arm64: split EL0/EL1 UNDEF handlers

In general, exceptions taken from EL1 need to be handled separately from
exceptions taken from EL0, as the logic to handle the two cases can be
significantly divergent, and exceptions taken from EL1 typically have
more stringent requirements on locking and instrumentation.

Subsequent patches will rework the way EL1 UNDEFs are handled in order
to address longstanding soundness issues with instrumentation and RCU.
In preparation for that rework, this patch splits the existing
do_undefinstr() handler into separate do_el0_undef() and do_el1_undef()
handlers.

Prior to this patch, do_undefinstr() was marked with NOKPROBE_SYMBOL(),
preventing instrumentation via kprobes. However, do_undefinstr() invokes
other code which can be instrumented, and:

* For UNDEFINED exceptions taken from EL0, there is no risk of recursion
within kprobes. Therefore it is safe for do_el0_undef to be
instrumented with kprobes, and it does not need to be marked with
NOKPROBE_SYMBOL().

* For UNDEFINED exceptions taken from EL1, either:

(a) The exception is has been taken when manipulating SSBS; these cases
are limited and do not occur within code that can be invoked
recursively via kprobes. Hence, in these cases instrumentation
with kprobes is benign.

(b) The exception has been taken for an unknown reason, as other than
manipulating SSBS we do not expect to take UNDEFINED exceptions
from EL1. Any handling of these exception is best-effort.

... and in either case, marking do_el1_undef() with NOKPROBE_SYMBOL()
isn't sufficient to prevent recursion via kprobes as functions it
calls (including die()) are instrumentable via kprobes.

Hence, it's not worthwhile to mark do_el1_undef() with
NOKPROBE_SYMBOL(). The same applies to do_el1_bti() and do_el1_fpac(),
so their NOKPROBE_SYMBOL() annotations are also removed.

Aside from the new instrumentability, there should be no functional
change as a result of this patch.

Signed-off-by: Mark Rutland <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: James Morse <[e-mail]>
Cc: Joey Gouly <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Will Deacon <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Will Deacon <[e-mail]>

arch/arm64/include/asm/exception.h

···
 asmlinkage void asm_exit_to_user_mode(struct pt_regs *regs);
 
 void do_mem_abort(unsigned long far, unsigned long esr, struct pt_regs *regs);
-void do_undefinstr(struct pt_regs *regs, unsigned long esr);
+void do_el0_undef(struct pt_regs *regs, unsigned long esr);
+void do_el1_undef(struct pt_regs *regs, unsigned long esr);
 void do_el0_bti(struct pt_regs *regs);
 void do_el1_bti(struct pt_regs *regs, unsigned long esr);
 void do_debug_exception(unsigned long addr_if_watchpoint, unsigned long esr,

arch/arm64/kernel/entry-common.c

···
 {
 	enter_from_kernel_mode(regs);
 	local_daif_inherit(regs);
-	do_undefinstr(regs, esr);
+	do_el1_undef(regs, esr);
 	local_daif_mask();
 	exit_to_kernel_mode(regs);
 }
···
 {
 	enter_from_user_mode(regs);
 	local_daif_restore(DAIF_PROCCTX);
-	do_undefinstr(regs, esr);
+	do_el0_undef(regs, esr);
 	exit_to_user_mode(regs);
 }
 

arch/arm64/kernel/traps.c

···
 	force_signal_inject(SIGSEGV, code, addr, 0);
 }
 
-void do_undefinstr(struct pt_regs *regs, unsigned long esr)
+void do_el0_undef(struct pt_regs *regs, unsigned long esr)
 {
 	/* check for AArch32 breakpoint instructions */
 	if (!aarch32_break_handler(regs))
···
 	if (call_undef_hook(regs) == 0)
 		return;
 
-	if (!user_mode(regs))
-		die("Oops - Undefined instruction", regs, esr);
-
 	force_signal_inject(SIGILL, ILL_ILLOPC, regs->pc, 0);
 }
-NOKPROBE_SYMBOL(do_undefinstr);
+
+void do_el1_undef(struct pt_regs *regs, unsigned long esr)
+{
+	if (call_undef_hook(regs) == 0)
+		return;
+
+	die("Oops - Undefined instruction", regs, esr);
+}
 
 void do_el0_bti(struct pt_regs *regs)
 {
···
 {
 	die("Oops - BTI", regs, esr);
 }
-NOKPROBE_SYMBOL(do_el1_bti);
 
 void do_el0_fpac(struct pt_regs *regs, unsigned long esr)
 {
···
 	 */
 	die("Oops - FPAC", regs, esr);
 }
-NOKPROBE_SYMBOL(do_el1_fpac)
 
 #define __user_cache_maint(insn, address, res)			\
 	if (address >= TASK_SIZE_MAX) {				\
···
 		hook_base = cp15_64_hooks;
 		break;
 	default:
-		do_undefinstr(regs, esr);
+		do_el0_undef(regs, esr);
 		return;
 	}
 
···
 	 * EL0. Fall back to our usual undefined instruction handler
 	 * so that we handle these consistently.
 	 */
-	do_undefinstr(regs, esr);
+	do_el0_undef(regs, esr);
 }
 #endif
 
···
 	 * back to our usual undefined instruction handler so that we handle
 	 * these consistently.
 	 */
-	do_undefinstr(regs, esr);
+	do_el0_undef(regs, esr);
 }
 
 static const char *esr_class_str[] = {