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>
[ bp: Adjust for the fact that vmexit is in inline assembly ]
Signed-off-by: Suraj Jitindar Singh <surajjs@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Suleiman Souhlal <suleiman@xxxxxxxxxx>
---
arch/x86/include/asm/cpufeatures.h | 2 +-
arch/x86/include/asm/nospec-branch.h | 2 +-
arch/x86/kernel/cpu/bugs.c | 63 +++++++++++++++++++++++++---
arch/x86/kvm/vmx.c | 4 +-
4 files changed, 62 insertions(+), 9 deletions(-)
diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 23126290185d..0181091abf49 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -203,7 +203,7 @@
#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 */
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 3e1d2389c00d..8bce4004aab2 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -279,7 +279,7 @@ static __always_inline void vmexit_fill_RSB(void)
asm volatile (ANNOTATE_NOSPEC_ALTERNATIVE
ALTERNATIVE("jmp 910f",
__stringify(__FILL_RETURN_BUFFER(%0, RSB_CLEAR_LOOPS, %1)),
- X86_FEATURE_RETPOLINE)
+ X86_FEATURE_RSB_VMEXIT)
"910:"
: "=r" (loops), ASM_CALL_CONSTRAINT
: : "memory" );
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 022b9f31333f..525623aa2dcb 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1278,16 +1278,69 @@ static void __init spectre_v2_select_mitigation(void)
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
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index babb1e5a4dfa..30f0d55c8b2d 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -11018,8 +11018,8 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
* 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
--
2.38.1.431.g37b22c650d-goog
