Re: [Xen-devel] [PATCH v2 3/3] arm64: remove the rest of asm-uaccess.h

[Mark Rutland <mark.rutland@xxxxxxx>]

On Wed, Nov 27, 2019 at 11:09:35AM -0500, Pavel Tatashin wrote:
> On Wed, Nov 27, 2019 at 11:03 AM Mark Rutland <mark.rutland@xxxxxxx> wrote:
> >
> > On Wed, Nov 27, 2019 at 10:31:54AM -0500, Pavel Tatashin wrote:
> > > On Wed, Nov 27, 2019 at 10:12 AM Mark Rutland <mark.rutland@xxxxxxx> 
> > > wrote:
> > > >
> > > > On Thu, Nov 21, 2019 at 09:24:06PM -0500, Pavel Tatashin wrote:
> > > > > The __uaccess_ttbr0_disable and __uaccess_ttbr0_enable,
> > > > > are the last two macros defined in asm-uaccess.h.
> > > > >
> > > > > Replace them with C wrappers and call C functions from
> > > > > kernel_entry and kernel_exit.
> > > >
> > > > For now, please leave those as-is.
> > > >
> > > > I don't think we want to have out-of-line C wrappers in the middle of
> > > > the entry assembly where we don't have a complete kernel environment.
> > > > The use in entry code can also assume non-preemptibility, while the C
> > > > functions have to explcitily disable that.
> > >
> > > I do not understand, if C function is called form non-preemptible
> > > context it stays non-preemptible. kernel_exit already may call C
> > > functions around the time __uaccess_ttbr0_enable is called (it may
> > > call post_ttbr_update_workaround), and that C functions does not do
> > > explicit preempt disable:
> >
> > Sorry, I meant that IRQs are disabled here.
> >
> > The C wrapper calls __uaccess_ttbr0_enable(), which calls
> > local_irq_save() and local_irq_restore(). Those are pointless in the
> > bowels of the entry code, and potentially expensive if IRQ prio masking
> > is in use.
> >
> > I'd rather not add more out-of-line C code calls here right now as I'd
> > prefer to factor out the logic to C in a better way.
> 
> Ah, yes, this makes sense. I could certainly factor out C calls in a
> better way, or is this something you want to work on?

I'm hoping to do that as part of ongoing entry-deasm work, now that a
lot of the prerequisite work was merged in v5.4.

> Without removing these assembly macros I do not think we want to
> address this suggestion from Kees Cook:
> https://lore.kernel.org/lkml/CA+CK2bCBS2fKOTmTFm13iv3u5TBPwpoCsYeeP352DVE-gs9GJw@xxxxxxxxxxxxxx/

In the mean time, we could add checks around addr_limit_user_check(),
and in the context-switch path. I have some preparatory cleanup to allow
for the context-switch check, which I'll send out at -rc1. That was what
I used to detect the case you reported previously.

Thanks,
Mark.

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