x86/extable: Rework the exception table mechanics

The exception table entries contain the instruction address, the fixup
address and the handler address. All addresses are relative. Storing the
handler address has a few downsides:

 1) Most handlers need to be exported

 2) Handlers can be defined everywhere and there is no overview about the
    handler types

 3) MCE needs to check the handler type to decide whether an in kernel #MC
    can be recovered. The functionality of the handler itself is not in any
    way special, but for these checks there need to be separate functions
    which in the worst case have to be exported.

    Some of these 'recoverable' exception fixups are pretty obscure and
    just reuse some other handler to spare code. That obfuscates e.g. the
    #MC safe copy functions. Cleaning that up would require more handlers
    and exports

Rework the exception fixup mechanics by storing a fixup type number instead
of the handler address and invoke the proper handler for each fixup
type. Also teach the extable sort to leave the type field alone.

This makes most handlers static except for special cases like the MCE
MSR fixup and the BPF fixup. This allows to add more types for cleaning up
the obscure places without adding more handler code and exports.

There is a marginal code size reduction for a production config and it
removes _eight_ exported symbols.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lkml.kernel.org/r/20210908132525.211958725@linutronix.de

1 files changed, 52 insertions, 71 deletions

diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index d9a1046f3a98..5db46df409b5 100644
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -9,40 +9,25 @@
 #include <asm/traps.h>
 #include <asm/kdebug.h>
 
-typedef bool (*ex_handler_t)(const struct exception_table_entry *,
-			    struct pt_regs *, int, unsigned long,
-			    unsigned long);
-
 static inline unsigned long
 ex_fixup_addr(const struct exception_table_entry *x)
 {
 	return (unsigned long)&x->fixup + x->fixup;
 }
-static inline ex_handler_t
-ex_fixup_handler(const struct exception_table_entry *x)
-{
-	return (ex_handler_t)((unsigned long)&x->handler + x->handler);
-}
 
-__visible bool ex_handler_default(const struct exception_table_entry *fixup,
-				  struct pt_regs *regs, int trapnr,
-				  unsigned long error_code,
-				  unsigned long fault_addr)
+static bool ex_handler_default(const struct exception_table_entry *fixup,
+			       struct pt_regs *regs)
 {
 	regs->ip = ex_fixup_addr(fixup);
 	return true;
 }
-EXPORT_SYMBOL(ex_handler_default);
 
-__visible bool ex_handler_fault(const struct exception_table_entry *fixup,
-				struct pt_regs *regs, int trapnr,
-				unsigned long error_code,
-				unsigned long fault_addr)
+static bool ex_handler_fault(const struct exception_table_entry *fixup,
+			     struct pt_regs *regs, int trapnr)
 {
 	regs->ax = trapnr;
-	return ex_handler_default(fixup, regs, trapnr, error_code, fault_addr);
+	return ex_handler_default(fixup, regs);
 }
-EXPORT_SYMBOL_GPL(ex_handler_fault);
 
 /*
  * Handler for when we fail to restore a task's FPU state.  We should never get
@@ -54,10 +39,8 @@ EXPORT_SYMBOL_GPL(ex_handler_fault);
  * of vulnerability by restoring from the initial state (essentially, zeroing
  * out all the FPU registers) if we can't restore from the task's FPU state.
  */
-__visible bool ex_handler_fprestore(const struct exception_table_entry *fixup,
-				    struct pt_regs *regs, int trapnr,
-				    unsigned long error_code,
-				    unsigned long fault_addr)
+static bool ex_handler_fprestore(const struct exception_table_entry *fixup,
+				 struct pt_regs *regs)
 {
 	regs->ip = ex_fixup_addr(fixup);
 
@@ -67,32 +50,23 @@ __visible bool ex_handler_fprestore(const struct exception_table_entry *fixup,
 	__restore_fpregs_from_fpstate(&init_fpstate, xfeatures_mask_fpstate());
 	return true;
 }
-EXPORT_SYMBOL_GPL(ex_handler_fprestore);
 
-__visible bool ex_handler_uaccess(const struct exception_table_entry *fixup,
-				  struct pt_regs *regs, int trapnr,
-				  unsigned long error_code,
-				  unsigned long fault_addr)
+static bool ex_handler_uaccess(const struct exception_table_entry *fixup,
+			       struct pt_regs *regs, int trapnr)
 {
 	WARN_ONCE(trapnr == X86_TRAP_GP, "General protection fault in user access. Non-canonical address?");
-	return ex_handler_default(fixup, regs, trapnr, error_code, fault_addr);
+	return ex_handler_default(fixup, regs);
 }
-EXPORT_SYMBOL(ex_handler_uaccess);
 
-__visible bool ex_handler_copy(const struct exception_table_entry *fixup,
-			       struct pt_regs *regs, int trapnr,
-			       unsigned long error_code,
-			       unsigned long fault_addr)
+static bool ex_handler_copy(const struct exception_table_entry *fixup,
+			    struct pt_regs *regs, int trapnr)
 {
 	WARN_ONCE(trapnr == X86_TRAP_GP, "General protection fault in user access. Non-canonical address?");
-	return ex_handler_fault(fixup, regs, trapnr, error_code, fault_addr);
+	return ex_handler_fault(fixup, regs, trapnr);
 }
-EXPORT_SYMBOL(ex_handler_copy);
 
-__visible bool ex_handler_rdmsr_unsafe(const struct exception_table_entry *fixup,
-				       struct pt_regs *regs, int trapnr,
-				       unsigned long error_code,
-				       unsigned long fault_addr)
+static bool ex_handler_rdmsr_unsafe(const struct exception_table_entry *fixup,
+				    struct pt_regs *regs)
 {
 	if (pr_warn_once("unchecked MSR access error: RDMSR from 0x%x at rIP: 0x%lx (%pS)\n",
 			 (unsigned int)regs->cx, regs->ip, (void *)regs->ip))
@@ -101,14 +75,11 @@ __visible bool ex_handler_rdmsr_unsafe(const struct exception_table_entry *fixup
 	/* Pretend that the read succeeded and returned 0. */
 	regs->ax = 0;
 	regs->dx = 0;
-	return ex_handler_default(fixup, regs, trapnr, error_code, fault_addr);
+	return ex_handler_default(fixup, regs);
 }
-EXPORT_SYMBOL(ex_handler_rdmsr_unsafe);
 
-__visible bool ex_handler_wrmsr_unsafe(const struct exception_table_entry *fixup,
-				       struct pt_regs *regs, int trapnr,
-				       unsigned long error_code,
-				       unsigned long fault_addr)
+static bool ex_handler_wrmsr_unsafe(const struct exception_table_entry *fixup,
+				    struct pt_regs *regs)
 {
 	if (pr_warn_once("unchecked MSR access error: WRMSR to 0x%x (tried to write 0x%08x%08x) at rIP: 0x%lx (%pS)\n",
 			 (unsigned int)regs->cx, (unsigned int)regs->dx,
@@ -116,44 +87,29 @@ __visible bool ex_handler_wrmsr_unsafe(const struct exception_table_entry *fixup
 		show_stack_regs(regs);
 
 	/* Pretend that the write succeeded. */
-	return ex_handler_default(fixup, regs, trapnr, error_code, fault_addr);
+	return ex_handler_default(fixup, regs);
 }
-EXPORT_SYMBOL(ex_handler_wrmsr_unsafe);
 
-__visible bool ex_handler_clear_fs(const struct exception_table_entry *fixup,
-				   struct pt_regs *regs, int trapnr,
-				   unsigned long error_code,
-				   unsigned long fault_addr)
+static bool ex_handler_clear_fs(const struct exception_table_entry *fixup,
+				struct pt_regs *regs)
 {
 	if (static_cpu_has(X86_BUG_NULL_SEG))
 		asm volatile ("mov %0, %%fs" : : "rm" (__USER_DS));
 	asm volatile ("mov %0, %%fs" : : "rm" (0));
-	return ex_handler_default(fixup, regs, trapnr, error_code, fault_addr);
+	return ex_handler_default(fixup, regs);
 }
-EXPORT_SYMBOL(ex_handler_clear_fs);
 
-enum handler_type ex_get_fault_handler_type(unsigned long ip)
+int ex_get_fixup_type(unsigned long ip)
 {
-	const struct exception_table_entry *e;
-	ex_handler_t handler;
+	const struct exception_table_entry *e = search_exception_tables(ip);
 
-	e = search_exception_tables(ip);
-	if (!e)
-		return EX_HANDLER_NONE;
-	handler = ex_fixup_handler(e);
-	if (handler == ex_handler_fault)
-		return EX_HANDLER_FAULT;
-	else if (handler == ex_handler_uaccess || handler == ex_handler_copy)
-		return EX_HANDLER_UACCESS;
-	else
-		return EX_HANDLER_OTHER;
+	return e ? e->type : EX_TYPE_NONE;
 }
 
 int fixup_exception(struct pt_regs *regs, int trapnr, unsigned long error_code,
 		    unsigned long fault_addr)
 {
 	const struct exception_table_entry *e;
-	ex_handler_t handler;
 
 #ifdef CONFIG_PNPBIOS
 	if (unlikely(SEGMENT_IS_PNP_CODE(regs->cs))) {
@@ -173,8 +129,33 @@ int fixup_exception(struct pt_regs *regs, int trapnr, unsigned long error_code,
 	if (!e)
 		return 0;
 
-	handler = ex_fixup_handler(e);
-	return handler(e, regs, trapnr, error_code, fault_addr);
+	switch (e->type) {
+	case EX_TYPE_DEFAULT:
+		return ex_handler_default(e, regs);
+	case EX_TYPE_FAULT:
+		return ex_handler_fault(e, regs, trapnr);
+	case EX_TYPE_UACCESS:
+		return ex_handler_uaccess(e, regs, trapnr);
+	case EX_TYPE_COPY:
+		return ex_handler_copy(e, regs, trapnr);
+	case EX_TYPE_CLEAR_FS:
+		return ex_handler_clear_fs(e, regs);
+	case EX_TYPE_FPU_RESTORE:
+		return ex_handler_fprestore(e, regs);
+	case EX_TYPE_RDMSR:
+		return ex_handler_rdmsr_unsafe(e, regs);
+	case EX_TYPE_WRMSR:
+		return ex_handler_wrmsr_unsafe(e, regs);
+	case EX_TYPE_BPF:
+		return ex_handler_bpf(e, regs);
+	case EX_TYPE_RDMSR_IN_MCE:
+		ex_handler_msr_mce(regs, false);
+		break;
+	case EX_TYPE_WRMSR_IN_MCE:
+		ex_handler_msr_mce(regs, true);
+		break;
+	}
+	BUG();
 }
 
 extern unsigned int early_recursion_flag;