x86/debug: Remove the historical junk (389cd0cd) · Commits · 戴 / test

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

arch/x86/kernel/traps.c

+12 −11

Original line number	Diff line number	Diff line
		@@ -843,18 +843,19 @@ static __always_inline void exc_debug_kernel(struct pt_regs *regs,
		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.
		* 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.
		*/
		dr6 &= ~DR_STEP;
		set_thread_flag(TIF_SINGLESTEP);
		if (WARN_ON_ONCE(current->thread.debugreg6 & DR_STEP))
		regs->flags &= ~X86_EFLAGS_TF;
		}

		out:
		instrumentation_end();
		idtentry_exit_nmi(regs, irq_state);

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