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x86/debug: Remove the historical junk

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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
This commit is contained in:
Peter Zijlstra 2020-09-02 15:25:58 +02:00 committed by Thomas Gleixner
parent f0b67c39c1
commit 389cd0cd8b
1 changed files with 12 additions and 11 deletions
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23
arch/x86/kernel/traps.c
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@ -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.
*/
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;
}
out:
instrumentation_end();
idtentry_exit_nmi(regs, irq_state);