Commit 22dd8365 authored by Thomas Gleixner's avatar Thomas Gleixner

x86/speculation/mds: Add mitigation mode VMWERV

In virtualized environments it can happen that the host has the microcode
update which utilizes the VERW instruction to clear CPU buffers, but the
hypervisor is not yet updated to expose the X86_FEATURE_MD_CLEAR CPUID bit
to guests.

Introduce an internal mitigation mode VMWERV which enables the invocation
of the CPU buffer clearing even if X86_FEATURE_MD_CLEAR is not set. If the
system has no updated microcode this results in a pointless execution of
the VERW instruction wasting a few CPU cycles. If the microcode is updated,
but not exposed to a guest then the CPU buffers will be cleared.

That said: Virtual Machines Will Eventually Receive Vaccine
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Reviewed-by: default avatarBorislav Petkov <bp@suse.de>
Reviewed-by: default avatarJon Masters <jcm@redhat.com>
Tested-by: default avatarJon Masters <jcm@redhat.com>
parent 8a4b06d3
......@@ -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
-----------------
......
......@@ -995,6 +995,7 @@ extern enum l1tf_mitigations l1tf_mitigation;
enum mds_mitigations {
MDS_MITIGATION_OFF,
MDS_MITIGATION_FULL,
MDS_MITIGATION_VMWERV,
};
#endif /* _ASM_X86_PROCESSOR_H */
......@@ -224,7 +224,8 @@ 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)
......@@ -235,10 +236,9 @@ 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]);
}
......@@ -705,8 +705,14 @@ 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);
}
......
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