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Date: Thu, 30 Jul 2020 10:14:10 -0700
From: "H.J. Lu" <hjl.tools@...il.com>
To: Florian Weimer <fweimer@...hat.com>
Cc: Jann Horn <jannh@...gle.com>, oss-security@...ts.openwall.com,
x86-64-abi <x86-64-abi@...glegroups.com>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>, Szabolcs Nagy <szabolcs.nagy@....com>
Subject: Re: Alternative CET ABI
On Thu, Jul 30, 2020 at 9:54 AM Florian Weimer <fweimer@...hat.com> wrote:
>
> * Jann Horn:
>
> > On Thu, Jul 30, 2020 at 6:02 PM Florian Weimer <fweimer@...hat.com> wrote:
> >> Functions no longer start with the ENDBR64 prefix. Instead, the link
> >> editor produces a PLT entry with an ENDBR64 prefix if it detects any
> >> address-significant relocation for it. The PLT entry performs a NOTRACK
> >> jump to the target address. This assumes that the target address is
> >> subject to RELRO, of course, so that redirection is not possible.
> >> Without address-significant relocations, the link editor produces a PLT
> >> entry without the ENDBR64 prefix (but still with the NOTRACK jump), or
> >> perhaps no PLT entry at all.
> >
> > How would this interact with function pointer comparisons? As in, if
> > library A exports a function func1 without referencing it, and
> > libraries B and C both take references to func1, would they end up
> > with different function pointers (pointing to their respective PLT
> > entries)?
>
> Same as today. ELF already deals with this by picking one canonical
> function address per process.
>
> Some targets already need PLTs for inter-DSO calls, so the problem is
> not new. It happens even on x86 because the main program can refer to
> its PLT stubs without run-time relocations, so those determine the
> canonical address of those functions, and not the actual implementation
> in a shared object.
>
> > Would this mean that the behavior of a program that compares
> > function pointers obtained through different shared libraries might
> > change?
>
> Hopefully not, because that would break things quite horribly (as it's
> sometimes possible to observe if the RTLD_DEEPBIND flag is used).
>
> Both the canonicalization and the fact in order to observe the function
> pointer, you need to take its address should take care of this.
>
> > I guess you could maybe canonicalize function pointers somehow, but
> > that'd probably at least break dlclose(), right?
>
> Ahh, dlclose. I think in this case, my idea to generate a PLT stub
> locally in the address-generating DSO will not work because the
> canonical address must survive dlclose if it refers to another DSO.
> There are two ways to deal with this: do not unload the PLT stub until
> the target DSO is also unloaded (but make sure that the DSO can be
> reloaded at a different address; probably not worth the complexity),
> or use the dlsym hack I sketched for regular symbol binding as well.
> Even more room for experiments, I guess.
>
> Thanks,
> Florian
FWIW, we can introduce a different CET PLT as long as it is compatible
with the past, current and future binaries.
--
H.J.
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