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Message-ID: <87h7tpeyed.fsf@oldenburg2.str.redhat.com>
Date: Thu, 30 Jul 2020 18:54:50 +0200
From: Florian Weimer <fweimer@...hat.com>
To: Jann Horn <jannh@...gle.com>
Cc: oss-security@...ts.openwall.com, x86-64-abi@...glegroups.com, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Szabolcs Nagy <szabolcs.nagy@....com>
Subject: Re: Alternative CET ABI
* 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
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