File xsa456-0i.patch of Package xen.33138
# Commit 94896de1a98c4289fe6fef9e16ef99fc6ef2efc4
# Date 2024-04-09 16:37:30 +0100
# Author Andrew Cooper <andrew.cooper3@citrix.com>
# Committer Andrew Cooper <andrew.cooper3@citrix.com>
x86/spec-ctrl: Rework conditional safety for SPEC_CTRL_ENTRY_*
Right now, we have a mix of safety strategies in different blocks, making the
logic fragile and hard to follow.
Start addressing this by having a safety LFENCE at the end of the blocks,
which can be patched out if other safety criteria are met. This will allow us
to simplify the sub-blocks. For SPEC_CTRL_ENTRY_FROM_IST, simply leave an
LFENCE unconditionally at the end; the IST path is not a fast-path by any
stretch of the imagination.
For SPEC_CTRL_ENTRY_FROM_INTR, the existing description was incorrect. The
IRET #GP path is non-fatal but can occur with the guest's choice of
MSR_SPEC_CTRL. It is safe to skip the flush/barrier-like protections when
interrupting Xen, but we must run DO_SPEC_CTRL_ENTRY irrespective.
This will skip RSB stuffing which was previously unconditional even when
interrupting Xen.
AFAICT, this is a missing cleanup from commit 3fffaf9c13e9 ("x86/entry: Avoid
using alternatives in NMI/#MC paths") where we split the IST entry path out of
the main INTR entry path.
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
--- a/xen/arch/x86/hvm/vmx/entry.S
+++ b/xen/arch/x86/hvm/vmx/entry.S
@@ -36,6 +36,7 @@ ENTRY(vmx_asm_vmexit_handler)
/* SPEC_CTRL_ENTRY_FROM_VMX Req: b=curr %rsp=regs/cpuinfo, Clob: acd */
ALTERNATIVE "", DO_OVERWRITE_RSB, X86_FEATURE_SC_RSB_HVM
ALTERNATIVE "", DO_SPEC_CTRL_ENTRY_FROM_HVM, X86_FEATURE_SC_MSR_HVM
+ ALTERNATIVE "lfence", "", X86_SPEC_NO_LFENCE_ENTRY_VMX
/* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */
/* Hardware clears MSR_DEBUGCTL on VMExit. Reinstate it if debugging Xen. */
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -1991,6 +1991,58 @@ void __init init_speculation_mitigations
print_details(thunk);
/*
+ * With the alternative blocks now chosen, see if we need any other
+ * adjustments for safety.
+ *
+ * We compile the LFENCE in, and patch it out if it's not needed.
+ *
+ * Notes:
+ * - SPEC_CTRL_ENTRY_FROM_SVM doesn't need an LFENCE because it has an
+ * unconditional STGI.
+ * - SPEC_CTRL_ENTRY_FROM_IST handles its own safety, without the use of
+ * alternatives.
+ * - DO_OVERWRITE_RSB has conditional branches in it, but it's an inline
+ * sequence. It is considered safe for uarch reasons.
+ */
+ {
+ /*
+ * SPEC_CTRL_ENTRY_FROM_PV conditional safety
+ *
+ * DO_SPEC_CTRL_ENTRY (X86_FEATURE_SC_MSR_PV if used) is an
+ * unconditional WRMSR as the last action.
+ *
+ * If we have it, or we're not using any prior conditional mitigation,
+ * then it's safe to drop the LFENCE.
+ */
+ if ( boot_cpu_has(X86_FEATURE_SC_MSR_PV) ||
+ !boot_cpu_has(X86_FEATURE_IBPB_ENTRY_PV) )
+ setup_force_cpu_cap(X86_SPEC_NO_LFENCE_ENTRY_PV);
+
+ /*
+ * SPEC_CTRL_ENTRY_FROM_INTR conditional safety
+ *
+ * DO_SPEC_CTRL_ENTRY (X86_FEATURE_SC_MSR_PV if used) is an
+ * unconditional WRMSR as the last action.
+ *
+ * If we have it, or we have no protections active in the block that
+ * is skipped when interrupting guest context, then it's safe to drop
+ * the LFENCE.
+ */
+ if ( boot_cpu_has(X86_FEATURE_SC_MSR_PV) ||
+ (!boot_cpu_has(X86_FEATURE_IBPB_ENTRY_PV) &&
+ !boot_cpu_has(X86_FEATURE_SC_RSB_PV)) )
+ setup_force_cpu_cap(X86_SPEC_NO_LFENCE_ENTRY_INTR);
+
+ /*
+ * SPEC_CTRL_ENTRY_FROM_VMX conditional safety
+ *
+ * Currently there are no safety actions with conditional branches, so
+ * no need for the extra safety LFENCE.
+ */
+ setup_force_cpu_cap(X86_SPEC_NO_LFENCE_ENTRY_VMX);
+ }
+
+ /*
* If MSR_SPEC_CTRL is available, apply Xen's default setting and discard
* any firmware settings. For performance reasons, when safe to do so, we
* delay applying non-zero settings until after dom0 has been constructed.
--- a/xen/include/asm-x86/cpufeatures.h
+++ b/xen/include/asm-x86/cpufeatures.h
@@ -49,5 +49,9 @@ XEN_CPUFEATURE(IBPB_ENTRY_HVM, X86_SY
#define X86_BUG_CLFLUSH_MFENCE X86_BUG( 2) /* MFENCE needed to serialise CLFLUSH */
#define X86_BUG_IBPB_NO_RET X86_BUG( 3) /* IBPB doesn't flush the RSB/RAS */
+#define X86_SPEC_NO_LFENCE_ENTRY_PV X86_BUG(16) /* (No) safety LFENCE for SPEC_CTRL_ENTRY_PV. */
+#define X86_SPEC_NO_LFENCE_ENTRY_INTR X86_BUG(17) /* (No) safety LFENCE for SPEC_CTRL_ENTRY_INTR. */
+#define X86_SPEC_NO_LFENCE_ENTRY_VMX X86_BUG(18) /* (No) safety LFENCE for SPEC_CTRL_ENTRY_VMX. */
+
/* Total number of capability words, inc synth and bug words. */
#define NCAPINTS (FSCAPINTS + X86_NR_SYNTH + X86_NR_BUG) /* N 32-bit words worth of info */
--- a/xen/include/asm-x86/spec_ctrl_asm.h
+++ b/xen/include/asm-x86/spec_ctrl_asm.h
@@ -280,25 +280,37 @@
ALTERNATIVE "", __stringify(DO_SPEC_CTRL_ENTRY maybexen=0), \
X86_FEATURE_SC_MSR_PV
+
+ ALTERNATIVE "lfence", "", X86_SPEC_NO_LFENCE_ENTRY_PV
.endm
/*
* Used after an exception or maskable interrupt, hitting Xen or PV context.
- * There will either be a guest speculation context, or (barring fatal
- * exceptions) a well-formed Xen speculation context.
+ * There will either be a guest speculation context, or a well-formed Xen
+ * speculation context, with the exception of one case. IRET #GP handling may
+ * have a guest choice of MSR_SPEC_CTRL.
+ *
+ * Therefore, we can skip the flush/barrier-like protections when hitting Xen,
+ * but we must still run the mode-based protections.
*/
.macro SPEC_CTRL_ENTRY_FROM_INTR
/*
* Requires %rsp=regs, %r14=stack_end, %rdx=0
* Clobbers %rax, %rcx, %rdx
*/
+ testb $3, UREGS_cs(%rsp)
+ jz .L\@_skip
+
ALTERNATIVE "", __stringify(DO_SPEC_CTRL_COND_IBPB maybexen=1), \
X86_FEATURE_IBPB_ENTRY_PV
ALTERNATIVE "", DO_OVERWRITE_RSB, X86_FEATURE_SC_RSB_PV
+.L\@_skip:
ALTERNATIVE "", __stringify(DO_SPEC_CTRL_ENTRY maybexen=1), \
X86_FEATURE_SC_MSR_PV
+
+ ALTERNATIVE "lfence", "", X86_SPEC_NO_LFENCE_ENTRY_INTR
.endm
/*
@@ -367,18 +379,9 @@
movzbl STACK_CPUINFO_FIELD(xen_spec_ctrl)(%r14), %eax
wrmsr
- /* Opencoded UNLIKELY_START() with no condition. */
-UNLIKELY_DISPATCH_LABEL(\@_serialise):
- .subsection 1
- /*
- * In the case that we might need to set SPEC_CTRL.IBRS for safety, we
- * need to ensure that an attacker can't poison the `jz .L\@_skip_wrmsr`
- * to speculate around the WRMSR. As a result, we need a dispatch
- * serialising instruction in the else clause.
- */
.L\@_skip_msr_spec_ctrl:
+
lfence
- UNLIKELY_END(\@_serialise)
.endm
/*
