diff options
| -rw-r--r-- | Documentation/x86/mds.rst | 27 | ||||
| -rw-r--r-- | arch/x86/include/asm/processor.h | 1 | ||||
| -rw-r--r-- | arch/x86/kernel/cpu/bugs.c | 18 |
3 files changed, 40 insertions, 6 deletions
diff --git a/Documentation/x86/mds.rst b/Documentation/x86/mds.rst index 87ce8ac9f36e..3d6f943f1afb 100644 --- a/Documentation/x86/mds.rst +++ b/Documentation/x86/mds.rst @@ -93,11 +93,38 @@ The kernel provides a function to invoke the buffer clearing: The mitigation is invoked on kernel/userspace, hypervisor/guest and C-state (idle) transitions. +As a special quirk to address virtualization scenarios where the host has +the microcode updated, but the hypervisor does not (yet) expose the +MD_CLEAR CPUID bit to guests, the kernel issues the VERW instruction in the +hope that it might actually clear the buffers. The state is reflected +accordingly. + According to current knowledge additional mitigations inside the kernel itself are not required because the necessary gadgets to expose the leaked data cannot be controlled in a way which allows exploitation from malicious user space or VM guests. +Kernel internal mitigation modes +-------------------------------- + + ======= ============================================================ + off Mitigation is disabled. Either the CPU is not affected or + mds=off is supplied on the kernel command line + + full Mitigation is eanbled. CPU is affected and MD_CLEAR is + advertised in CPUID. + + vmwerv Mitigation is enabled. CPU is affected and MD_CLEAR is not + advertised in CPUID. That is mainly for virtualization + scenarios where the host has the updated microcode but the + hypervisor does not expose MD_CLEAR in CPUID. It's a best + effort approach without guarantee. + ======= ============================================================ + +If the CPU is affected and mds=off is not supplied on the kernel command +line then the kernel selects the appropriate mitigation mode depending on +the availability of the MD_CLEAR CPUID bit. + Mitigation points ----------------- diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h index 5e9f953face0..b54f25697beb 100644 --- a/arch/x86/include/asm/processor.h +++ b/arch/x86/include/asm/processor.h @@ -1000,6 +1000,7 @@ extern enum l1tf_mitigations l1tf_mitigation; enum mds_mitigations { MDS_MITIGATION_OFF, MDS_MITIGATION_FULL, + MDS_MITIGATION_VMWERV, }; #endif /* _ASM_X86_PROCESSOR_H */ diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c index 50b7d2a980e8..053d71a3b9cc 100644 --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -222,7 +222,8 @@ static enum mds_mitigations mds_mitigation __ro_after_init = MDS_MITIGATION_FULL static const char * const mds_strings[] = { [MDS_MITIGATION_OFF] = "Vulnerable", - [MDS_MITIGATION_FULL] = "Mitigation: Clear CPU buffers" + [MDS_MITIGATION_FULL] = "Mitigation: Clear CPU buffers", + [MDS_MITIGATION_VMWERV] = "Vulnerable: Clear CPU buffers attempted, no microcode", }; static void __init mds_select_mitigation(void) @@ -233,10 +234,9 @@ static void __init mds_select_mitigation(void) } if (mds_mitigation == MDS_MITIGATION_FULL) { - if (boot_cpu_has(X86_FEATURE_MD_CLEAR)) - static_branch_enable(&mds_user_clear); - else - mds_mitigation = MDS_MITIGATION_OFF; + if (!boot_cpu_has(X86_FEATURE_MD_CLEAR)) + mds_mitigation = MDS_MITIGATION_VMWERV; + static_branch_enable(&mds_user_clear); } pr_info("%s\n", mds_strings[mds_mitigation]); } @@ -690,8 +690,14 @@ void arch_smt_update(void) break; } - if (mds_mitigation == MDS_MITIGATION_FULL) + switch (mds_mitigation) { + case MDS_MITIGATION_FULL: + case MDS_MITIGATION_VMWERV: update_mds_branch_idle(); + break; + case MDS_MITIGATION_OFF: + break; + } mutex_unlock(&spec_ctrl_mutex); } |