Date:   Mon, 7 Nov 2022 13:21:49 -0800
From:   "H.J. Lu" <hjl.tools@...il.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc:     "bsingharora@...il.com" <bsingharora@...il.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "Syromiatnikov, Eugene" <esyr@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "rdunlap@...radead.org" <rdunlap@...radead.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "Yu, Yu-cheng" <yu-cheng.yu@...el.com>,
        "Eranian, Stephane" <eranian@...gle.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "fweimer@...hat.com" <fweimer@...hat.com>,
        "nadav.amit@...il.com" <nadav.amit@...il.com>,
        "jannh@...gle.com" <jannh@...gle.com>,
        "dethoma@...rosoft.com" <dethoma@...rosoft.com>,
        "kcc@...gle.com" <kcc@...gle.com>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "bp@...en8.de" <bp@...en8.de>, "oleg@...hat.com" <oleg@...hat.com>,
        "Yang, Weijiang" <weijiang.yang@...el.com>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "jamorris@...ux.microsoft.com" <jamorris@...ux.microsoft.com>,
        "arnd@...db.de" <arnd@...db.de>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "pavel@....cz" <pavel@....cz>,
        "mike.kravetz@...cle.com" <mike.kravetz@...cle.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "john.allen@....com" <john.allen@....com>,
        "rppt@...nel.org" <rppt@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "corbet@....net" <corbet@....net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "gorcunov@...il.com" <gorcunov@...il.com>
Subject: Re: [RFC 37/37] fs/binfmt_elf: Block old shstk elf bit

On Mon, Nov 7, 2022 at 1:11 PM Edgecombe, Rick P
<rick.p.edgecombe@...el.com> wrote:
>
> On Mon, 2022-11-07 at 11:10 -0800, H.J. Lu wrote:
> > On Mon, Nov 7, 2022 at 9:47 AM Edgecombe, Rick P
> > <rick.p.edgecombe@...el.com> wrote:
> > >
> > > On Mon, 2022-11-07 at 17:55 +0100, Florian Weimer wrote:
> > > > * Rick P. Edgecombe:
> > > >
> > > > > On Sun, 2022-11-06 at 10:33 +0100, Florian Weimer wrote:
> > > > > > * H. J. Lu:
> > > > > >
> > > > > > > This change doesn't make a binary CET compatible.  It just
> > > > > > > requires
> > > > > > > that the toolchain must be updated and all binaries have to
> > > > > > > be
> > > > > > > recompiled with the new toolchain to enable CET.  It
> > > > > > > doesn't
> > > > > > > solve
> > > > > > > any
> > > > > > > issue which can't be solved by not updating glibc.
> > > > > >
> > > > > > Right, and it doesn't even address the library case (the
> > > > > > kernel
> > > > > > would
> > > > > > have to hook into mmap for that).  The kernel shouldn't do
> > > > > > this.
> > > > >
> > > > > Shadow stack shouldn't enable as a result of loading a library,
> > > > > if
> > > > > that's what you mean.
> > > >
> > > > It's the opposite—loading incompatible libraries needs to disable
> > > > shadow
> > > > stack (or ideally, not enable it in the first place).
> > >
> > > The glibc changes I have been using would not have enabled shadow
> > > stack
> > > in the first place unless the execing binary has the elf bit. So
> > > the
> > > binary would run as if shadow stack was not enabled in the kernel
> > > and
> > > there should be nothing to disable when an incompatible binary is
> > > loaded. Glibc will have to detect this and act accordingly because
> > > not
> > > all kernels will have shadow stack configured.
> > >
> > > >   Technically, I
> > > > think most incompatible code resides in libraries, so this kernel
> > > > change
> > > > achieves nothing besides punishing early implementations of the
> > > > published-as-finalized x86-64 ABI.
> > >
> > > It's under the assumption that not breaking things is more
> > > important
> > > than having shadow stack enabled. So it is not intended as a
> > > punishment
> > > for users at all, rather the opposite.
> > >
> > > I'm not sure how much the spec mandates things by the letter of it,
> > > but
> > > in any case things have gone wrong in the real world. I am very
> > > open to
> > > discussion here. I only went this way as a last resort because I
> > > didn't
> > > hear back on the last thread.
> >
> > Some applications and libraries are compiled with -fcf-protection,
> > but
> > they manipulate the stack in such a way that they aren't compatible
> > with the shadow stack.   However, if the build/test setup doesn't
> > support
> > shadow stack, it is impossible to validate.
> >
>
> When we have everything in place, the problems would be much more
> obvious when distros started turning it on. But we can't turn it on as

Not necessarily.  The problem will show up only in a CET enabled
environment since build/test setup may not be on a CET capable
hardware.

> planned without breaking things for existing binaries. We can have both
> by:
> 1. Choosing a new bit, adding it to the tools, and never supporting the
> old bit in glibc.
> 2. Providing the option to have the kernel block the old bit, so
> upgraded users can decide what experience they would like. Then distros
> can find the problems and adjust their packages. I'm starting to think
> a default off sysctl toggle might be better than a Kconfig.
> 3. Any other ideas?

Don't enable CET in glibc until we can validate CET functionality.

-- 
H.J.

Powered by blists - more mailing lists