[PATCHv4 07/30] x86/traps: Add #VE support for TDX guest

Kirill A. Shutemov
2022-02-24T15:57:09Z

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 unmapped pages (EPT violation)

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

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.

If a guest kernel action which would normally cause a #VE occurs in
the interrupt-disabled region before TDGETVEINFO, a #DF (fault
exception) is delivered to the guest which will result in an oops.

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

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>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Co-developed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
arch/x86/coco/tdx.c | 60 ++++++++++++++
arch/x86/include/asm/idtentry.h | 4 +
arch/x86/include/asm/tdx.h | 21 +++++
arch/x86/kernel/idt.c | 3 +
arch/x86/kernel/traps.c | 138 ++++++++++++++++++++++++++------
5 files changed, 203 insertions(+), 23 deletions(-)

diff --git a/arch/x86/coco/tdx.c b/arch/x86/coco/tdx.c
index 14c085930b5f..86a2f35e7308 100644
--- a/arch/x86/coco/tdx.c
+++ b/arch/x86/coco/tdx.c
@@ -10,6 +10,7 @@

/* TDX module Call Leaf IDs */
#define TDX_GET_INFO 1
+#define TDX_GET_VEINFO 3

static struct {
unsigned int gpa_width;
@@ -58,6 +59,65 @@ static void get_info(void)
td_info.attributes = out.rdx;
}

+void tdx_get_ve_info(struct ve_info *ve)
+{
+ struct tdx_module_output out;
+
+ /*
+ * Retrieve 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, i.e. before the previous #VE info
+ * was consumed, is morphed to a #DF by the TDX module. Note, the TDX
+ * module also treats virtual NMIs as inhibited if the #VE valid flag is
+ * set, e.g. so that NMI=>#VE will not result in a #DF.
+ */
+ tdx_module_call(TDX_GET_VEINFO, 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 = lower_32_bits(out.r10);
+ ve->instr_info = upper_32_bits(out.r10);
+}
+
+/*
+ * Handle the user initiated #VE.
+ *
+ * For example, executing the CPUID instruction from user space
+ * is a valid case and hence the resulting #VE has to be handled.
+ *
+ * For dis-allowed or invalid #VE just return failure.
+ */
+static bool virt_exception_user(struct pt_regs *regs, struct ve_info *ve)
+{
+ pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
+ return false;
+}
+
+/* Handle the kernel #VE */
+static bool virt_exception_kernel(struct pt_regs *regs, struct ve_info *ve)
+{
+ pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
+ return false;
+}
+
+bool tdx_handle_virt_exception(struct pt_regs *regs, struct ve_info *ve)
+{
+ bool ret;
+
+ if (user_mode(regs))
+ ret = virt_exception_user(regs, ve);
+ else
+ ret = virt_exception_kernel(regs, ve);
+
+ /* After successful #VE handling, move the IP */
+ if (ret)
+ regs->ip += ve->instr_len;
+
+ return ret;
+}
+
void __init tdx_early_init(void)
{
u32 eax, sig[3];
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 557227e40da9..34cf998ad534 100644
--- a/arch/x86/include/asm/tdx.h
+++ b/arch/x86/include/asm/tdx.h
@@ -5,6 +5,7 @@

#include <linux/bits.h>
#include <linux/init.h>
+#include <asm/ptrace.h>

#define TDX_CPUID_LEAF_ID 0x21
#define TDX_IDENT "IntelTDX "
@@ -47,6 +48,22 @@ struct tdx_hypercall_args {
u64 r15;
};

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

void __init tdx_early_init(void);
@@ -58,6 +75,10 @@ u64 __tdx_module_call(u64 fn, u64 rcx, u64 rdx, u64 r8, u64 r9,
/* Used to request services from the VMM */
u64 __tdx_hypercall(struct tdx_hypercall_args *args, unsigned long flags);

+void tdx_get_ve_info(struct ve_info *ve);
+
+bool tdx_handle_virt_exception(struct pt_regs *regs, struct ve_info *ve);
+
#else

static inline void tdx_early_init(void) { };
diff --git a/arch/x86/kernel/idt.c b/arch/x86/kernel/idt.c
index df0fa695bb09..1da074123c16 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
};

/*
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 7ef00dee35be..b2510af38158 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -62,6 +62,7 @@
#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>
@@ -611,13 +612,43 @@ static bool try_fixup_enqcmd_gp(void)
#endif
}

+static bool gp_try_fixup_and_notify(struct pt_regs *regs, int trapnr,
+ unsigned long error_code, const char *str)
+{
+ int ret;
+
+ if (fixup_exception(regs, trapnr, error_code, 0))
+ return true;
+
+ current->thread.error_code = error_code;
+ current->thread.trap_nr = trapnr;
+
+ /*
+ * To be potentially processing a kprobe fault and to trust the result
+ * from kprobe_running(), we have to be non-preemptible.
+ */
+ if (!preemptible() && kprobe_running() &&
+ kprobe_fault_handler(regs, trapnr))
+ return true;
+
+ ret = notify_die(DIE_GPF, str, regs, error_code, trapnr, SIGSEGV);
+ return ret == NOTIFY_STOP;
+}
+
+static void gp_user_force_sig_segv(struct pt_regs *regs, int trapnr,
+ unsigned long error_code, const char *str)
+{
+ current->thread.error_code = error_code;
+ current->thread.trap_nr = trapnr;
+ show_signal(current, SIGSEGV, "", str, regs, error_code);
+ force_sig(SIGSEGV);
+}
+
DEFINE_IDTENTRY_ERRORCODE(exc_general_protection)
{
char desc[sizeof(GPFSTR) + 50 + 2*sizeof(unsigned long) + 1] = GPFSTR;
enum kernel_gp_hint hint = GP_NO_HINT;
- struct task_struct *tsk;
unsigned long gp_addr;
- int ret;

if (user_mode(regs) && try_fixup_enqcmd_gp())
return;
@@ -636,40 +667,21 @@ DEFINE_IDTENTRY_ERRORCODE(exc_general_protection)
return;
}

- tsk = current;
-
if (user_mode(regs)) {
if (fixup_iopl_exception(regs))
goto exit;

- tsk->thread.error_code = error_code;
- tsk->thread.trap_nr = X86_TRAP_GP;
-
if (fixup_vdso_exception(regs, X86_TRAP_GP, error_code, 0))
goto exit;

- show_signal(tsk, SIGSEGV, "", desc, regs, error_code);
- force_sig(SIGSEGV);
+ gp_user_force_sig_segv(regs, X86_TRAP_GP, error_code, desc);
goto exit;
}

if (fixup_exception(regs, X86_TRAP_GP, error_code, 0))
goto exit;

- tsk->thread.error_code = error_code;
- tsk->thread.trap_nr = X86_TRAP_GP;
-
- /*
- * To be potentially processing a kprobe fault and to trust the result
- * from kprobe_running(), we have to be non-preemptible.
- */
- if (!preemptible() &&
- kprobe_running() &&
- kprobe_fault_handler(regs, X86_TRAP_GP))
- goto exit;
-
- ret = notify_die(DIE_GPF, desc, regs, error_code, X86_TRAP_GP, SIGSEGV);
- if (ret == NOTIFY_STOP)
+ if (gp_try_fixup_and_notify(regs, X86_TRAP_GP, error_code, desc))
goto exit;

if (error_code)
@@ -1267,6 +1279,86 @@ DEFINE_IDTENTRY(exc_device_not_available)
}
}

+#ifdef CONFIG_INTEL_TDX_GUEST
+
+#define VE_FAULT_STR "VE fault"
+
+static void ve_raise_fault(struct pt_regs *regs, long error_code)
+{
+ if (user_mode(regs)) {
+ gp_user_force_sig_segv(regs, X86_TRAP_VE, error_code, VE_FAULT_STR);
+ return;
+ }
+
+ if (gp_try_fixup_and_notify(regs, X86_TRAP_VE, error_code, VE_FAULT_STR))
+ return;
+
+ die_addr(VE_FAULT_STR, regs, error_code, 0);
+}
+
+/*
+ * 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 unmapped pages (EPT violation)
+ *
+ * In the settings that Linux will run in, virtualization exceptions are
+ * never generated on accesses to normal, TD-private memory that has been
+ * accepted.
+ *
+ * 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.
+ *
+ * If a guest kernel action which would normally cause a #VE occurs in
+ * the interrupt-disabled region before TDGETVEINFO, a #DF (fault
+ * exception) is delivered to the guest which will result in an oops.
+ */
+DEFINE_IDTENTRY(exc_virtualization_exception)
+{
+ struct ve_info ve;
+
+ /*
+ * NMIs/Machine-checks/Interrupts will be in a disabled state
+ * till TDGETVEINFO TDCALL is executed. This ensures that VE
+ * info cannot be overwritten by a nested #VE.
+ */
+ tdx_get_ve_info(&ve);
+
+ cond_local_irq_enable(regs);
+
+ /*
+ * If tdx_handle_virt_exception() could not process
+ * it successfully, treat it as #GP(0) and handle it.
+ */
+ if (!tdx_handle_virt_exception(regs, &ve))
+ ve_raise_fault(regs, 0);
+
+ cond_local_irq_disable(regs);
+}
+
+#endif
+
#ifdef CONFIG_X86_32
DEFINE_IDTENTRY_SW(iret_error)
{
--
2.34.1