x86/spec-ctrl: Mitigate the Zen1 DIV leakage

In the Zen1 microarchitecure, there is one divider in the pipeline which
services uops from both threads.  In the case of #DE, the latched result from
the previous DIV to execute will be forwarded speculatively.

This is an interesting covert channel that allows two threads to communicate
without any system calls.  In also allows userspace to obtain the result of
the most recent DIV instruction executed (even speculatively) in the core,
which can be from a higher privilege context.

Scrub the result from the divider by executing a non-faulting divide.  This
needs performing on the exit-to-guest paths, and ist_exit-to-Xen.

Alternatives in IST context is believed safe now that it's done in NMI
context.

This is XSA-439 / CVE-2023-20588.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit b5926c6)

docs/misc/xen-command-line.pandoc

@@ -2189,7 +2189,7 @@ By default SSBD will be mitigated at runtime (i.e `ssbd=runtime`).
 >              {msr-sc,rsb,md-clear,ibpb-entry}=<bool>|{pv,hvm}=<bool>,
 >              bti-thunk=retpoline|lfence|jmp, {ibrs,ibpb,ssbd,psfd,
 >              eager-fpu,l1d-flush,branch-harden,srb-lock,
->              unpriv-mmio,gds-mit}=<bool> ]`
+>              unpriv-mmio,gds-mit,div-scrub}=<bool> ]`
 
 Controls for speculative execution sidechannel mitigations.  By default, Xen
 will pick the most appropriate mitigations based on compiled in support,
@@ -2309,6 +2309,10 @@ has elected not to lock the configuration, Xen will use GDS_CTRL to mitigate
 GDS with.  Otherwise, Xen will mitigate by disabling AVX, which blocks the use
 of the AVX2 Gather instructions.
 
+On all hardware, the `div-scrub=` option can be used to force or prevent Xen
+from mitigating the DIV-leakage vulnerability.  By default, Xen will mitigate
+DIV-leakage on hardware believed to be vulnerable.
+
 ### sync_console
 > `= <boolean>`
 

xen/arch/x86/hvm/svm/entry.S

@@ -72,6 +72,7 @@ __UNLIKELY_END(nsvm_hap)
 1:          /* No Spectre v1 concerns.  Execution will hit VMRUN imminently. */
         .endm
         ALTERNATIVE "", svm_vmentry_spec_ctrl, X86_FEATURE_SC_MSR_HVM
+        ALTERNATIVE "", DO_SPEC_CTRL_DIV, X86_FEATURE_SC_DIV
 
         pop  %r15
         pop  %r14

xen/arch/x86/spec_ctrl.c

@@ -22,6 +22,7 @@
 #include <xen/param.h>
 #include <xen/warning.h>
 
+#include <asm/amd.h>
 #include <asm/hvm/svm/svm.h>
 #include <asm/microcode.h>
 #include <asm/msr.h>
@@ -78,6 +79,7 @@ static int8_t __initdata opt_srb_lock = -1;
 static bool __initdata opt_unpriv_mmio;
 static bool __read_mostly opt_fb_clear_mmio;
 static int8_t __initdata opt_gds_mit = -1;
+static int8_t __initdata opt_div_scrub = -1;
 
 static int __init parse_spec_ctrl(const char *s)
 {
@@ -132,6 +134,7 @@ static int __init parse_spec_ctrl(const char *s)
             opt_srb_lock = 0;
             opt_unpriv_mmio = false;
             opt_gds_mit = 0;
+            opt_div_scrub = 0;
         }
         else if ( val > 0 )
             rc = -EINVAL;
@@ -284,6 +287,8 @@ static int __init parse_spec_ctrl(const char *s)
             opt_unpriv_mmio = val;
         else if ( (val = parse_boolean("gds-mit", s, ss)) >= 0 )
             opt_gds_mit = val;
+        else if ( (val = parse_boolean("div-scrub", s, ss)) >= 0 )
+            opt_div_scrub = val;
         else
             rc = -EINVAL;
 
@@ -484,7 +489,7 @@ static void __init print_details(enum ind_thunk thunk)
                "\n");
 
     /* Settings for Xen's protection, irrespective of guests. */
-    printk("  Xen settings: BTI-Thunk %s, SPEC_CTRL: %s%s%s%s%s, Other:%s%s%s%s%s\n",
+    printk("  Xen settings: BTI-Thunk %s, SPEC_CTRL: %s%s%s%s%s, Other:%s%s%s%s%s%s\n",
            thunk == THUNK_NONE      ? "N/A" :
            thunk == THUNK_RETPOLINE ? "RETPOLINE" :
            thunk == THUNK_LFENCE    ? "LFENCE" :
@@ -509,6 +514,7 @@ static void __init print_details(enum ind_thunk thunk)
            opt_l1d_flush                             ? " L1D_FLUSH" : "",
            opt_md_clear_pv || opt_md_clear_hvm ||
            opt_fb_clear_mmio                         ? " VERW"  : "",
+           opt_div_scrub                             ? " DIV" : "",
            opt_branch_harden                         ? " BRANCH_HARDEN" : "");
 
     /* L1TF diagnostics, printed if vulnerable or PV shadowing is in use. */
@@ -933,6 +939,45 @@ static void __init srso_calculations(bool hw_smt_enabled)
         setup_force_cpu_cap(X86_FEATURE_SRSO_NO);
 }
 
+/*
+ * The Div leakage issue is specific to the AMD Zen1 microarchitecure.
+ *
+ * However, there's no $FOO_NO bit defined, so if we're virtualised we have no
+ * hope of spotting the case where we might move to vulnerable hardware.  We
+ * also can't make any useful conclusion about SMT-ness.
+ *
+ * Don't check the hypervisor bit, so at least we do the safe thing when
+ * booting on something that looks like a Zen1 CPU.
+ */
+static bool __init has_div_vuln(void)
+{
+    if ( !(boot_cpu_data.x86_vendor &
+           (X86_VENDOR_AMD | X86_VENDOR_HYGON)) )
+        return false;
+
+    if ( boot_cpu_data.x86 != 0x17 && boot_cpu_data.x86 != 0x18 )
+        return false;
+
+    return is_zen1_uarch();
+}
+
+static void __init div_calculations(bool hw_smt_enabled)
+{
+    bool cpu_bug_div = has_div_vuln();
+
+    if ( opt_div_scrub == -1 )
+        opt_div_scrub = cpu_bug_div;
+
+    if ( opt_div_scrub )
+        setup_force_cpu_cap(X86_FEATURE_SC_DIV);
+
+    if ( opt_smt == -1 && !cpu_has_hypervisor && cpu_bug_div && hw_smt_enabled )
+        warning_add(
+            "Booted on leaky-DIV hardware with SMT/Hyperthreading\n"
+            "enabled.  Please assess your configuration and choose an\n"
+            "explicit 'smt=<bool>' setting.  See XSA-439.\n");
+}
+
 static void __init ibpb_calculations(void)
 {
     bool def_ibpb_entry = false;
@@ -1644,6 +1689,8 @@ void __init init_speculation_mitigations(void)
 
     ibpb_calculations();
 
+    div_calculations(hw_smt_enabled);
+
     /* Check whether Eager FPU should be enabled by default. */
     if ( opt_eager_fpu == -1 )
         opt_eager_fpu = should_use_eager_fpu();

xen/include/asm-x86/cpufeatures.h

@@ -35,7 +35,8 @@ XEN_CPUFEATURE(SC_RSB_HVM,        X86_SYNTH(19)) /* RSB overwrite needed for HVM
 XEN_CPUFEATURE(XEN_SELFSNOOP,     X86_SYNTH(20)) /* SELFSNOOP gets used by Xen itself */
 XEN_CPUFEATURE(SC_MSR_IDLE,       X86_SYNTH(21)) /* Clear MSR_SPEC_CTRL on idle */
 XEN_CPUFEATURE(XEN_LBR,           X86_SYNTH(22)) /* Xen uses MSR_DEBUGCTL.LBR */
-/* Bits 23,24 unused. */
+XEN_CPUFEATURE(SC_DIV,            X86_SYNTH(23)) /* DIV scrub needed */
+/* Bit 24 unused. */
 XEN_CPUFEATURE(SC_VERW_IDLE,      X86_SYNTH(25)) /* VERW used by Xen for idle */
 XEN_CPUFEATURE(XEN_SHSTK,         X86_SYNTH(26)) /* Xen uses CET Shadow Stacks */
 XEN_CPUFEATURE(XEN_IBT,           X86_SYNTH(27)) /* Xen uses CET Indirect Branch Tracking */

xen/include/asm-x86/spec_ctrl_asm.h

@@ -177,6 +177,19 @@
 .L\@_verw_skip:
 .endm
 
+.macro DO_SPEC_CTRL_DIV
+/*
+ * Requires nothing
+ * Clobbers %rax
+ *
+ * Issue a DIV for its flushing side effect (Zen1 uarch specific).  Any
+ * non-faulting DIV will do; a byte DIV has least latency, and doesn't clobber
+ * %rdx.
+ */
+    mov $1, %eax
+    div %al
+.endm
+
 .macro DO_SPEC_CTRL_ENTRY maybexen:req
 /*
  * Requires %rsp=regs (also cpuinfo if !maybexen)
@@ -279,6 +292,8 @@
     ALTERNATIVE "", DO_SPEC_CTRL_EXIT_TO_GUEST, X86_FEATURE_SC_MSR_PV
 
     DO_SPEC_CTRL_COND_VERW
+
+    ALTERNATIVE "", DO_SPEC_CTRL_DIV, X86_FEATURE_SC_DIV
 .endm
 
 /*
@@ -391,6 +406,8 @@ UNLIKELY_DISPATCH_LABEL(\@_serialise):
     verw STACK_CPUINFO_FIELD(verw_sel)(%r14)
 .L\@_skip_verw:
 
+    ALTERNATIVE "", DO_SPEC_CTRL_DIV, X86_FEATURE_SC_DIV
+
 .L\@_skip_ist_exit:
 .endm