Re: [PATCH v10 02/16] x86/virt/tdx: Detect TDX during kernel boot

From: David Hildenbrand
Date: Thu Mar 23 2023 - 13:04:03 EST

On 16.03.23 23:37, Huang, Kai wrote:
On Thu, 2023-03-16 at 13:48 +0100, David Hildenbrand wrote:
On 06.03.23 15:13, Kai Huang wrote:
Intel Trust Domain Extensions (TDX) protects guest VMs from malicious
host and certain physical attacks. A CPU-attested software module
called 'the TDX module' runs inside a new isolated memory range as a
trusted hypervisor to manage and run protected VMs.

Pre-TDX Intel hardware has support for a memory encryption architecture
called MKTME. The memory encryption hardware underpinning MKTME is also
used for Intel TDX. TDX ends up "stealing" some of the physical address
space from the MKTME architecture for crypto-protection to VMs. The
BIOS is responsible for partitioning the "KeyID" space between legacy
MKTME and TDX. The KeyIDs reserved for TDX are called 'TDX private
KeyIDs' or 'TDX KeyIDs' for short.

TDX doesn't trust the BIOS. During machine boot, TDX verifies the TDX
private KeyIDs are consistently and correctly programmed by the BIOS
across all CPU packages before it enables TDX on any CPU core. A valid
TDX private KeyID range on BSP indicates TDX has been enabled by the
BIOS, otherwise the BIOS is buggy.


Sorry for the late reply!

So we don't trust the BIOS, but trust the BIOS that it won't hot-remove
physical memory or hotplug physical CPUS (if I understood the cover
letter correctly)? :)

The "trust" in this context means security, but not functionality. BIOS needs
to do the right thing in order to make things work correctly in terms of
functionality.

For physical memory hotplug or CPU hotplug, we don't have patch to _explicitly_
distinguish them (from logical memory hotplug and logical cpu online/offline),
but actually they are kinda also handled: For memory hotplug, and hot-added
memory is rejected to go online (because they cannot be in TDX's convertible
memory ranges). For CPU hotplug, we have a function to do per-cpu
initialization (tdx_cpu_enable() in patch 5), and it will return error for hot-
added physical cpu.

Make sense, thanks!




The TDX module is expected to be loaded by the BIOS when it enables TDX,
but the kernel needs to properly initialize it before it can be used to
create and run any TDX guests. The TDX module will be initialized by
the KVM subsystem when KVM wants to use TDX.

Add a new early_initcall(tdx_init) to detect the TDX by detecting TDX
private KeyIDs. Also add a function to report whether TDX is enabled by
the BIOS. Similar to AMD SME, kexec() will use it to determine whether
cache flush is needed.

The TDX module itself requires one TDX KeyID as the 'TDX global KeyID'
to protect its metadata. Each TDX guest also needs a TDX KeyID for its
own protection. Just use the first TDX KeyID as the global KeyID and
leave the rest for TDX guests. If no TDX KeyID is left for TDX guests,
disable TDX as initializing the TDX module alone is useless.

Does that really happen in practice that we care about that at all?
Seems weird and rather like a broken firmware or sth like that ...

No it doesn't happen in practice, because the BIOS is sane enough.

But since the public spec doesn't explicitly say it is guaranteed this doesn't
happen when TDX is enabled, I just added this sanity check.

Okay!




To start to support TDX, create a new arch/x86/virt/vmx/tdx/tdx.c for
TDX host kernel support. Add a new Kconfig option CONFIG_INTEL_TDX_HOST
to opt-in TDX host kernel support (to distinguish with TDX guest kernel
support). So far only KVM uses TDX. Make the new config option depend
on KVM_INTEL.

Signed-off-by: Kai Huang <kai.huang@xxxxxxxxx>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>


[...]

---
arch/x86/Kconfig | 12 ++++
arch/x86/Makefile | 2 +
arch/x86/include/asm/msr-index.h | 3 +
arch/x86/include/asm/tdx.h | 7 +++
arch/x86/virt/Makefile | 2 +
arch/x86/virt/vmx/Makefile | 2 +
arch/x86/virt/vmx/tdx/Makefile | 2 +
arch/x86/virt/vmx/tdx/tdx.c | 105 +++++++++++++++++++++++++++++++
8 files changed, 135 insertions(+)
create mode 100644 arch/x86/virt/Makefile
create mode 100644 arch/x86/virt/vmx/Makefile
create mode 100644 arch/x86/virt/vmx/tdx/Makefile
create mode 100644 arch/x86/virt/vmx/tdx/tdx.c

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 3604074a878b..fc010973a6ff 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1952,6 +1952,18 @@ config X86_SGX
If unsure, say N.
+config INTEL_TDX_HOST
+ bool "Intel Trust Domain Extensions (TDX) host support"
+ depends on CPU_SUP_INTEL
+ depends on X86_64
+ depends on KVM_INTEL
+ help
+ Intel Trust Domain Extensions (TDX) protects guest VMs from malicious
+ host and certain physical attacks. This option enables necessary TDX
+ support in host kernel to run protected VMs.

s/in host/in the host/ ?

Sure.


Also, is "protected VMs" the right term to use here? "Encrypted VMs",
"Confidential VMs" ... ?

"Encrypted VM" perhaps is not a good choice, because there are more things than
encryption. I am also OK with "Confidential VMs", but "protected VMs" is also
used in the KVM series (not upstreamed yet), and also used by s390 by looking at
the git log.

So both "protected VM" and "confidential VM" work for me.

Not sure anyone else wants to comment?

I'm fine as long as it's used consistently. "Protected VM" would have been the one out of the 3 alternatives that I have heard least frequently.



[...]

+static u32 tdx_global_keyid __ro_after_init;
+static u32 tdx_guest_keyid_start __ro_after_init;
+static u32 tdx_nr_guest_keyids __ro_after_init;
+
+/*
+ * Use tdx_global_keyid to indicate that TDX is uninitialized.
+ * This is used in TDX initialization error paths to take it from
+ * initialized -> uninitialized.
+ */
+static void __init clear_tdx(void)
+{
+ tdx_global_keyid = 0;
+}

Why not set "tdx_global_keyid" last, such that you don't have to clear
when anything goes wrong before that? Seems more straight forward.

My thinking was by reserving the global keyid and taking it out first, I can
check the remaining keyids for TDX guests easily:


+ if (!nr_tdx_keyids) {
+ pr_info("initialization failed: too few private KeyIDs
available.\n");
+ goto no_tdx;
+ }

Otherwise need to do:

if (nr_tdx_keyids < 2) {
...
}

Also, in the later patch to handle memory hotplug we will add an additional step
to register_memory_notifier() which can also fail, so I just introduced
clear_tdx() here.

But nothing is big deal, and yes we can set the global keyid at last and remove
clear_tdx().

Good, that simplifies things, thanks!

--
Thanks,

David / dhildenb