Messages in this thread Patch in this message | | | Date | Thu, 20 Aug 2020 20:19:46 +0200 | From | peterz@infradea ... | Subject | Re: [RFC][PATCH 4/7] x86/debug: Move historical SYSENTER junk into exc_debug_kernel() |
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On Thu, Aug 20, 2020 at 09:43:15AM -0700, Andy Lutomirski wrote: > I’ve lost track of how many bugs QEMU and KVM have in this space. > Let’s keep it as a warning, but a bug. But let’s get rid of the > totally bogus TIF_SINGLESTEP manipulation.
OK, I've shuffled the series around to fix that ordering problem in patch 4 and added the below patch at the end.
Although I'm not entirely sure it actually leaks a #DB or just wrecks the state.. *shrug*.
--- Subject: x86/debug: Remove the historical junk From: Peter Zijlstra <peterz@infradead.org> Date: Thu Aug 20 18:28:37 CEST 2020
Suggested-by: Brian Gerst <brgerst@gmail.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> --- arch/x86/kernel/traps.c | 24 ++++++++++-------------- 1 file changed, 10 insertions(+), 14 deletions(-)
--- a/arch/x86/kernel/traps.c +++ b/arch/x86/kernel/traps.c @@ -839,22 +839,18 @@ static __always_inline void exc_debug_ke goto out; /* - * Reload dr6, the notifier might have changed it. + * 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 = current->thread.debugreg6; - - 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); + 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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