Remove the historical junk and replace it with a WARN and a comment.
The problem is that even though the kernel only uses TF single-step in
kprobes and KGDB, both of which consume the event before this, QEMU/KVM has
bugs in this area that can trigger this state so it has to be dealt with.
Suggested-by: Brian Gerst <brgerst@gmail.com>
Suggested-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20200902133201.170216274@infradead.org
if (notify_debug(regs, &dr6))
goto out;
if (notify_debug(regs, &dr6))
goto out;
- if (WARN_ON_ONCE(dr6 & DR_STEP)) {
- /*
- * Historical junk that used to handle SYSENTER single-stepping.
- * This should be unreachable now. If we survive for a while
- * without anyone hitting this warning, we'll turn this into
- * an oops.
- */
- dr6 &= ~DR_STEP;
- set_thread_flag(TIF_SINGLESTEP);
+ /*
+ * The kernel doesn't use TF single-step outside of:
+ *
+ * - Kprobes, consumed through kprobe_debug_handler()
+ * - KGDB, consumed through notify_debug()
+ *
+ * So if we get here with DR_STEP set, something is wonky.
+ *
+ * A known way to trigger this is through QEMU's GDB stub,
+ * which leaks #DB into the guest and causes IST recursion.
+ */
+ if (WARN_ON_ONCE(current->thread.debugreg6 & DR_STEP))
regs->flags &= ~X86_EFLAGS_TF;
regs->flags &= ~X86_EFLAGS_TF;
out:
instrumentation_end();
idtentry_exit_nmi(regs, irq_state);
out:
instrumentation_end();
idtentry_exit_nmi(regs, irq_state);