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: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/f8fe40e0088749734b4435b554f73eee53dcf7a8.1591932307.git.luto@kernel.org

diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
index d203c54..2fc6b0c 100644 (file)
--- a/arch/x86/include/asm/idtentry.h
+++ b/arch/x86/include/asm/idtentry.h
@@ -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);
 

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 7febae3..af75109 100644 (file)
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -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)

diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index b991aa4..1d6cb07 100644 (file)
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -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))
-               return;
+       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",