APIC at some other address */
--- a/xen/arch/x86/cpu/common.c
+++ b/xen/arch/x86/cpu/common.c
@@ -54,6 +54,7 @@ unsigned int vaddr_bits __read_mostly =
 u64 host_pat = 0x050100070406;
 
 static unsigned int cleared_caps[NCAPINTS];
+static unsigned int forced_caps[NCAPINTS];
 
 void __init setup_clear_cpu_cap(unsigned int cap)
 {
@@ -63,6 +64,10 @@ void __init setup_clear_cpu_cap(unsigned
        if (__test_and_set_bit(cap, cleared_caps))
                return;
 
+       if (test_bit(cap, forced_caps))
+               printk("%pS clearing previously forced feature %#x\n",
+                      __builtin_return_address(0), cap);
+
        __clear_bit(cap, boot_cpu_data.x86_capability);
        dfs = lookup_deep_deps(cap);
 
@@ -72,7 +77,26 @@ void __init setup_clear_cpu_cap(unsigned
        for (i = 0; i < FSCAPINTS; ++i) {
                cleared_caps[i] |= dfs[i];
                boot_cpu_data.x86_capability[i] &= ~dfs[i];
+               if (!(forced_caps[i] & dfs[i]))
+                       continue;
+               printk("%pS implicitly clearing previously forced feature(s) 
%u:%#x\n",
+                      __builtin_return_address(0),
+                      i, forced_caps[i] & dfs[i]);
+       }
+}
+
+void __init setup_force_cpu_cap(unsigned int cap)
+{
+       if (__test_and_set_bit(cap, forced_caps))
+               return;
+
+       if (test_bit(cap, cleared_caps)) {
+               printk("%pS tries to force previously cleared feature %#x\n",
+                      __builtin_return_address(0), cap);
+               return;
        }
+
+       __set_bit(cap, boot_cpu_data.x86_capability);
 }
 
 static void default_init(struct cpuinfo_x86 * c)
@@ -375,8 +399,10 @@ void identify_cpu(struct cpuinfo_x86 *c)
        for (i = 0; i < FSCAPINTS; ++i)
                c->x86_capability[i] &= known_features[i];
 
-       for (i = 0 ; i < NCAPINTS ; ++i)
+       for (i = 0 ; i < NCAPINTS ; ++i) {
+               c->x86_capability[i] |= forced_caps[i];
                c->x86_capability[i] &= ~cleared_caps[i];
+       }
 
        /* If the model name is still unset, do table lookup. */
        if ( !c->x86_model_id[0] ) {
--- a/xen/arch/x86/cpu/intel.c
+++ b/xen/arch/x86/cpu/intel.c
@@ -27,7 +27,7 @@ static bool __init probe_intel_cpuid_fau
 
        expected_levelling_cap |= LCAP_faulting;
        levelling_caps |=  LCAP_faulting;
-       __set_bit(X86_FEATURE_CPUID_FAULTING, boot_cpu_data.x86_capability);
+       setup_force_cpu_cap(X86_FEATURE_CPUID_FAULTING);
        return 1;
 }
 
@@ -320,9 +320,6 @@ static void early_init_intel(struct cpui
        if (c == &boot_cpu_data)
                intel_init_levelling();
 
-       if (test_bit(X86_FEATURE_CPUID_FAULTING, boot_cpu_data.x86_capability))
-               __set_bit(X86_FEATURE_CPUID_FAULTING, c->x86_capability);
-
        intel_ctxt_switch_levelling(NULL);
 }
 
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -1485,14 +1485,14 @@ void __init noreturn __start_xen(unsigne
     if ( !opt_smep )
         setup_clear_cpu_cap(X86_FEATURE_SMEP);
     if ( cpu_has_smep && opt_smep != SMEP_HVM_ONLY )
-        __set_bit(X86_FEATURE_XEN_SMEP, boot_cpu_data.x86_capability);
+        setup_force_cpu_cap(X86_FEATURE_XEN_SMEP);
     if ( boot_cpu_has(X86_FEATURE_XEN_SMEP) )
         set_in_cr4(X86_CR4_SMEP);
 
     if ( !opt_smap )
         setup_clear_cpu_cap(X86_FEATURE_SMAP);
     if ( cpu_has_smap && opt_smap != SMAP_HVM_ONLY )
-        __set_bit(X86_FEATURE_XEN_SMAP, boot_cpu_data.x86_capability);
+        setup_force_cpu_cap(X86_FEATURE_XEN_SMAP);
     if ( boot_cpu_has(X86_FEATURE_XEN_SMAP) )
         set_in_cr4(X86_CR4_SMAP);
 
--- a/xen/include/asm-x86/processor.h
+++ b/xen/include/asm-x86/processor.h
@@ -169,6 +169,7 @@ extern const struct x86_cpu_id *x86_matc
 
 extern void identify_cpu(struct cpuinfo_x86 *);
 extern void setup_clear_cpu_cap(unsigned int);
+extern void setup_force_cpu_cap(unsigned int);
 extern void print_cpu_info(unsigned int cpu);
 extern unsigned int init_intel_cacheinfo(struct cpuinfo_x86 *c);
 



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