[tip: x86/tdx] x86/tdx: Port I/O: Add runtime hypercalls

From: tip-bot2 for Kuppuswamy Sathyanarayanan
Date: Fri Apr 08 2022 - 21:28:44 EST


The following commit has been merged into the x86/tdx branch of tip:

Commit-ID: 03149948832a078f759022ed5b92e722d8d23c26
Gitweb: https://git.kernel.org/tip/03149948832a078f759022ed5b92e722d8d23c26
Author: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
AuthorDate: Wed, 06 Apr 2022 02:29:26 +03:00
Committer: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
CommitterDate: Thu, 07 Apr 2022 08:27:52 -07:00

x86/tdx: Port I/O: Add runtime hypercalls

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@xxxxxxxxxxxxxxx>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>
Reviewed-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20220405232939.73860-18-kirill.shutemov@xxxxxxxxxxxxxxx
---
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 3f30087..df08edc 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 ab10bc7..e47e2ed 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.
@@ -341,6 +351,73 @@ 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,
+ };
+ u64 mask = GENMASK(BITS_PER_BYTE * size, 0);
+ bool success;
+
+ /*
+ * 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)
+{
+ int size, port;
+ bool in;
+
+ 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;
@@ -397,6 +474,8 @@ 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;