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# Commit 6a49b9a7920c82015381740905582b666160d955
# Date 2020-06-09 17:42:18 +0100
# Author Andrew Cooper <andrew.cooper3@citrix.com>
# Committer Andrew Cooper <andrew.cooper3@citrix.com>
x86/spec-ctrl: Mitigate the Special Register Buffer Data Sampling sidechannel
See patch documentation and comments.
This is part of XSA-320 / CVE-2020-0543
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -1995,7 +1995,7 @@ By default SSBD will be mitigated at run
### spec-ctrl (x86)
> `= List of [ <bool>, xen=<bool>, {pv,hvm,msr-sc,rsb,md-clear}=<bool>,
> bti-thunk=retpoline|lfence|jmp, {ibrs,ibpb,ssbd,eager-fpu,
-> l1d-flush,branch-harden}=<bool> ]`
+> l1d-flush,branch-harden,srb-lock}=<bool> ]`
Controls for speculative execution sidechannel mitigations. By default, Xen
will pick the most appropriate mitigations based on compiled in support,
@@ -2072,6 +2072,12 @@ If Xen is compiled with `CONFIG_SPECULAT
speculation barriers to protect selected conditional branches. By default,
Xen will enable this mitigation.
+On hardware supporting SRBDS_CTRL, the `srb-lock=` option can be used to force
+or prevent Xen from protect the Special Register Buffer from leaking stale
+data. By default, Xen will enable this mitigation, except on parts where MDS
+is fixed and TAA is fixed/mitigated (in which case, there is believed to be no
+way for an attacker to obtain the stale data).
+
### sync_console
> `= <boolean>`
--- a/xen/arch/x86/acpi/power.c
+++ b/xen/arch/x86/acpi/power.c
@@ -295,6 +295,9 @@ static int enter_state(u32 state)
ci->spec_ctrl_flags |= (default_spec_ctrl_flags & SCF_ist_wrmsr);
spec_ctrl_exit_idle(ci);
+ if ( boot_cpu_has(X86_FEATURE_SRBDS_CTRL) )
+ wrmsrl(MSR_MCU_OPT_CTRL, default_xen_mcu_opt_ctrl);
+
done:
spin_debug_enable();
local_irq_restore(flags);
--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -361,12 +361,14 @@ void start_secondary(void *unused)
microcode_update_one();
/*
- * If MSR_SPEC_CTRL is available, apply Xen's default setting and discard
- * any firmware settings. Note: MSR_SPEC_CTRL may only become available
- * after loading microcode.
+ * If any speculative control MSRs are available, apply Xen's default
+ * settings. Note: These MSRs may only become available after loading
+ * microcode.
*/
if ( boot_cpu_has(X86_FEATURE_IBRSB) )
wrmsrl(MSR_SPEC_CTRL, default_xen_spec_ctrl);
+ if ( boot_cpu_has(X86_FEATURE_SRBDS_CTRL) )
+ wrmsrl(MSR_MCU_OPT_CTRL, default_xen_mcu_opt_ctrl);
tsx_init(); /* Needs microcode. May change HLE/RTM feature bits. */
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -65,6 +65,9 @@ static unsigned int __initdata l1d_maxph
static bool __initdata cpu_has_bug_msbds_only; /* => minimal HT impact. */
static bool __initdata cpu_has_bug_mds; /* Any other M{LP,SB,FB}DS combination. */
+static int8_t __initdata opt_srb_lock = -1;
+uint64_t __read_mostly default_xen_mcu_opt_ctrl;
+
static int __init parse_spec_ctrl(const char *s)
{
const char *ss;
@@ -112,6 +115,7 @@ static int __init parse_spec_ctrl(const
opt_ssbd = false;
opt_l1d_flush = 0;
opt_branch_harden = false;
+ opt_srb_lock = 0;
}
else if ( val > 0 )
rc = -EINVAL;
@@ -178,6 +182,8 @@ static int __init parse_spec_ctrl(const
opt_l1d_flush = val;
else if ( (val = parse_boolean("branch-harden", s, ss)) >= 0 )
opt_branch_harden = val;
+ else if ( (val = parse_boolean("srb-lock", s, ss)) >= 0 )
+ opt_srb_lock = val;
else
rc = -EINVAL;
@@ -341,7 +347,7 @@ static void __init print_details(enum in
"\n");
/* Settings for Xen's protection, irrespective of guests. */
- printk(" Xen settings: BTI-Thunk %s, SPEC_CTRL: %s%s%s, Other:%s%s%s%s\n",
+ printk(" Xen settings: BTI-Thunk %s, SPEC_CTRL: %s%s%s, Other:%s%s%s%s%s\n",
thunk == THUNK_NONE ? "N/A" :
thunk == THUNK_RETPOLINE ? "RETPOLINE" :
thunk == THUNK_LFENCE ? "LFENCE" :
@@ -352,6 +358,8 @@ static void __init print_details(enum in
(default_xen_spec_ctrl & SPEC_CTRL_SSBD) ? " SSBD+" : " SSBD-",
!(caps & ARCH_CAPS_TSX_CTRL) ? "" :
(opt_tsx & 1) ? " TSX+" : " TSX-",
+ !boot_cpu_has(X86_FEATURE_SRBDS_CTRL) ? "" :
+ opt_srb_lock ? " SRB_LOCK+" : " SRB_LOCK-",
opt_ibpb ? " IBPB" : "",
opt_l1d_flush ? " L1D_FLUSH" : "",
opt_md_clear_pv || opt_md_clear_hvm ? " VERW" : "",
@@ -1149,6 +1157,34 @@ void __init init_speculation_mitigations
tsx_init();
}
+ /* Calculate suitable defaults for MSR_MCU_OPT_CTRL */
+ if ( boot_cpu_has(X86_FEATURE_SRBDS_CTRL) )
+ {
+ uint64_t val;
+
+ rdmsrl(MSR_MCU_OPT_CTRL, val);
+
+ /*
+ * On some SRBDS-affected hardware, it may be safe to relax srb-lock
+ * by default.
+ *
+ * On parts which enumerate MDS_NO and not TAA_NO, TSX is the only way
+ * to access the Fill Buffer. If TSX isn't available (inc. SKU
+ * reasons on some models), or TSX is explicitly disabled, then there
+ * is no need for the extra overhead to protect RDRAND/RDSEED.
+ */
+ if ( opt_srb_lock == -1 &&
+ (caps & (ARCH_CAPS_MDS_NO|ARCH_CAPS_TAA_NO)) == ARCH_CAPS_MDS_NO &&
+ (!cpu_has_hle || ((caps & ARCH_CAPS_TSX_CTRL) && opt_tsx == 0)) )
+ opt_srb_lock = 0;
+
+ val &= ~MCU_OPT_CTRL_RNGDS_MITG_DIS;
+ if ( !opt_srb_lock )
+ val |= MCU_OPT_CTRL_RNGDS_MITG_DIS;
+
+ default_xen_mcu_opt_ctrl = val;
+ }
+
print_details(thunk, caps);
/*
@@ -1180,6 +1216,9 @@ void __init init_speculation_mitigations
wrmsrl(MSR_SPEC_CTRL, bsp_delay_spec_ctrl ? 0 : default_xen_spec_ctrl);
}
+
+ if ( boot_cpu_has(X86_FEATURE_SRBDS_CTRL) )
+ wrmsrl(MSR_MCU_OPT_CTRL, default_xen_mcu_opt_ctrl);
}
static void __init __maybe_unused build_assertions(void)
--- a/xen/include/asm-x86/spec_ctrl.h
+++ b/xen/include/asm-x86/spec_ctrl.h
@@ -54,6 +54,8 @@ extern int8_t opt_pv_l1tf_hwdom, opt_pv_
*/
extern paddr_t l1tf_addr_mask, l1tf_safe_maddr;
+extern uint64_t default_xen_mcu_opt_ctrl;
+
static inline void init_shadow_spec_ctrl_state(void)
{
struct cpu_info *info = get_cpu_info();
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