x86/traps: Add #VE support for TDX guest

Virtualization Exceptions (#VE) are delivered to TDX guests due to
specific guest actions which may happen in either user space or the
kernel:

 * Specific instructions (WBINVD, for example)
 * Specific MSR accesses
 * Specific CPUID leaf accesses
 * Access to specific guest physical addresses

Syscall entry code has a critical window where the kernel stack is not
yet set up. Any exception in this window leads to hard to debug issues
and can be exploited for privilege escalation. Exceptions in the NMI
entry code also cause issues. Returning from the exception handler with
IRET will re-enable NMIs and nested NMI will corrupt the NMI stack.

For these reasons, the kernel avoids #VEs during the syscall gap and
the NMI entry code. Entry code paths do not access TD-shared memory,
MMIO regions, use #VE triggering MSRs, instructions, or CPUID leaves
that might generate #VE. VMM can remove memory from TD at any point,
but access to unaccepted (or missing) private memory leads to VM
termination, not to #VE.

Similarly to page faults and breakpoints, #VEs are allowed in NMI
handlers once the kernel is ready to deal with nested NMIs.

During #VE delivery, all interrupts, including NMIs, are blocked until
TDGETVEINFO is called. It prevents #VE nesting until the kernel reads
the VE info.

TDGETVEINFO retrieves the #VE info from the TDX module, which also
clears the "#VE valid" flag.  This must be done before anything else as
any #VE that occurs while the valid flag is set escalates to #DF by TDX
module. It will result in an oops.

Virtual NMIs are inhibited if the #VE valid flag is set. NMI will not be
delivered until TDGETVEINFO is called.

For now, convert unhandled #VE's (everything, until later in this
series) so that they appear just like a #GP by calling the
ve_raise_fault() directly. The ve_raise_fault() function is similar
to #GP handler and is responsible for sending SIGSEGV to userspace
and CPU die and notifying debuggers and other die chain users.

Co-developed-by: Sean Christopherson <sean.j.christopherson@intel.com>
Co-developed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lkml.kernel.org/r/20220405232939.73860-8-kirill.shutemov@linux.intel.com

1 files changed, 38 insertions, 0 deletions

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index e84f6dd3ed2a..60a3f2ff5b95 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -10,6 +10,7 @@
 
 /* TDX module Call Leaf IDs */
 #define TDX_GET_INFO			1
+#define TDX_GET_VEINFO			3
 
 /*
  * Wrapper for standard use of __tdx_hypercall with no output aside from
@@ -73,6 +74,43 @@ static u64 get_cc_mask(void)
 	return BIT_ULL(gpa_width - 1);
 }
 
+void tdx_get_ve_info(struct ve_info *ve)
+{
+	struct tdx_module_output out;
+
+	/*
+	 * Called during #VE handling to retrieve the #VE info from the
+	 * TDX module.
+	 *
+	 * This has to be called early in #VE handling.  A "nested" #VE which
+	 * occurs before this will raise a #DF and is not recoverable.
+	 *
+	 * The call retrieves the #VE info from the TDX module, which also
+	 * clears the "#VE valid" flag. This must be done before anything else
+	 * because any #VE that occurs while the valid flag is set will lead to
+	 * #DF.
+	 *
+	 * Note, the TDX module treats virtual NMIs as inhibited if the #VE
+	 * valid flag is set. It means that NMI=>#VE will not result in a #DF.
+	 */
+	tdx_module_call(TDX_GET_VEINFO, 0, 0, 0, 0, &out);
+
+	/* Transfer the output parameters */
+	ve->exit_reason = out.rcx;
+	ve->exit_qual   = out.rdx;
+	ve->gla         = out.r8;
+	ve->gpa         = out.r9;
+	ve->instr_len   = lower_32_bits(out.r10);
+	ve->instr_info  = upper_32_bits(out.r10);
+}
+
+bool tdx_handle_virt_exception(struct pt_regs *regs, struct ve_info *ve)
+{
+	pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
+
+	return false;
+}
+
 void __init tdx_early_init(void)
 {
 	u64 cc_mask;