This is a note to let you know that I've just added the patch titled
x86/speculation: Fill RSB on vmexit for IBRS
to the 5.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
x86-speculation-fill-rsb-on-vmexit-for-ibrs.patch
and it can be found in the queue-5.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From foo@baz Wed Oct 5 12:28:43 PM CEST 2022
From: Thadeu Lima de Souza Cascardo <cascardo@xxxxxxxxxxxxx>
Date: Mon, 3 Oct 2022 10:10:31 -0300
Subject: x86/speculation: Fill RSB on vmexit for IBRS
To: stable@xxxxxxxxxxxxxxx
Cc: x86@xxxxxxxxxx, kvm@xxxxxxxxxxxxxxx, bp@xxxxxxxxx, pbonzini@xxxxxxxxxx, peterz@xxxxxxxxxxxxx, jpoimboe@xxxxxxxxxx
Message-ID: <20221003131038.12645-31-cascardo@xxxxxxxxxxxxx>
From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
commit 9756bba28470722dacb79ffce554336dd1f6a6cd upstream.
Prevent RSB underflow/poisoning attacks with RSB. While at it, add a
bunch of comments to attempt to document the current state of tribal
knowledge about RSB attacks and what exactly is being mitigated.
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/x86/include/asm/cpufeatures.h | 2 -
arch/x86/kernel/cpu/bugs.c | 63 ++++++++++++++++++++++++++++++++++---
arch/x86/kvm/vmx/vmenter.S | 6 +--
3 files changed, 62 insertions(+), 9 deletions(-)
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -204,7 +204,7 @@
#define X86_FEATURE_SME ( 7*32+10) /* AMD Secure Memory Encryption */
#define X86_FEATURE_PTI ( 7*32+11) /* Kernel Page Table Isolation enabled */
#define X86_FEATURE_KERNEL_IBRS ( 7*32+12) /* "" Set/clear IBRS on kernel entry/exit */
-/* FREE! ( 7*32+13) */
+#define X86_FEATURE_RSB_VMEXIT ( 7*32+13) /* "" Fill RSB on VM-Exit */
#define X86_FEATURE_INTEL_PPIN ( 7*32+14) /* Intel Processor Inventory Number */
#define X86_FEATURE_CDP_L2 ( 7*32+15) /* Code and Data Prioritization L2 */
#define X86_FEATURE_MSR_SPEC_CTRL ( 7*32+16) /* "" MSR SPEC_CTRL is implemented */
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1278,17 +1278,70 @@ static void __init spectre_v2_select_mit
pr_info("%s\n", spectre_v2_strings[mode]);
/*
- * If spectre v2 protection has been enabled, unconditionally fill
- * RSB during a context switch; this protects against two independent
- * issues:
+ * If Spectre v2 protection has been enabled, fill the RSB during a
+ * context switch. In general there are two types of RSB attacks
+ * across context switches, for which the CALLs/RETs may be unbalanced.
*
- * - RSB underflow (and switch to BTB) on Skylake+
- * - SpectreRSB variant of spectre v2 on X86_BUG_SPECTRE_V2 CPUs
+ * 1) RSB underflow
+ *
+ * Some Intel parts have "bottomless RSB". When the RSB is empty,
+ * speculated return targets may come from the branch predictor,
+ * which could have a user-poisoned BTB or BHB entry.
+ *
+ * AMD has it even worse: *all* returns are speculated from the BTB,
+ * regardless of the state of the RSB.
+ *
+ * When IBRS or eIBRS is enabled, the "user -> kernel" attack
+ * scenario is mitigated by the IBRS branch prediction isolation
+ * properties, so the RSB buffer filling wouldn't be necessary to
+ * protect against this type of attack.
+ *
+ * The "user -> user" attack scenario is mitigated by RSB filling.
+ *
+ * 2) Poisoned RSB entry
+ *
+ * If the 'next' in-kernel return stack is shorter than 'prev',
+ * 'next' could be tricked into speculating with a user-poisoned RSB
+ * entry.
+ *
+ * The "user -> kernel" attack scenario is mitigated by SMEP and
+ * eIBRS.
+ *
+ * The "user -> user" scenario, also known as SpectreBHB, requires
+ * RSB clearing.
+ *
+ * So to mitigate all cases, unconditionally fill RSB on context
+ * switches.
+ *
+ * FIXME: Is this pointless for retbleed-affected AMD?
*/
setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
pr_info("Spectre v2 / SpectreRSB mitigation: Filling RSB on context switch\n");
/*
+ * Similar to context switches, there are two types of RSB attacks
+ * after vmexit:
+ *
+ * 1) RSB underflow
+ *
+ * 2) Poisoned RSB entry
+ *
+ * When retpoline is enabled, both are mitigated by filling/clearing
+ * the RSB.
+ *
+ * When IBRS is enabled, while #1 would be mitigated by the IBRS branch
+ * prediction isolation protections, RSB still needs to be cleared
+ * because of #2. Note that SMEP provides no protection here, unlike
+ * user-space-poisoned RSB entries.
+ *
+ * eIBRS, on the other hand, has RSB-poisoning protections, so it
+ * doesn't need RSB clearing after vmexit.
+ */
+ if (boot_cpu_has(X86_FEATURE_RETPOLINE) ||
+ boot_cpu_has(X86_FEATURE_KERNEL_IBRS))
+ setup_force_cpu_cap(X86_FEATURE_RSB_VMEXIT);
+
+ /*
* Retpoline protects the kernel, but doesn't protect firmware. IBRS
* and Enhanced IBRS protect firmware too, so enable IBRS around
* firmware calls only when IBRS / Enhanced IBRS aren't otherwise
--- a/arch/x86/kvm/vmx/vmenter.S
+++ b/arch/x86/kvm/vmx/vmenter.S
@@ -181,15 +181,15 @@ SYM_INNER_LABEL(vmx_vmexit, SYM_L_GLOBAL
* IMPORTANT: RSB filling and SPEC_CTRL handling must be done before
* the first unbalanced RET after vmexit!
*
- * For retpoline, RSB filling is needed to prevent poisoned RSB entries
- * and (in some cases) RSB underflow.
+ * For retpoline or IBRS, RSB filling is needed to prevent poisoned RSB
+ * entries and (in some cases) RSB underflow.
*
* eIBRS has its own protection against poisoned RSB, so it doesn't
* need the RSB filling sequence. But it does need to be enabled
* before the first unbalanced RET.
*/
- FILL_RETURN_BUFFER %_ASM_CX, RSB_CLEAR_LOOPS, X86_FEATURE_RETPOLINE
+ FILL_RETURN_BUFFER %_ASM_CX, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_VMEXIT
pop %_ASM_ARG2 /* @flags */
pop %_ASM_ARG1 /* @vmx */
Patches currently in stable-queue which might be from cascardo@xxxxxxxxxxxxx are
queue-5.4/x86-speculation-disable-rrsba-behavior.patch
queue-5.4/kvm-vmx-flatten-__vmx_vcpu_run.patch
queue-5.4/x86-kvm-vmx-make-noinstr-clean.patch
queue-5.4/revert-x86-speculation-add-rsb-vm-exit-protections.patch
queue-5.4/kvm-vmx-fix-ibrs-handling-after-vmexit.patch
queue-5.4/kvm-vmx-prevent-guest-rsb-poisoning-attacks-with-eibrs.patch
queue-5.4/kvm-nvmx-use-__vmx_vcpu_run-in-nested_vmx_check_vmentry_hw.patch
queue-5.4/x86-bugs-keep-a-per-cpu-ia32_spec_ctrl-value.patch
queue-5.4/x86-cpu-amd-enumerate-btc_no.patch
queue-5.4/x86-speculation-fix-firmware-entry-spec_ctrl-handling.patch
queue-5.4/x86-speculation-add-spectre_v2-ibrs-option-to-support-kernel-ibrs.patch
queue-5.4/x86-cpu-add-consistent-cpu-match-macros.patch
queue-5.4/x86-speculation-remove-x86_spec_ctrl_mask.patch
queue-5.4/x86-bugs-add-cannon-lake-to-retbleed-affected-cpu-list.patch
queue-5.4/x86-bugs-warn-when-ibrs-mitigation-is-selected-on-enhanced-ibrs-parts.patch
queue-5.4/x86-speculation-fill-rsb-on-vmexit-for-ibrs.patch
queue-5.4/x86-cpu-add-a-steppings-field-to-struct-x86_cpu_id.patch
queue-5.4/kvm-vmx-convert-launched-argument-to-flags.patch
queue-5.4/x86-common-stamp-out-the-stepping-madness.patch
queue-5.4/x86-bugs-split-spectre_v2_select_mitigation-and-spectre_v2_user_select_mitigation.patch
queue-5.4/x86-bugs-report-intel-retbleed-vulnerability.patch
queue-5.4/x86-speculation-change-fill_return_buffer-to-work-with-objtool.patch
queue-5.4/x86-cpufeatures-move-retpoline-flags-to-word-11.patch
queue-5.4/x86-speculation-fix-spec_ctrl-write-on-smt-state-change.patch
queue-5.4/kvm-vmx-use-test-reg-reg-instead-of-cmp-0-reg-in-vmenter.s.patch
queue-5.4/x86-bugs-optimize-spec_ctrl-msr-writes.patch
queue-5.4/x86-bugs-report-amd-retbleed-vulnerability.patch
queue-5.4/x86-speculation-fix-rsb-filling-with-config_retpoline-n.patch
queue-5.4/intel_idle-disable-ibrs-during-long-idle.patch
queue-5.4/x86-speculation-use-declare_per_cpu-for-x86_spec_ctrl_current.patch
queue-5.4/x86-entry-remove-skip_r11rcx.patch
queue-5.4/x86-speculation-use-cached-host-spec_ctrl-value-for-guest-entry-exit.patch
queue-5.4/x86-devicetable-move-x86-specific-macro-out-of-generic-code.patch
queue-5.4/x86-bugs-add-amd-retbleed-boot-parameter.patch
queue-5.4/x86-entry-add-kernel-ibrs-implementation.patch
queue-5.4/revert-x86-cpu-add-a-steppings-field-to-struct-x86_cpu_id.patch
queue-5.4/x86-speculation-add-rsb-vm-exit-protections.patch
