[PATCH v8 06/11] x86/traps: Add #VE support for TDX guest

From: Kuppuswamy Sathyanarayanan
Date: Mon Oct 04 2021 - 22:52:43 EST

From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>

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 TD-shared memory, which includes MMIO

In the settings that Linux will run in, virtual exceptions are never
generated on accesses to normal, TD-private memory that has been
accepted.

The entry paths do not access TD-shared memory, MMIO regions or use
those specific MSRs, instructions, CPUID leaves that might generate #VE.
In addition, all interrupts including NMIs are blocked by the hardware
starting with #VE delivery until TDGETVEINFO is called.  This eliminates
the chance of a #VE during the syscall gap or paranoid entry paths and
simplifies #VE handling.

After TDGETVEINFO #VE could happen in theory (e.g. through an NMI),
but it is expected not to happen because TDX expects NMIs not to
trigger #VEs. Another case where they could happen is if the #VE
exception panics, but in this case there are no guarantees on anything
anyways.

If a guest kernel action which would normally cause a #VE occurs in the
interrupt-disabled region before TDGETVEINFO, a #DF is delivered to the
guest which will result in an oops (and should eventually be a panic, as
it is expected panic_on_oops is set to 1 for TDX guests).

Add basic infrastructure to handle any #VE which occurs in the kernel or
userspace.  Later patches will add handling for specific #VE scenarios.

Convert unhandled #VE's (everything, until later in this series) so that
they appear just like a #GP by calling ve_raise_fault() directly.
ve_raise_fault() 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@xxxxxxxxx>
Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
---

Changes since v7:
* None

Changes since v6:
* None

Changes since v5:
* Fixed "We" usage in commit log and replaced cpu -> CPU.
* Renamed "tdg_" prefix with "tdx_".
* Removed TODO comment in tdg_handle_virtualization_exception() as
per Boris review comments.
* Added comments for ve_raise_fault().

Changes since v4:
* Since ve_raise_fault() is used only by TDX code, moved it
within #ifdef CONFIG_INTEL_TDX_GUEST.

Changes since v3:
* None

arch/x86/include/asm/idtentry.h | 4 ++
arch/x86/include/asm/tdx.h | 19 ++++++++
arch/x86/kernel/idt.c | 6 +++
arch/x86/kernel/tdx.c | 33 ++++++++++++++
arch/x86/kernel/traps.c | 77 +++++++++++++++++++++++++++++++++
5 files changed, 139 insertions(+)

diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
index 1345088e9902..8ccc81d653b3 100644
--- a/arch/x86/include/asm/idtentry.h
+++ b/arch/x86/include/asm/idtentry.h
@@ -625,6 +625,10 @@ DECLARE_IDTENTRY_XENCB(X86_TRAP_OTHER, exc_xen_hypervisor_callback);
DECLARE_IDTENTRY_RAW(X86_TRAP_OTHER, exc_xen_unknown_trap);
#endif

+#ifdef CONFIG_INTEL_TDX_GUEST
+DECLARE_IDTENTRY(X86_TRAP_VE, exc_virtualization_exception);
+#endif
+
/* Device interrupts common/spurious */
DECLARE_IDTENTRY_IRQ(X86_TRAP_OTHER, common_interrupt);
#ifdef CONFIG_X86_LOCAL_APIC
diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
index 8d83a719b90b..458a564dd4c2 100644
--- a/arch/x86/include/asm/tdx.h
+++ b/arch/x86/include/asm/tdx.h
@@ -39,6 +39,20 @@ struct tdx_hypercall_output {
u64 r15;
};

+/*
+ * Used by #VE exception handler to gather the #VE exception
+ * info from the TDX module. This is software only structure
+ * and not related to TDX module/VMM.
+ */
+struct ve_info {
+ u64 exit_reason;
+ u64 exit_qual;
+ u64 gla; /* Guest Linear (virtual) Address */
+ u64 gpa; /* Guest Physical (virtual) Address */
+ u32 instr_len;
+ u32 instr_info;
+};
+
#ifdef CONFIG_INTEL_TDX_GUEST

extern u64 is_tdx_guest;
@@ -52,6 +66,11 @@ u64 __tdx_module_call(u64 fn, u64 rcx, u64 rdx, u64 r8, u64 r9,
u64 __tdx_hypercall(u64 type, u64 fn, u64 r12, u64 r13, u64 r14,
u64 r15, struct tdx_hypercall_output *out);

+unsigned long tdx_get_ve_info(struct ve_info *ve);
+
+int tdx_handle_virtualization_exception(struct pt_regs *regs,
+ struct ve_info *ve);
+
#else

#define is_tdx_guest 0ULL
diff --git a/arch/x86/kernel/idt.c b/arch/x86/kernel/idt.c
index df0fa695bb09..a5eaae8e6c44 100644
--- a/arch/x86/kernel/idt.c
+++ b/arch/x86/kernel/idt.c
@@ -68,6 +68,9 @@ static const __initconst struct idt_data early_idts[] = {
*/
INTG(X86_TRAP_PF, asm_exc_page_fault),
#endif
+#ifdef CONFIG_INTEL_TDX_GUEST
+ INTG(X86_TRAP_VE, asm_exc_virtualization_exception),
+#endif
};

/*
@@ -91,6 +94,9 @@ static const __initconst struct idt_data def_idts[] = {
INTG(X86_TRAP_MF, asm_exc_coprocessor_error),
INTG(X86_TRAP_AC, asm_exc_alignment_check),
INTG(X86_TRAP_XF, asm_exc_simd_coprocessor_error),
+#ifdef CONFIG_INTEL_TDX_GUEST
+ INTG(X86_TRAP_VE, asm_exc_virtualization_exception),
+#endif

#ifdef CONFIG_X86_32
TSKG(X86_TRAP_DF, GDT_ENTRY_DOUBLEFAULT_TSS),
diff --git a/arch/x86/kernel/tdx.c b/arch/x86/kernel/tdx.c
index d5fc2904facf..f7885c777a09 100644
--- a/arch/x86/kernel/tdx.c
+++ b/arch/x86/kernel/tdx.c
@@ -6,6 +6,9 @@

#include <asm/tdx.h>

+/* TDX Module call Leaf IDs */
+#define TDGETVEINFO 3
+
/*
* Allocate it in the data region to avoid zeroing it during
* BSS initialization. It is mainly used in cc_platform_has()
@@ -50,6 +53,36 @@ static inline u64 _tdx_hypercall(u64 fn, u64 r12, u64 r13, u64 r14,
return out->r10;
}

+unsigned long tdx_get_ve_info(struct ve_info *ve)
+{
+ struct tdx_module_output out = {0};
+ u64 ret;
+
+ /*
+ * NMIs and machine checks are suppressed. Before this point any
+ * #VE is fatal. After this point (TDGETVEINFO call), NMIs and
+ * additional #VEs are permitted (but it is expected not to
+ * happen unless kernel panics).
+ */
+ ret = __tdx_module_call(TDGETVEINFO, 0, 0, 0, 0, &out);
+
+ ve->exit_reason = out.rcx;
+ ve->exit_qual = out.rdx;
+ ve->gla = out.r8;
+ ve->gpa = out.r9;
+ ve->instr_len = out.r10 & UINT_MAX;
+ ve->instr_info = out.r10 >> 32;
+
+ return ret;
+}
+
+int tdx_handle_virtualization_exception(struct pt_regs *regs,
+ struct ve_info *ve)
+{
+ pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
+ return -EFAULT;
+}
+
void __init tdx_early_init(void)
{
is_tdx_guest_init();
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index a58800973aed..152d1d3b9dc8 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -61,6 +61,7 @@
#include <asm/insn.h>
#include <asm/insn-eval.h>
#include <asm/vdso.h>
+#include <asm/tdx.h>

#ifdef CONFIG_X86_64
#include <asm/x86_init.h>
@@ -1140,6 +1141,82 @@ DEFINE_IDTENTRY(exc_device_not_available)
}
}

+#ifdef CONFIG_INTEL_TDX_GUEST
+#define VEFSTR "VE fault"
+static void ve_raise_fault(struct pt_regs *regs, long error_code)
+{
+ struct task_struct *tsk = current;
+
+ if (user_mode(regs)) {
+ tsk->thread.error_code = error_code;
+ tsk->thread.trap_nr = X86_TRAP_VE;
+
+ /*
+ * Not fixing up VDSO exceptions similar to #GP handler
+ * because it is expected that VDSO doesn't trigger #VE.
+ */
+ show_signal(tsk, SIGSEGV, "", VEFSTR, regs, error_code);
+ force_sig(SIGSEGV);
+ return;
+ }
+
+ /*
+ * Attempt to recover from #VE exception failure without
+ * triggering OOPS (useful for MSR read/write failures)
+ */
+ if (fixup_exception(regs, X86_TRAP_VE, error_code, 0))
+ return;
+
+ tsk->thread.error_code = error_code;
+ tsk->thread.trap_nr = X86_TRAP_VE;
+
+ /*
+ * To be potentially processing a kprobe fault and to trust the result
+ * from kprobe_running(), it should be non-preemptible.
+ */
+ if (!preemptible() &&
+ kprobe_running() &&
+ kprobe_fault_handler(regs, X86_TRAP_VE))
+ return;
+
+ /* Notify about #VE handling failure, useful for debugger hooks */
+ if (notify_die(DIE_GPF, VEFSTR, regs, error_code,
+ X86_TRAP_VE, SIGSEGV) == NOTIFY_STOP)
+ return;
+
+ /* Trigger OOPS and panic */
+ die_addr(VEFSTR, regs, error_code, 0);
+}
+
+DEFINE_IDTENTRY(exc_virtualization_exception)
+{
+ struct ve_info ve;
+ int ret;
+
+ RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
+
+ /*
+ * NMIs/Machine-checks/Interrupts will be in a disabled state
+ * till TDGETVEINFO TDCALL is executed. This prevents #VE
+ * nesting issue.
+ */
+ ret = tdx_get_ve_info(&ve);
+
+ cond_local_irq_enable(regs);
+
+ if (!ret)
+ ret = tdx_handle_virtualization_exception(regs, &ve);
+ /*
+ * If tdx_handle_virtualization_exception() could not process
+ * it successfully, treat it as #GP(0) and handle it.
+ */
+ if (ret)
+ ve_raise_fault(regs, 0);
+
+ cond_local_irq_disable(regs);
+}
+#endif
+
#ifdef CONFIG_X86_32
DEFINE_IDTENTRY_SW(iret_error)
{
--
2.25.1