The following commit has been merged into the x86/entry branch of tip: Commit-ID: 389cd0cd8b3790b555c3679da946f4aa4fba3bab Gitweb: https://git.kernel.org/tip/389cd0cd8b3790b555c3679da946f4aa4fba3bab Author: Peter Zijlstra <[email protected]> AuthorDate: Wed, 02 Sep 2020 15:25:58 +02:00 Committer: Thomas Gleixner <[email protected]> CommitterDate: Fri, 04 Sep 2020 15:12:55 +02:00
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 <[email protected]> Suggested-by: Andy Lutomirski <[email protected]> Signed-off-by: Peter Zijlstra (Intel) <[email protected]> Signed-off-by: Thomas Gleixner <[email protected]> Tested-by: Daniel Thompson <[email protected]> Link: https://lore.kernel.org/r/[email protected] --- arch/x86/kernel/traps.c | 23 ++++++++++++----------- 1 file changed, 12 insertions(+), 11 deletions(-) diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c index 682af24..1e89001 100644 --- a/arch/x86/kernel/traps.c +++ b/arch/x86/kernel/traps.c @@ -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);
