linux-kernel - Re: [PATCH v7 28/41] x86: Introduce userspace API for shadow stack

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Date:   Fri, 10 Mar 2023 12:43:26 -0800
From:   "H.J. Lu" <hjl.tools@...il.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH v7 28/41] x86: Introduce userspace API for shadow stack

On Fri, Mar 10, 2023 at 12:27 PM Edgecombe, Rick P
<rick.p.edgecombe@...el.com> wrote:
>
> On Fri, 2023-03-10 at 12:00 -0800, H.J. Lu wrote:
> > > > So it does:
> > > > 1. Enable shadow stack
> > > > 2. Call elf libs checking functions
> > > > 3. If all good, lock shadow stack. Else, disable shadow stack.
> > > > 4. Return from elf checking functions and if shstk is enabled,
> > > > don't
> > > > underflow because it was enabled in step 1 and we have return
> > > > addresses
> > > > from 2 on the shadow stack
> > > >
> > > > I'm wondering if this can't be improved in glibc to look like:
> > > > 1. Check elf libs, and record it somewhere
> > > > 2. Wait until just the right spot
> > > > 3. If all good, enable and lock shadow stack.
> > >
> > > I will try it out.
> > >
> >
> > Currently glibc enables shadow stack as early as possible.  There
> > are only a few places where a function call in glibc never returns.
> > We can enable shadow stack just before calling main.   There are
> > quite some code paths without shadow stack protection.   Is this
> > an issue?
>
> Thanks for checking. Hmm, does the loader get attacked?

Not I know of.  But there are user codes from .init_array
and .preinit_array which are executed before main.   In theory,
an attack can happen before main.

-- 
H.J.