linux-kernel - Re: [RFC 37/37] fs/binfmt_elf: Block old shstk elf bit

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Message-ID: <87h6zaiu05.fsf@oldenburg.str.redhat.com>
Date:   Mon, 07 Nov 2022 17:55:54 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [RFC 37/37] fs/binfmt_elf: Block old shstk elf bit

* 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).  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.

Thanks,
Florian