linux-kernel - Re: [patch V2 09/16] x86/ioperm: Move TSS bitmap update to exit to user work

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Date:   Tue, 12 Nov 2019 09:41:58 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Willy Tarreau <w@....eu>, Juergen Gross <jgross@...e.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [patch V2 09/16] x86/ioperm: Move TSS bitmap update to exit to
 user work

On Tue, Nov 12, 2019 at 9:20 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Tue, 12 Nov 2019, Andy Lutomirski wrote:
> > On Mon, Nov 11, 2019 at 2:35 PM Thomas Gleixner <tglx@...utronix.de> wrote:
> > >
> > > There is no point to update the TSS bitmap for tasks which use I/O bitmaps
> > > on every context switch. It's enough to update it right before exiting to
> > > user space.
> > +
> > > +static inline void switch_to_bitmap(unsigned long tifp)
> > > +{
> > > +       /*
> > > +        * Invalidate I/O bitmap if the previous task used it. If the next
> > > +        * task has an I/O bitmap it will handle it on exit to user mode.
> > > +        */
> > > +       if (tifp & _TIF_IO_BITMAP)
> > > +               tss_invalidate_io_bitmap(this_cpu_ptr(&cpu_tss_rw));
> > > +}
> >
> > Shouldn't you be invalidating the io bitmap if the *next* task doesn't
> > use?  Or is the rule that, when a non-io-bitmap-using task is running,
> > even in kernel mode, the io bitmap is always invalid.
>
> Well it does not make much of a difference whether we do the above or
> !(tifn & _TIF_IO_BITMAP). We always end up in that code when one of the
> involved tasks has TIF_IO_BITMAP set. I decided to use the sched out check
> because that makes it clear that this is the end of the valid I/O
> bitmap. If the next task has TIF_IO_BITMAP set as well, then it will anyway
> end up in the exit to user mode update code. Clearing it here ensures that
> even if the exit to user mode malfunctions the bitmap cannot be leaked.
>
> > As it stands, you need exit_thread() to invalidate the bitmap.  I
> > assume it does, but I can't easily see it in the middle of the series
> > like this.
>
> It does.
>
> > IOW your code might be fine, but it could at least use some comments
> > in appropriate places (exit_to_usermode_loop()?) that we guarantee
> > that, if the bit is *clear*, then the TSS has the io bitmap marked
> > invalid.  And add an assertion under CONFIG_DEBUG_ENTRY.
> >
> > Also, do you need to update EXIT_TO_USERMODE_LOOP_FLAGS?
>
> No, the TIF_IO_BITMAP check is done once after the loop has run and it
> would not make any sense in the loop as TIF_IO_BITMAP cannot be cleared
> there and we'd loop forever. The other usermode loop flags are transient.
>

Right.  But your diff tool *said* the diff was in
exit_to_usermode_loop().  Can you look at your .gitconfig and see if
you have something weird going on?

--Andy