Subject: x86/pv: Don't have %cr4.fsgsbase active behind a guest kernels back
From: Andrew Cooper <[email-masked]>

Currently, a 64bit PV guest can appear to set and clear FSGSBASE in %cr4, but
the bit remains set in hardware.  Therefore, the {RD,WR}{FS,GS}BASE are usable
even when the guest kernel believes that they are disabled.

The FSGSBASE feature isn't currently supported in Linux, and its context
switch path has some optimisations which rely on userspace being unable to use
the WR{FS,GS}BASE instructions.  Xen's current behaviour undermines this
expectation.

In 64bit PV guest context, always load the guest kernels setting of FSGSBASE
into %cr4.  This requires adjusting how Xen uses the {RD,WR}{FS,GS}BASE
instructions.

 * Delete the cpu_has_fsgsbase helper.  It is no longer safe, as users need to
   check %cr4 directly.
 * The raw __rd{fs,gs}base() helpers are only safe to use when %cr4.fsgsbase
   is set.  Comment this property.
 * The {rd,wr}{fs,gs}{base,shadow}() and read_msr() helpers are updated to use
   the current %cr4 value to determine which mechanism to use.
 * toggle_guest_mode() and save_segments() are update to avoid reading
   fs/gsbase if the values in hardware cannot be stale WRT struct vcpu.  A
   consequence of this is that the write_cr() path needs to cache the current
   bases, as subsequent context switches will skip saving the values.
 * write_cr4() is updated to ensure that the shadow %cr4.fsgsbase value is
   observed in a safe way WRT the hardware setting, if an interrupt happens to
   hit in the middle.
 * pv_make_cr4() is updated for 64bit PV guests to use the guest kernels
   choice of FSGSBASE.

This is part of XSA-293

Signed-off-by: Andrew Cooper <[email-masked]>
Reviewed-by: Jan Beulich <[email-masked]>

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index 120f1e3..d246e40 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -428,6 +428,16 @@ unsigned long pv_make_cr4(const struct vcpu *v)
     if ( d->arch.vtsc || (v->arch.pv_vcpu.ctrlreg[4] & X86_CR4_TSD) )
         cr4 |= X86_CR4_TSD;
 
+    /*
+     * The {RD,WR}{FS,GS}BASE are only useable in 64bit code segments.  While
+     * we must not have CR4.FSGSBASE set behind the back of a 64bit PV kernel,
+     * we do leave it set in 32bit PV context to speed up Xen's context switch
+     * path.
+     */
+    if ( !is_pv_32bit_domain(d) &&
+         !(v->arch.pv_vcpu.ctrlreg[4] & X86_CR4_FSGSBASE) )
+        cr4 &= ~X86_CR4_FSGSBASE;
+
     return cr4;
 }
 
@@ -2022,7 +2032,8 @@ static void save_segments(struct vcpu *v)
     regs->fs = read_sreg(fs);
     regs->gs = read_sreg(gs);
 
-    if ( cpu_has_fsgsbase && !is_pv_32bit_vcpu(v) )
+    /* %fs/%gs bases can only be stale if WR{FS,GS}BASE are usable. */
+    if ( (read_cr4() & X86_CR4_FSGSBASE) && !is_pv_32bit_vcpu(v) )
     {
         v->arch.pv_vcpu.fs_base = __rdfsbase();
         if ( v->arch.flags & TF_kernel_mode )
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index 550435c..3a7b362 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -1504,7 +1504,7 @@ void __init noreturn __start_xen(unsigned long mbi_p)
 
     cr4_pv32_mask = mmu_cr4_features & XEN_CR4_PV32_BITS;
 
-    if ( cpu_has_fsgsbase )
+    if ( boot_cpu_has(X86_FEATURE_FSGSBASE) )
         set_in_cr4(X86_CR4_FSGSBASE);
 
     if ( opt_invpcid && cpu_has_invpcid )
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index 7d44a4c..b1cff5e 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -2372,6 +2372,17 @@ static int priv_op_write_cr(unsigned int reg, unsigned long val,
     }
 
     case 4: /* Write CR4 */
+        /*
+         * If this write will disable FSGSBASE, refresh Xen's idea of the
+         * guest bases now that they can no longer change.
+         */
+        if ( (curr->arch.pv_vcpu.ctrlreg[4] & X86_CR4_FSGSBASE) &&
+             !(val & X86_CR4_FSGSBASE) )
+        {
+            curr->arch.pv_vcpu.fs_base = __rdfsbase();
+            curr->arch.pv_vcpu.gs_base_kernel = __rdgsbase();
+        }
+
         curr->arch.pv_vcpu.ctrlreg[4] = pv_fixup_guest_cr4(curr, val);
         write_cr4(pv_make_cr4(curr));
         ctxt_switch_levelling(curr);
@@ -2432,14 +2443,15 @@ static int priv_op_read_msr(unsigned int reg, uint64_t *val,
     case MSR_FS_BASE:
         if ( is_pv_32bit_domain(currd) )
             break;
-        *val = cpu_has_fsgsbase ? __rdfsbase() : curr->arch.pv_vcpu.fs_base;
+        *val = (read_cr4() & X86_CR4_FSGSBASE) ? __rdfsbase()
+                                               : curr->arch.pv_vcpu.fs_base;
         return X86EMUL_OKAY;
 
     case MSR_GS_BASE:
         if ( is_pv_32bit_domain(currd) )
             break;
-        *val = cpu_has_fsgsbase ? __rdgsbase()
-                                : curr->arch.pv_vcpu.gs_base_kernel;
+        *val = (read_cr4() & X86_CR4_FSGSBASE) ? __rdgsbase()
+                                               : curr->arch.pv_vcpu.gs_base_kernel;
         return X86EMUL_OKAY;
 
     case MSR_SHADOW_GS_BASE:
diff --git a/xen/arch/x86/x86_64/traps.c b/xen/arch/x86/x86_64/traps.c
index 396e677..cb2abb1 100644
--- a/xen/arch/x86/x86_64/traps.c
+++ b/xen/arch/x86/x86_64/traps.c
@@ -267,7 +267,9 @@ void toggle_guest_mode(struct vcpu *v)
 {
     if ( is_pv_32bit_vcpu(v) )
         return;
-    if ( cpu_has_fsgsbase )
+
+    /* %fs/%gs bases can only be stale if WR{FS,GS}BASE are usable. */
+    if ( read_cr4() & X86_CR4_FSGSBASE )
     {
         if ( v->arch.flags & TF_kernel_mode )
             v->arch.pv_vcpu.gs_base_kernel = __rdgsbase();
diff --git a/xen/include/asm-x86/cpufeature.h b/xen/include/asm-x86/cpufeature.h
index af2a892..89ff249 100644
--- a/xen/include/asm-x86/cpufeature.h
+++ b/xen/include/asm-x86/cpufeature.h
@@ -80,7 +80,6 @@ XEN_CPUFEATURE(XEN_LBR,         (FSCAPINTS+0)*32+24) /* Xen uses MSR_DEBUGCTL.LB
 #define cpu_has_nx		boot_cpu_has(X86_FEATURE_NX)
 #define cpu_has_clflush		boot_cpu_has(X86_FEATURE_CLFLUSH)
 #define cpu_has_page1gb		boot_cpu_has(X86_FEATURE_PAGE1GB)
-#define cpu_has_fsgsbase	boot_cpu_has(X86_FEATURE_FSGSBASE)
 #define cpu_has_aperfmperf	boot_cpu_has(X86_FEATURE_APERFMPERF)
 #define cpu_has_smep            boot_cpu_has(X86_FEATURE_SMEP)
 #define cpu_has_invpcid         boot_cpu_has(X86_FEATURE_INVPCID)
diff --git a/xen/include/asm-x86/msr.h b/xen/include/asm-x86/msr.h
index 5e1df8f..08eec30 100644
--- a/xen/include/asm-x86/msr.h
+++ b/xen/include/asm-x86/msr.h
@@ -109,6 +109,14 @@ static inline uint64_t rdtsc_ordered(void)
 			  : "=a" (low), "=d" (high) \
 			  : "c" (counter))
 
+/*
+ * On hardware supporting FSGSBASE, the value loaded into hardware is the
+ * guest kernel's choice for 64bit PV guests (Xen's choice for Idle, HVM and
+ * 32bit PV).
+ *
+ * Therefore, the {RD,WR}{FS,GS}BASE instructions are only safe to use if
+ * %cr4.fsgsbase is set.
+ */
 static inline unsigned long __rdfsbase(void)
 {
     unsigned long base;
@@ -139,7 +147,7 @@ static inline unsigned long rdfsbase(void)
 {
     unsigned long base;
 
-    if ( cpu_has_fsgsbase )
+    if ( read_cr4() & X86_CR4_FSGSBASE )
         return __rdfsbase();
 
     rdmsrl(MSR_FS_BASE, base);
@@ -151,7 +159,7 @@ static inline unsigned long rdgsbase(void)
 {
     unsigned long base;
 
-    if ( cpu_has_fsgsbase )
+    if ( read_cr4() & X86_CR4_FSGSBASE )
         return __rdgsbase();
 
     rdmsrl(MSR_GS_BASE, base);
@@ -161,7 +169,7 @@ static inline unsigned long rdgsbase(void)
 
 static inline void wrfsbase(unsigned long base)
 {
-    if ( cpu_has_fsgsbase )
+    if ( read_cr4() & X86_CR4_FSGSBASE )
 #ifdef HAVE_GAS_FSGSBASE
         asm volatile ( "wrfsbase %0" :: "r" (base) );
 #else
@@ -173,7 +181,7 @@ static inline void wrfsbase(unsigned long base)
 
 static inline void wrgsbase(unsigned long base)
 {
-    if ( cpu_has_fsgsbase )
+    if ( read_cr4() & X86_CR4_FSGSBASE )
 #ifdef HAVE_GAS_FSGSBASE
         asm volatile ( "wrgsbase %0" :: "r" (base) );
 #else
diff --git a/xen/include/asm-x86/processor.h b/xen/include/asm-x86/processor.h
index 50badab..a5319e3 100644
--- a/xen/include/asm-x86/processor.h
+++ b/xen/include/asm-x86/processor.h
@@ -380,11 +380,31 @@ static inline unsigned long read_cr4(void)
 
 static inline void write_cr4(unsigned long val)
 {
+    struct cpu_info *info = get_cpu_info();
+
     /* No global pages in case of PCIDs enabled! */
     ASSERT(!(val & X86_CR4_PGE) || !(val & X86_CR4_PCIDE));
 
-    get_cpu_info()->cr4 = val;
-    asm volatile ( "mov %0,%%cr4" : : "r" (val) );
+    /*
+     * On hardware supporting FSGSBASE, the value in %cr4 is the kernel's
+     * choice for 64bit PV guests, which impacts whether Xen can use the
+     * instructions.
+     *
+     * The {rd,wr}{fs,gs}base() helpers use info->cr4 to work out whether it
+     * is safe to execute the {RD,WR}{FS,GS}BASE instruction, falling back to
+     * the MSR path if not.  Some users require interrupt safety.
+     *
+     * If FSGSBASE is currently or about to become clear, reflect this in
+     * info->cr4 before updating %cr4, so an interrupt which hits in the
+     * middle won't observe FSGSBASE set in info->cr4 but clear in %cr4.
+     */
+    info->cr4 = val & (info->cr4 | ~X86_CR4_FSGSBASE);
+
+    asm volatile ( "mov %[val], %%cr4"
+                   : "+m" (info->cr4) /* Force ordering without a barrier. */
+                   : [val] "r" (val) );
+
+    info->cr4 = val;
 }
 
 /* Clear and set 'TS' bit respectively */