Re: [PATCH v3 0/6] Tracing vs CR2

[Andy Lutomirski]

On Fri, Jul 19, 2019 at 8:59 PM Eiichi Tsukata <devel@xxxxxxxxxxxx> wrote:
>
>
> On 2019/07/19 5:27, Andy Lutomirski wrote:
> > Hi all-
> >
> > I suspect that a bunch of the bugs you're all finding boil down to:
> >
> >  - Nested debug exceptions could corrupt the outer exception's DR6.
> >  - Nested debug exceptions in which *both* exceptions came from the
> > kernel were probably all kinds of buggy
> >  - Data breakpoints in bad places in the kernel were bad news
> >
> > Could you give this not-quite-finished series a try?
> >
> > https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/
> >
>
> Though I'm still trying to find out other cases(other areas which could
> be buggy if we set hw breakpoints), as far as I tested, there is
> no problem so far.
>
> If I understand correctly, the call trace and the dr6 value will be:
>
> ====
>
> debug() // dr6: 0xffff4ff0, user_mode: 1
>   TRACE_IRQS_OFF
>     arch_stack_user_walk()
>       debug()  // dr6: 0xffff4ff1 == 0xffff4ff0 | 0xffff0ff1 ... (*)
>         do_debug()
>           WARN_ON_ONCE
>   do_debug() // dr6: 0xffff0ff0(cleared in the above do_debug())

The dr6 register will indeed be cleared like this, but the dr6
variable should still be 0xffff4ff0.

>
> (*) :
> >   * The Intel SDM says:
> >   *
> >   *   Certain debug exceptions may clear bits 0-3. The remaining
> >   *   contents of the DR6 register are never cleared by the
> >   *   processor. To avoid confusion in identifying debug
> >   *   exceptions, debug handlers should clear the register before
> >   *   returning to the interrupted task.
>
> ====
>
> Note: printk() in do_debug() can cause infinite loop(printk() ->
> irq_disable() -> do_debug() -> printk() ...), so printk_deferred()
> was preferable.
>

Shouldn't that be fixed with my patches?  It should only be able to
recurse two deep: do_debug() from user mode can indeed trip
breakpoints, but the next do_debug() will clear DR7 in paranoid_entry.