[PATCH v4 11/12] x86/virt/tdx: Make TDX_MODULE_CALL handle SEAMCALL #UD and #GP

Kai Huang
2023-08-15T11:04:01Z

SEAMCALL instruction causes #UD if the CPU isn't in VMX operation.
Currently the TDX_MODULE_CALL assembly doesn't handle #UD, thus making
SEAMCALL when VMX is disabled would cause Oops.

Unfortunately, there are legal cases that SEAMCALL can be made when VMX
is disabled. For instance, VMX can be disabled due to emergency reboot
while there are still TDX guests running.

Extend the TDX_MODULE_CALL assembly to return an error code for #UD to
handle this case gracefully, e.g., KVM can then quietly eat all SEAMCALL
errors caused by emergency reboot.

SEAMCALL instruction also causes #GP when TDX isn't enabled by the BIOS.
Use _ASM_EXTABLE_FAULT() to catch both exceptions with the trap number
recorded, and define two new error codes by XORing the trap number to
the TDX_SW_ERROR. This opportunistically handles #GP too while using
the same simple assembly code.

A bonus is when kernel mistakenly calls SEAMCALL when CPU isn't in VMX
operation, or when TDX isn't enabled by the BIOS, or when the BIOS is
buggy, the kernel can get a nicer error code rather than a less
understandable Oops.

This is basically based on Peter's code.

Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Kai Huang <kai.huang@intel.com>
---

v3 -> v4:
- Changed changelog to use emergency reboot as justification instead of
the erratum.
- Slight changed patch title to be more precise.

v2 -> v3:
- Added more material to the changelog to explain the TDX erratum
(hoping it can be merged together with this series).
- Changed to use %rdi to hold $TDX_SW_ERROR instead of %r12 as the
latter is callee-saved.

v1 -> v2:
- Skip saving output registers when SEAMCALL #UD/#GP

---
arch/x86/include/asm/tdx.h | 4 ++++
arch/x86/virt/vmx/tdx/tdxcall.S | 19 +++++++++++++++++++
2 files changed, 23 insertions(+)

diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
index a69bb7d3061b..adcbe3f1de30 100644
--- a/arch/x86/include/asm/tdx.h
+++ b/arch/x86/include/asm/tdx.h
@@ -8,6 +8,7 @@

#include <asm/errno.h>
#include <asm/ptrace.h>
+#include <asm/trapnr.h>
#include <asm/shared/tdx.h>

/*
@@ -20,6 +21,9 @@
#define TDX_SW_ERROR (TDX_ERROR | GENMASK_ULL(47, 40))
#define TDX_SEAMCALL_VMFAILINVALID (TDX_SW_ERROR | _UL(0xFFFF0000))

+#define TDX_SEAMCALL_GP (TDX_SW_ERROR | X86_TRAP_GP)
+#define TDX_SEAMCALL_UD (TDX_SW_ERROR | X86_TRAP_UD)
+
#ifndef __ASSEMBLY__

/*
diff --git a/arch/x86/virt/vmx/tdx/tdxcall.S b/arch/x86/virt/vmx/tdx/tdxcall.S
index 3f0b83a9977e..016a2a1ec1d6 100644
--- a/arch/x86/virt/vmx/tdx/tdxcall.S
+++ b/arch/x86/virt/vmx/tdx/tdxcall.S
@@ -1,6 +1,7 @@
/* SPDX-License-Identifier: GPL-2.0 */
#include <asm/asm-offsets.h>
#include <asm/frame.h>
+#include <asm/asm.h>
#include <asm/tdx.h>

/*
@@ -85,6 +86,7 @@
.endif /* \saved */

.if \host
+.Lseamcall\@:
seamcall
/*
* SEAMCALL instruction is essentially a VMExit from VMX root
@@ -191,11 +193,28 @@
.if \host
.Lseamcall_vmfailinvalid\@:
mov $TDX_SEAMCALL_VMFAILINVALID, %rax
+ jmp .Lseamcall_fail\@
+
+.Lseamcall_trap\@:
+ /*
+ * SEAMCALL caused #GP or #UD. By reaching here RAX contains
+ * the trap number. Convert the trap number to the TDX error
+ * code by setting TDX_SW_ERROR to the high 32-bits of RAX.
+ *
+ * Note cannot OR TDX_SW_ERROR directly to RAX as OR instruction
+ * only accepts 32-bit immediate at most.
+ */
+ movq $TDX_SW_ERROR, %rdi
+ orq %rdi, %rax
+
+.Lseamcall_fail\@:
.if \ret && \saved
/* pop the unused structure pointer back to RSI */
popq %rsi
.endif
jmp .Lout\@
+
+ _ASM_EXTABLE_FAULT(.Lseamcall\@, .Lseamcall_trap\@)
.endif /* \host */

.endm
--
2.41.0