x86/tdx: Disable NOTIFY_ENABLES

== Background ==

There is a class of side-channel attacks against SGX enclaves called
"SGX Step"[1]. These attacks create lots of exceptions inside of
enclaves. Basically, run an in-enclave instruction, cause an exception.
Over and over.

There is a concern that a VMM could attack a TDX guest in the same way
by causing lots of #VE's. The TDX architecture includes new
countermeasures for these attacks. It basically counts the number of
exceptions and can send another *special* exception once the number of
VMM-induced #VE's hits a critical threshold[2].

== Problem ==

But, these special exceptions are independent of any action that the
guest takes. They can occur anywhere that the guest executes. This
includes sensitive areas like the entry code. The (non-paranoid) #VE
handler is incapable of handling exceptions in these areas.

== Solution ==

Fortunately, the special exceptions can be disabled by the guest via
write to NOTIFY_ENABLES TDCS field. NOTIFY_ENABLES is disabled by
default, but might be enabled by a bootloader, firmware or an earlier
kernel before the current kernel runs.

Disable NOTIFY_ENABLES feature explicitly and unconditionally. Any
NOTIFY_ENABLES-based #VE's that occur before this point will end up
in the early #VE exception handler and die due to unexpected exit
reason.

[1] https://github.com/jovanbulck/sgx-step
[2] https://intel.github.io/ccc-linux-guest-hardening-docs/security-spec.html#safety-against-ve-in-kernel-code

Signed-off-by: Kirill A. Shutemov <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Reviewed-by: Dave Hansen <[email protected]>
Link: https://lore.kernel.org/all/20230126221159.8635-8-kirill.shutemov%40linux.intel.com

Author: kiryl

arch/x86/coco/tdx/tdx.c

@@ -19,6 +19,10 @@
 #define TDX_GET_VEINFO			3
 #define TDX_GET_REPORT			4
 #define TDX_ACCEPT_PAGE			6
+#define TDX_WR				8
+
+/* TDCS fields. To be used by TDG.VM.WR and TDG.VM.RD module calls */
+#define TDCS_NOTIFY_ENABLES		0x9100000000000010
 
 /* TDX hypercall Leaf IDs */
 #define TDVMCALL_MAP_GPA		0x10001
@@ -863,6 +867,9 @@ void __init tdx_early_init(void)
 	tdx_parse_tdinfo(&cc_mask);
 	cc_set_mask(cc_mask);
 
+	/* Kernel does not use NOTIFY_ENABLES and does not need random #VEs */
+	tdx_module_call(TDX_WR, 0, TDCS_NOTIFY_ENABLES, 0, -1ULL, NULL);
+
 	/*
 	 * All bits above GPA width are reserved and kernel treats shared bit
 	 * as flag, not as part of physical address.