[PATCHv6 17/30] x86/tdx: Port I/O: add runtime hypercalls

Kirill A. Shutemov
2022-03-16T02:12:04Z

From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>

TDX hypervisors cannot emulate instructions directly. This includes
port I/O which is normally emulated in the hypervisor. All port I/O
instructions inside TDX trigger the #VE exception in the guest and
would be normally emulated there.

Use a hypercall to emulate port I/O. Extend the
tdx_handle_virt_exception() and add support to handle the #VE due to
port I/O instructions.

String I/O operations are not supported in TDX. Unroll them by declaring
CC_ATTR_GUEST_UNROLL_STRING_IO confidential computing attribute.

== Userspace Implications ==

The ioperm() facility allows userspace access to I/O instructions like
inb/outb. Among other things, this allows writing userspace device
drivers.

This series has no special handling for ioperm(). Users will be able to
successfully request I/O permissions but will induce a #VE on their
first I/O instruction which leads SIGSEGV. If this is undesirable users
can enable kernel lockdown feature with 'lockdown=integrity' kernel
command line option. It makes ioperm() fail.

More robust handling of this situation (denying ioperm() in all TDX
guests) will be addressed in follow-on work.

Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
---
arch/x86/coco/core.c | 7 +++-
arch/x86/coco/tdx/tdx.c | 79 +++++++++++++++++++++++++++++++++++++++++
2 files changed, 85 insertions(+), 1 deletion(-)

diff --git a/arch/x86/coco/core.c b/arch/x86/coco/core.c
index 3f3008783e05..df08edc94f9b 100644
--- a/arch/x86/coco/core.c
+++ b/arch/x86/coco/core.c
@@ -18,7 +18,12 @@ static u64 cc_mask __ro_after_init;

static bool intel_cc_platform_has(enum cc_attr attr)
{
- return false;
+ switch (attr) {
+ case CC_ATTR_GUEST_UNROLL_STRING_IO:
+ return true;
+ default:
+ return false;
+ }
}

/*
diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index 68da658f563f..3caa5f551d90 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -19,6 +19,16 @@
#define EPT_READ 0
#define EPT_WRITE 1

+/* Port I/O direction */
+#define PORT_READ 0
+#define PORT_WRITE 1
+
+/* See Exit Qualification for I/O Instructions in VMX documentation */
+#define VE_IS_IO_IN(e) ((e) & BIT(3))
+#define VE_GET_IO_SIZE(e) (((e) & GENMASK(2, 0)) + 1)
+#define VE_GET_PORT_NUM(e) ((e) >> 16)
+#define VE_IS_IO_STRING(e) ((e) & BIT(4))
+
/*
* Wrapper for standard use of __tdx_hypercall with no output aside from
* return code.
@@ -334,6 +344,73 @@ static bool handle_mmio(struct pt_regs *regs, struct ve_info *ve)
return true;
}

+static bool handle_in(struct pt_regs *regs, int size, int port)
+{
+ struct tdx_hypercall_args args = {
+ .r10 = TDX_HYPERCALL_STANDARD,
+ .r11 = hcall_func(EXIT_REASON_IO_INSTRUCTION),
+ .r12 = size,
+ .r13 = PORT_READ,
+ .r14 = port,
+ };
+ bool success;
+ u64 mask = GENMASK(BITS_PER_BYTE * size, 0);
+
+ /*
+ * Emulate the I/O read via hypercall. More info about ABI can be found
+ * in TDX Guest-Host-Communication Interface (GHCI) section titled
+ * "TDG.VP.VMCALL<Instruction.IO>".
+ */
+ success = !__tdx_hypercall(&args, TDX_HCALL_HAS_OUTPUT);
+
+ /* Update part of the register affected by the emulated instruction */
+ regs->ax &= ~mask;
+ if (success)
+ regs->ax |= args.r11 & mask;
+
+ return success;
+}
+
+static bool handle_out(struct pt_regs *regs, int size, int port)
+{
+ u64 mask = GENMASK(BITS_PER_BYTE * size, 0);
+
+ /*
+ * Emulate the I/O write via hypercall. More info about ABI can be found
+ * in TDX Guest-Host-Communication Interface (GHCI) section titled
+ * "TDG.VP.VMCALL<Instruction.IO>".
+ */
+ return !_tdx_hypercall(hcall_func(EXIT_REASON_IO_INSTRUCTION), size,
+ PORT_WRITE, port, regs->ax & mask);
+}
+
+/*
+ * Emulate I/O using hypercall.
+ *
+ * Assumes the IO instruction was using ax, which is enforced
+ * by the standard io.h macros.
+ *
+ * Return True on success or False on failure.
+ */
+static bool handle_io(struct pt_regs *regs, u32 exit_qual)
+{
+ bool in;
+ int size, port;
+
+ if (VE_IS_IO_STRING(exit_qual))
+ return false;
+
+ in = VE_IS_IO_IN(exit_qual);
+ size = VE_GET_IO_SIZE(exit_qual);
+ port = VE_GET_PORT_NUM(exit_qual);
+
+
+ if (in)
+ return handle_in(regs, size, port);
+ else
+ return handle_out(regs, size, port);
+}
+
void tdx_get_ve_info(struct ve_info *ve)
{
struct tdx_module_output out;
@@ -383,6 +460,8 @@ static bool virt_exception_kernel(struct pt_regs *regs, struct ve_info *ve)
return handle_cpuid(regs);
case EXIT_REASON_EPT_VIOLATION:
return handle_mmio(regs, ve);
+ case EXIT_REASON_IO_INSTRUCTION:
+ return handle_io(regs, ve->exit_qual);
default:
pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
return false;
--
2.34.1