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From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 10 Jun 2019 12:55:48 -0700
Message-ID: <CALCETrVzgkhu=kjF4U5MEc+TJmsDJf8pVgnoPH5F4gTdsDF4rQ@mail.gmail.com>
Subject: Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup function
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Yu-cheng Yu <yu-cheng.yu@intel.com>,
        Peter Zijlstra <peterz@infradead.org>, X86 ML <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
        Florian Weimer <fweimer@redhat.com>,
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        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Randy Dunlap <rdunlap@infradead.org>,
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        Dave Martin <Dave.Martin@arm.com>
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On Mon, Jun 10, 2019 at 12:52 PM Dave Hansen <dave.hansen@intel.com> wrote:
>
> On 6/10/19 12:38 PM, Yu-cheng Yu wrote:
> >>> When an application starts, its highest stack address is determined.
> >>> It uses that as the maximum the bitmap needs to cover.
> >> Huh, I didn't think we ran code from the stack. ;)
> >>
> >> Especially given the way that we implemented the new 5-level-paging
> >> address space, I don't think that expecting code to be below the stack
> >> is a good universal expectation.
> > Yes, you make a good point.  However, allowing the application manage the bitmap
> > is the most efficient and flexible.  If the loader finds a legacy lib is beyond
> > the bitmap can cover, it can deal with the problem by moving the lib to a lower
> > address; or re-allocate the bitmap.
>
> How could the loader reallocate the bitmap and coordinate with other
> users of the bitmap?
>
> > If the loader cannot allocate a big bitmap to cover all 5-level
> > address space (the bitmap will be large), it can put all legacy lib's
> > at lower address.  We cannot do these easily in the kernel.
>
> This is actually an argument to do it in the kernel.  The kernel can
> always allocate the virtual space however it wants, no matter how large.
>  If we hide the bitmap behind a kernel API then we can put it at high
> 5-level user addresses because we also don't have to worry about the
> high bits confusing userspace.
>

That's a fairly compelling argument.

The bitmap is one bit per page, right?  So it's smaller than the
address space by a factor of 8*2^12 == 2^15.  This means that, if we
ever get full 64-bit linear addresses reserved entirely for userspace
(which could happen if my perennial request to Intel to split user and
kernel addresses completely happens), then we'll need 2^48 bytes for
the bitmap, which simply does not fit in the address space of a legacy
application.