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x86/debug: Remove the historical junk
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
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1 changed files with 12 additions and 11 deletions
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@ -843,18 +843,19 @@ static __always_inline void exc_debug_kernel(struct pt_regs *regs,
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if (notify_debug(regs, &dr6))
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goto out;
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if (WARN_ON_ONCE(dr6 & DR_STEP)) {
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/*
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* Historical junk that used to handle SYSENTER single-stepping.
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* This should be unreachable now. If we survive for a while
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* without anyone hitting this warning, we'll turn this into
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* an oops.
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*/
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dr6 &= ~DR_STEP;
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set_thread_flag(TIF_SINGLESTEP);
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/*
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* The kernel doesn't use TF single-step outside of:
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*
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* - Kprobes, consumed through kprobe_debug_handler()
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* - KGDB, consumed through notify_debug()
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*
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* So if we get here with DR_STEP set, something is wonky.
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*
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* A known way to trigger this is through QEMU's GDB stub,
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* which leaks #DB into the guest and causes IST recursion.
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*/
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if (WARN_ON_ONCE(current->thread.debugreg6 & DR_STEP))
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regs->flags &= ~X86_EFLAGS_TF;
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}
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out:
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instrumentation_end();
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idtentry_exit_nmi(regs, irq_state);
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