x86/pv: Rewrite guest %cr4 handling from scratch

The PV cr4 logic is almost impossible to follow, and leaks bits into guest
context which definitely shouldn't be visible (in particular, VMXE).

The biggest problem however, and source of the complexity, is that it derives
new real and guest cr4 values from the current value in hardware - this is
context dependent and an inappropriate source of information.

Rewrite the cr4 logic to be invariant of the current value in hardware.

First of all, modify write_ptbase() to always use mmu_cr4_features for IDLE
and HVM contexts.  mmu_cr4_features *is* the correct value to use, and makes
the ASSERT() obviously redundant.

For PV guests, curr->arch.pv.ctrlreg[4] remains the guests view of cr4, but
all logic gets reworked in terms of this and mmu_cr4_features only.

Two masks are introduced; bits which the guest has control over, and bits
which are forwarded from Xen's settings.  One guest-visible change here is
that Xen's VMXE setting is no longer visible at all.

pv_make_cr4() follows fairly closely from pv_guest_cr4_to_real_cr4(), but
deliberately starts with mmu_cr4_features, and only alters the minimal subset
of bits.

The boot-time {compat_,}pv_cr4_mask variables are removed, as they are a
remnant of the pre-CPUID policy days.  pv_fixup_guest_cr4() gains a related
derivation from the policy.

Another guest visible change here is that a 32bit PV guest can now flip
FSGSBASE in its view of CR4.  While the {RD,WR}{FS,GS}BASE instructions are
unusable outside of a 64bit code segment, the ability to modify FSGSBASE
matches real hardware behaviour, and avoids the need for any 32bit/64bit
differences in the logic.

Overall, this patch shouldn't have a practical change in guest behaviour.
VMXE will disappear from view, and an inquisitive 32bit kernel can now see
FSGSBASE changing, but this new logic is otherwise bug-compatible with before.

This is part of XSA-293.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index b5febd67b4e5cbc2ed48f35b3de98cadb91d1b42..fcb565b0fb2b3a00f6d90fd23c57dbf92753927d 100644 (file)
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -768,49 +768,6 @@ void arch_domain_creation_finished(struct domain *d)
 {
 }
 
-/*
- * These are the masks of CR4 bits (subject to hardware availability) which a
- * PV guest may not legitimiately attempt to modify.
- */
-static unsigned long __read_mostly pv_cr4_mask, compat_pv_cr4_mask;
-
-static int __init init_pv_cr4_masks(void)
-{
-    unsigned long common_mask = ~X86_CR4_TSD;
-
-    /*
-     * All PV guests may attempt to modify TSD, DE and OSXSAVE.
-     */
-    if ( cpu_has_de )
-        common_mask &= ~X86_CR4_DE;
-    if ( cpu_has_xsave )
-        common_mask &= ~X86_CR4_OSXSAVE;
-
-    pv_cr4_mask = compat_pv_cr4_mask = common_mask;
-
-    /*
-     * 64bit PV guests may attempt to modify FSGSBASE.
-     */
-    if ( cpu_has_fsgsbase )
-        pv_cr4_mask &= ~X86_CR4_FSGSBASE;
-
-    return 0;
-}
-__initcall(init_pv_cr4_masks);
-
-unsigned long pv_guest_cr4_fixup(const struct vcpu *v, unsigned long guest_cr4)
-{
-    unsigned long hv_cr4 = real_cr4_to_pv_guest_cr4(read_cr4());
-    unsigned long mask = is_pv_32bit_vcpu(v) ? compat_pv_cr4_mask : pv_cr4_mask;
-
-    if ( (guest_cr4 & mask) != (hv_cr4 & mask) )
-        printk(XENLOG_G_WARNING
-               "d%d attempted to change %pv's CR4 flags %08lx -> %08lx\n",
-               current->domain->domain_id, v, hv_cr4, guest_cr4);
-
-    return (hv_cr4 & mask) | (guest_cr4 & ~mask);
-}
-
 #define xen_vcpu_guest_context vcpu_guest_context
 #define fpu_ctxt fpu_ctxt.x
 CHECK_FIELD_(struct, vcpu_guest_context, fpu_ctxt);
@@ -828,7 +785,6 @@ int arch_set_info_guest(
 #ifdef CONFIG_PV
     unsigned long cr3_gfn;
     struct page_info *cr3_page;
-    unsigned long cr4;
     int rc = 0;
 #endif
 
@@ -1004,9 +960,7 @@ int arch_set_info_guest(
     v->arch.pv.ctrlreg[0] &= X86_CR0_TS;
     v->arch.pv.ctrlreg[0] |= read_cr0() & ~X86_CR0_TS;
 
-    cr4 = v->arch.pv.ctrlreg[4];
-    v->arch.pv.ctrlreg[4] = cr4 ? pv_guest_cr4_fixup(v, cr4) :
-        real_cr4_to_pv_guest_cr4(mmu_cr4_features);
+    v->arch.pv.ctrlreg[4] = pv_fixup_guest_cr4(v, v->arch.pv.ctrlreg[4]);
 
     memset(v->arch.dr, 0, sizeof(v->arch.dr));
     v->arch.dr6 = X86_DR6_DEFAULT;

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index a63040a07109d8b3f9952b7713a27eb5a0bce5cc..3557cd1178989d30d6860896d4fe71aaac45e5af 100644 (file)
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -545,33 +545,13 @@ void make_cr3(struct vcpu *v, mfn_t mfn)
         v->arch.cr3 |= get_pcid_bits(v, false);
 }
 
-unsigned long pv_guest_cr4_to_real_cr4(const struct vcpu *v)
-{
-    const struct domain *d = v->domain;
-    unsigned long cr4;
-
-    cr4 = v->arch.pv.ctrlreg[4] & ~X86_CR4_DE;
-    cr4 |= mmu_cr4_features & (X86_CR4_PSE | X86_CR4_SMEP | X86_CR4_SMAP |
-                               X86_CR4_OSXSAVE | X86_CR4_FSGSBASE);
-
-    if ( d->arch.pv.pcid )
-        cr4 |= X86_CR4_PCIDE;
-    else if ( !d->arch.pv.xpti )
-        cr4 |= X86_CR4_PGE;
-
-    cr4 |= d->arch.vtsc ? X86_CR4_TSD : 0;
-
-    return cr4;
-}
-
 void write_ptbase(struct vcpu *v)
 {
     struct cpu_info *cpu_info = get_cpu_info();
     unsigned long new_cr4;
 
     new_cr4 = (is_pv_vcpu(v) && !is_idle_vcpu(v))
-              ? pv_guest_cr4_to_real_cr4(v)
-              : ((read_cr4() & ~(X86_CR4_PCIDE | X86_CR4_TSD)) | X86_CR4_PGE);
+              ? pv_make_cr4(v) : mmu_cr4_features;
 
     if ( is_pv_vcpu(v) && v->domain->arch.pv.xpti )
     {
@@ -590,8 +570,6 @@ void write_ptbase(struct vcpu *v)
         switch_cr3_cr4(v->arch.cr3, new_cr4);
         cpu_info->pv_cr3 = 0;
     }
-
-    ASSERT(is_pv_vcpu(v) || read_cr4() == mmu_cr4_features);
 }
 
 /*

diff --git a/xen/arch/x86/pv/domain.c b/xen/arch/x86/pv/domain.c
index 7e84b04082a504d5c226b933cefc2b6243021eea..91123a8b6c5ea6538179a30e8af59e441d8354a5 100644 (file)
--- a/xen/arch/x86/pv/domain.c
+++ b/xen/arch/x86/pv/domain.c
@@ -97,6 +97,52 @@ static void release_compat_l4(struct vcpu *v)
     v->arch.guest_table_user = pagetable_null();
 }
 
+unsigned long pv_fixup_guest_cr4(const struct vcpu *v, unsigned long cr4)
+{
+    const struct cpuid_policy *p = v->domain->arch.cpuid;
+
+    /* Discard attempts to set guest controllable bits outside of the policy. */
+    cr4 &= ~((p->basic.tsc     ? 0 : X86_CR4_TSD)      |
+             (p->basic.de      ? 0 : X86_CR4_DE)       |
+             (p->feat.fsgsbase ? 0 : X86_CR4_FSGSBASE) |
+             (p->basic.xsave   ? 0 : X86_CR4_OSXSAVE));
+
+    /* Masks expected to be disjoint sets. */
+    BUILD_BUG_ON(PV_CR4_GUEST_MASK & PV_CR4_GUEST_VISIBLE_MASK);
+
+    /*
+     * A guest sees the policy subset of its own choice of guest controllable
+     * bits, and a subset of Xen's choice of certain hardware settings.
+     */
+    return ((cr4 & PV_CR4_GUEST_MASK) |
+            (mmu_cr4_features & PV_CR4_GUEST_VISIBLE_MASK));
+}
+
+unsigned long pv_make_cr4(const struct vcpu *v)
+{
+    const struct domain *d = v->domain;
+    unsigned long cr4 = mmu_cr4_features &
+        ~(X86_CR4_PCIDE | X86_CR4_PGE | X86_CR4_TSD);
+
+    /*
+     * PCIDE or PGE depends on the PCID/XPTI settings, but must not both be
+     * set, as it impacts the safety of TLB flushing.
+     */
+    if ( d->arch.pv.pcid )
+        cr4 |= X86_CR4_PCIDE;
+    else if ( !d->arch.pv.xpti )
+        cr4 |= X86_CR4_PGE;
+
+    /*
+     * TSD is needed if either the guest has elected to use it, or Xen is
+     * virtualising the TSC value the guest sees.
+     */
+    if ( d->arch.vtsc || (v->arch.pv.ctrlreg[4] & X86_CR4_TSD) )
+        cr4 |= X86_CR4_TSD;
+
+    return cr4;
+}
+
 int switch_compat(struct domain *d)
 {
     struct vcpu *v;
@@ -191,7 +237,7 @@ int pv_vcpu_initialise(struct vcpu *v)
     /* PV guests by default have a 100Hz ticker. */
     v->periodic_period = MILLISECS(10);
 
-    v->arch.pv.ctrlreg[4] = real_cr4_to_pv_guest_cr4(mmu_cr4_features);
+    v->arch.pv.ctrlreg[4] = pv_fixup_guest_cr4(v, 0);
 
     if ( is_pv_32bit_domain(d) )
     {

diff --git a/xen/arch/x86/pv/emul-priv-op.c b/xen/arch/x86/pv/emul-priv-op.c
index 942ece2ca0c37a50e812db8158e420655a45be03..b4a56f9c81579e93fca503508088308eb1d2b40c 100644 (file)
--- a/xen/arch/x86/pv/emul-priv-op.c
+++ b/xen/arch/x86/pv/emul-priv-op.c
@@ -33,6 +33,7 @@
 #include <asm/hypercall.h>
 #include <asm/mc146818rtc.h>
 #include <asm/p2m.h>
+#include <asm/pv/domain.h>
 #include <asm/pv/traps.h>
 #include <asm/shared.h>
 #include <asm/traps.h>
@@ -779,8 +780,8 @@ static int write_cr(unsigned int reg, unsigned long val,
     }
 
     case 4: /* Write CR4 */
-        curr->arch.pv.ctrlreg[4] = pv_guest_cr4_fixup(curr, val);
-        write_cr4(pv_guest_cr4_to_real_cr4(curr));
+        curr->arch.pv.ctrlreg[4] = pv_fixup_guest_cr4(curr, val);
+        write_cr4(pv_make_cr4(curr));
         ctxt_switch_levelling(curr);
         return X86EMUL_OKAY;
     }

diff --git a/xen/include/asm-x86/domain.h b/xen/include/asm-x86/domain.h
index a8711bf599ef00a0ff32b23d7f7e96d555972281..214e44ce1cf6727a57e9d169a2d634e1d5f366b7 100644 (file)
--- a/xen/include/asm-x86/domain.h
+++ b/xen/include/asm-x86/domain.h
@@ -643,17 +643,6 @@ bool update_secondary_system_time(struct vcpu *,
 void vcpu_show_execution_state(struct vcpu *);
 void vcpu_show_registers(const struct vcpu *);
 
-/* Clean up CR4 bits that are not under guest control. */
-unsigned long pv_guest_cr4_fixup(const struct vcpu *, unsigned long guest_cr4);
-
-/* Convert between guest-visible and real CR4 values. */
-unsigned long pv_guest_cr4_to_real_cr4(const struct vcpu *v);
-
-#define real_cr4_to_pv_guest_cr4(c)                         \
-    ((c) & ~(X86_CR4_PGE | X86_CR4_PSE | X86_CR4_TSD |      \
-             X86_CR4_OSXSAVE | X86_CR4_SMEP |               \
-             X86_CR4_FSGSBASE | X86_CR4_SMAP | X86_CR4_PCIDE))
-
 static inline struct vcpu_guest_context *alloc_vcpu_guest_context(void)
 {
     return vmalloc(sizeof(struct vcpu_guest_context));

diff --git a/xen/include/asm-x86/pv/domain.h b/xen/include/asm-x86/pv/domain.h
index 7d9d09dd131bff21c4f6b9eda4ec7f3901a8fa43..99a0fe7b6a56b0334d102595144477383076af1c 100644 (file)
--- a/xen/include/asm-x86/pv/domain.h
+++ b/xen/include/asm-x86/pv/domain.h
@@ -61,6 +61,23 @@ int pv_vcpu_initialise(struct vcpu *v);
 void pv_domain_destroy(struct domain *d);
 int pv_domain_initialise(struct domain *d);
 
+/*
+ * Bits which a PV guest can toggle in its view of cr4.  Some are loaded into
+ * hardware, while some are fully emulated.
+ */
+#define PV_CR4_GUEST_MASK \
+    (X86_CR4_TSD | X86_CR4_DE | X86_CR4_FSGSBASE | X86_CR4_OSXSAVE)
+
+/* Bits which a PV guest may observe from the real hardware settings. */
+#define PV_CR4_GUEST_VISIBLE_MASK \
+    (X86_CR4_PAE | X86_CR4_MCE | X86_CR4_OSFXSR | X86_CR4_OSXMMEXCPT)
+
+/* Given a new cr4 value, construct the resulting guest-visible cr4 value. */
+unsigned long pv_fixup_guest_cr4(const struct vcpu *v, unsigned long cr4);
+
+/* Create a cr4 value to load into hardware, based on vcpu settings. */
+unsigned long pv_make_cr4(const struct vcpu *v);
+
 bool xpti_pcid_enabled(void);
 
 #else  /* !CONFIG_PV */
@@ -72,6 +89,8 @@ static inline int pv_vcpu_initialise(struct vcpu *v) { return -EOPNOTSUPP; }
 static inline void pv_domain_destroy(struct domain *d) {}
 static inline int pv_domain_initialise(struct domain *d) { return -EOPNOTSUPP; }
 
+static inline unsigned long pv_make_cr4(const struct vcpu *v) { return ~0ul; }
+
 #endif /* CONFIG_PV */
 
 void paravirt_ctxt_switch_from(struct vcpu *v);