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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [xen staging] x86/spec-ctrl: Detail the safety properties in SPEC_CTRL_ENTRY_*
commit 40dea83b75386cb693481cf340024ce093be5c0f
Author: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon Mar 25 11:09:35 2024 +0000
Commit: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Apr 9 16:37:30 2024 +0100
x86/spec-ctrl: Detail the safety properties in SPEC_CTRL_ENTRY_*
The complexity is getting out of hand.
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
xen/arch/x86/hvm/svm/entry.S | 14 ++++++++
xen/arch/x86/hvm/vmx/entry.S | 14 ++++++++
xen/arch/x86/include/asm/spec_ctrl_asm.h | 59 ++++++++++++++++++++++++++++++++
3 files changed, 87 insertions(+)
diff --git a/xen/arch/x86/hvm/svm/entry.S b/xen/arch/x86/hvm/svm/entry.S
index ecd4db4c41..6c48118dc9 100644
--- a/xen/arch/x86/hvm/svm/entry.S
+++ b/xen/arch/x86/hvm/svm/entry.S
@@ -100,6 +100,11 @@ __UNLIKELY_END(nsvm_hap)
/* SPEC_CTRL_ENTRY_FROM_SVM Req: %rsp=regs/cpuinfo, %rdx=0 Clob:
acd */
+ /*
+ * IBPB is to mitigate BTC/SRSO on AMD/Hygon parts, in particular
+ * making type-confused RETs safe to use. This is not needed on Zen5
+ * and later parts when SRSO_MSR_FIX (BP-SPEC-REDUCE) is in use.
+ */
.macro svm_vmexit_cond_ibpb
testb $SCF_entry_ibpb, CPUINFO_scf(%rsp)
jz .L_skip_ibpb
@@ -111,8 +116,17 @@ __UNLIKELY_END(nsvm_hap)
.endm
ALTERNATIVE "", svm_vmexit_cond_ibpb, X86_FEATURE_IBPB_ENTRY_HVM
+ /*
+ * RSB (RAS/RAP) stuffing is to prevents RET predictions following
guest
+ * entries. This is not needed on Zen4 and later, when AutoIBRS is in
+ * use.
+ */
ALTERNATIVE "", DO_OVERWRITE_RSB, X86_FEATURE_SC_RSB_HVM
+ /*
+ * Restore Xen's MSR_SPEC_CTRL setting, making indirect CALLs/JMPs
+ * safe to use. The guest's setting resides in the VMCB.
+ */
.macro svm_vmexit_spec_ctrl
movzbl CPUINFO_xen_spec_ctrl(%rsp), %eax
movzbl CPUINFO_last_spec_ctrl(%rsp), %edx
diff --git a/xen/arch/x86/hvm/vmx/entry.S b/xen/arch/x86/hvm/vmx/entry.S
index 241ec5ce07..f52b3f0893 100644
--- a/xen/arch/x86/hvm/vmx/entry.S
+++ b/xen/arch/x86/hvm/vmx/entry.S
@@ -34,8 +34,22 @@ ENTRY(vmx_asm_vmexit_handler)
mov %rax,VCPU_hvm_guest_cr2(%rbx)
/* SPEC_CTRL_ENTRY_FROM_VMX Req: b=curr %rsp=regs/cpuinfo, Clob:
acd */
+ /*
+ * RSB stuffing is to prevents RET predictions following guest
+ * entries. This is *not* sufficient to flush all RSB entries on
+ * parts enumerating eIBRS, although the following restore_spec_ctrl
+ * does covers us.
+ */
ALTERNATIVE "", DO_OVERWRITE_RSB, X86_FEATURE_SC_RSB_HVM
+ /*
+ * Restore Xen's MSR_SPEC_CTRL setting. The guest's value resides in
+ * the MSR load/save list. For Legacy IBRS, this flushes/inhibits
+ * indirect predictions and does not flush the RSB. For eIBRS, this
+ * prevents CALLs/JMPs using predictions learnt at a lower predictor
+ * mode, and it flushes the RSB. On eIBRS parts that also suffer from
+ * PBRSB, the prior RSB stuffing suffices to make the RSB safe.
+ */
.macro restore_spec_ctrl
mov $MSR_SPEC_CTRL, %ecx
movzbl CPUINFO_xen_spec_ctrl(%rsp), %eax
diff --git a/xen/arch/x86/include/asm/spec_ctrl_asm.h
b/xen/arch/x86/include/asm/spec_ctrl_asm.h
index 5e6d085ec3..346e131a31 100644
--- a/xen/arch/x86/include/asm/spec_ctrl_asm.h
+++ b/xen/arch/x86/include/asm/spec_ctrl_asm.h
@@ -244,10 +244,32 @@
*/
movzbl STACK_CPUINFO_FIELD(scf)(%r14), %ebx
+ /*
+ * For all safety notes, 32bit PV guest kernels run in Ring 1 and are
+ * therefore supervisor (== Xen) in the architecture. As a result, most
+ * hardware isolation techniques do not work.
+ */
+
+ /*
+ * IBPB is to mitigate BTC/SRSO on AMD/Hygon parts, in particular making
+ * type-confused RETs safe to use. This is not needed on Zen5 and later
+ * parts when SRSO_U/S_NO is enumerated.
+ */
ALTERNATIVE "", DO_COND_IBPB, X86_FEATURE_IBPB_ENTRY_PV
+ /*
+ * RSB stuffing is to prevent RET predictions following guest entries.
+ * This is not needed if SMEP is active and the RSB is full-width.
+ */
ALTERNATIVE "", DO_OVERWRITE_RSB, X86_FEATURE_SC_RSB_PV
+ /*
+ * Only used on Intel parts. Restore Xen's MSR_SPEC_CTRL setting. The
+ * guest can't change it's value behind Xen's back. For Legacy IBRS, this
+ * flushes/inhibits indirect predictions and does not flush the RSB. For
+ * eIBRS, this prevents CALLs/JMPs using predictions learnt at a lower
+ * predictor mode, and it flushes the RSB.
+ */
ALTERNATIVE "", __stringify(DO_SPEC_CTRL_ENTRY maybexen=0), \
X86_FEATURE_SC_MSR_PV
@@ -270,6 +292,14 @@
*/
movzbl STACK_CPUINFO_FIELD(scf)(%r14), %ebx
+ /*
+ * All safety notes the same as SPEC_CTRL_ENTRY_FROM_PV, although there is
+ * a conditional jump skipping some actions when interrupting Xen.
+ *
+ * On Intel parts, the IRET #GP path ends up here with the guest's choice
+ * of MSR_SPEC_CTRL.
+ */
+
testb $3, UREGS_cs(%rsp)
jz .L\@_skip
@@ -320,6 +350,19 @@
*/
movzbl STACK_CPUINFO_FIELD(scf)(%r14), %ebx
+ /*
+ * For all safety notes, 32bit PV guest kernels run in Ring 1 and are
+ * therefore supervisor (== Xen) in the architecture. As a result, most
+ * hardware isolation techniques do not work.
+ */
+
+ /*
+ * IBPB is to mitigate BTC/SRSO on AMD/Hygon parts, in particular making
+ * type-confused RETs safe to use. This is not needed on Zen5 and later
+ * parts when SRSO_U/S_NO is enumerated. The SVM path takes care of
+ * Host/Guest interactions prior to clearing GIF, and it's not used on the
+ * VMX path.
+ */
test $SCF_ist_ibpb, %bl
jz .L\@_skip_ibpb
@@ -329,6 +372,12 @@
.L\@_skip_ibpb:
+ /*
+ * RSB stuffing is to prevent RET predictions following guest entries.
+ * SCF_ist_rsb is active if either PV or HVM protections are needed. The
+ * VMX path cannot guarantee to make the RSB safe ahead of taking an IST
+ * vector.
+ */
test $SCF_ist_rsb, %bl
jz .L\@_skip_rsb
@@ -336,6 +385,16 @@
.L\@_skip_rsb:
+ /*
+ * Only used on Intel parts. Restore Xen's MSR_SPEC_CTRL setting. PV
+ * guests can't change their value behind Xen's back. HVM guests have
+ * their value stored in the MSR load/save list. For Legacy IBRS, this
+ * flushes/inhibits indirect predictions and does not flush the RSB. For
+ * eIBRS, this prevents CALLs/JMPs using predictions learnt at a lower
+ * predictor mode, and it flushes the RSB. On eIBRS parts that also
+ * suffer from PBRSB, the prior RSB stuffing suffices to make the RSB
+ * safe.
+ */
test $SCF_ist_sc_msr, %bl
jz .L\@_skip_msr_spec_ctrl
--
generated by git-patchbot for /home/xen/git/xen.git#staging
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