Commit 15a416e8 authored by Andy Lutomirski's avatar Andy Lutomirski Committed by Thomas Gleixner
Browse files

x86/entry: Treat BUG/WARN as NMI-like entries 



BUG/WARN are cleverly optimized using UD2 to handle the BUG/WARN out of
line in an exception fixup.

But if BUG or WARN is issued in a funny RCU context, then the
idtentry_enter...() path might helpfully WARN that the RCU context is
invalid, which results in infinite recursion.

Split the BUG/WARN handling into an nmi_enter()/nmi_exit() path in
exc_invalid_op() to increase the chance to survive the experience.

[ tglx: Make the declaration match the implementation ]

Signed-off-by: default avatarAndy Lutomirski <luto@kernel.org>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/f8fe40e0088749734b4435b554f73eee53dcf7a8.1591932307.git.luto@kernel.org
parent f0178fc0

arch/x86/include/asm/idtentry.h

+1 −1
Original line number Diff line number Diff line
@@ -543,7 +543,6 @@ SYM_CODE_END(spurious_entries_start) 
DECLARE_IDTENTRY(X86_TRAP_DE,		exc_divide_error);

DECLARE_IDTENTRY(X86_TRAP_OF,		exc_overflow);

DECLARE_IDTENTRY(X86_TRAP_BR,		exc_bounds);

DECLARE_IDTENTRY(X86_TRAP_UD,		exc_invalid_op);

DECLARE_IDTENTRY(X86_TRAP_NM,		exc_device_not_available);

DECLARE_IDTENTRY(X86_TRAP_OLD_MF,	exc_coproc_segment_overrun);

DECLARE_IDTENTRY(X86_TRAP_SPURIOUS,	exc_spurious_interrupt_bug);
@@ -561,6 +560,7 @@ DECLARE_IDTENTRY_ERRORCODE(X86_TRAP_GP, exc_general_protection); 
DECLARE_IDTENTRY_ERRORCODE(X86_TRAP_AC,	exc_alignment_check);



/* Raw exception entries which need extra work */

DECLARE_IDTENTRY_RAW(X86_TRAP_UD,		exc_invalid_op);

DECLARE_IDTENTRY_RAW(X86_TRAP_BP,		exc_int3);

DECLARE_IDTENTRY_RAW_ERRORCODE(X86_TRAP_PF,	exc_page_fault);

arch/x86/kernel/traps.c

+38 −26
Original line number Diff line number Diff line
@@ -97,24 +97,6 @@ int is_valid_bugaddr(unsigned long addr) 
	return ud == INSN_UD0 || ud == INSN_UD2;

}



int fixup_bug(struct pt_regs *regs, int trapnr)

{

	if (trapnr != X86_TRAP_UD)

		return 0;



	switch (report_bug(regs->ip, regs)) {

	case BUG_TRAP_TYPE_NONE:

	case BUG_TRAP_TYPE_BUG:

		break;



	case BUG_TRAP_TYPE_WARN:

		regs->ip += LEN_UD2;

		return 1;

	}



	return 0;

}



static nokprobe_inline int

do_trap_no_signal(struct task_struct *tsk, int trapnr, const char *str,

		  struct pt_regs *regs,	long error_code)
@@ -190,13 +172,6 @@ static void do_error_trap(struct pt_regs *regs, long error_code, char *str, 
{

	RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");



	/*

	 * WARN*()s end up here; fix them up before we call the

	 * notifier chain.

	 */

	if (!user_mode(regs) && fixup_bug(regs, trapnr))

		return;



	if (notify_die(DIE_TRAP, str, regs, error_code, trapnr, signr) !=

			NOTIFY_STOP) {

		cond_local_irq_enable(regs);
@@ -241,9 +216,46 @@ static inline void handle_invalid_op(struct pt_regs *regs) 
		      ILL_ILLOPN, error_get_trap_addr(regs));

}



DEFINE_IDTENTRY(exc_invalid_op)

DEFINE_IDTENTRY_RAW(exc_invalid_op)

{

	bool rcu_exit;



	/*

	 * Handle BUG/WARN like NMIs instead of like normal idtentries:

	 * if we bugged/warned in a bad RCU context, for example, the last

	 * thing we want is to BUG/WARN again in the idtentry code, ad

	 * infinitum.

	 */

	if (!user_mode(regs) && is_valid_bugaddr(regs->ip)) {

		enum bug_trap_type type;



		nmi_enter();

		instrumentation_begin();

		trace_hardirqs_off_finish();

		type = report_bug(regs->ip, regs);

		if (regs->flags & X86_EFLAGS_IF)

			trace_hardirqs_on_prepare();

		instrumentation_end();

		nmi_exit();



		if (type == BUG_TRAP_TYPE_WARN) {

			/* Skip the ud2. */

			regs->ip += LEN_UD2;

			return;

		}



		/*

		 * Else, if this was a BUG and report_bug returns or if this

		 * was just a normal #UD, we want to continue onward and

		 * crash.

		 */

	}



	rcu_exit = idtentry_enter_cond_rcu(regs);

	instrumentation_begin();

	handle_invalid_op(regs);

	instrumentation_end();

	idtentry_exit_cond_rcu(regs, rcu_exit);

}



DEFINE_IDTENTRY(exc_coproc_segment_overrun)

arch/x86/mm/extable.c

+13 −2
Original line number Diff line number Diff line
@@ -204,8 +204,19 @@ void __init early_fixup_exception(struct pt_regs *regs, int trapnr) 
	if (fixup_exception(regs, trapnr, regs->orig_ax, 0))

		return;



	if (fixup_bug(regs, trapnr))

	if (trapnr == X86_TRAP_UD) {

		if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {

			/* Skip the ud2. */

			regs->ip += LEN_UD2;

			return;

		}



		/*

		 * If this was a BUG and report_bug returns or if this

		 * was just a normal #UD, we want to continue onward and

		 * crash.

		 */

	}



fail:

	early_printk("PANIC: early exception 0x%02x IP %lx:%lx error %lx cr2 0x%lx\n",

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