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Date: Mon, 12 Feb 2024 17:18:22 -0600
From: William Roberts <bill.c.roberts@...il.com>
To: enh <enh@...gle.com>
Cc: musl@...ts.openwall.com
Subject: Re: PAC/BTI Support on aarch64
On Mon, Feb 12, 2024 at 5:05 PM enh <enh@...gle.com> wrote:
>
> On Mon, Feb 12, 2024 at 2:46 PM Rich Felker <dalias@...c.org> wrote:
> >
> > On Mon, Feb 12, 2024 at 03:25:48PM -0600, William Roberts wrote:
> > > On Mon, Feb 12, 2024 at 12:42 PM Rich Felker <dalias@...c.org> wrote:
> > > >
> > > > On Mon, Feb 12, 2024 at 10:38:50AM -0600, William Roberts wrote:
> > > > > Hello,
> > > > >
> > > > > I was just wondering if there was any work being done to support PAC
> > > > > and BTI in aarch64? I could add support but didn't want to duplicate
> > > > > the work.
> > > >
> > > > I'm not aware of any active work on this, but before writing a full
> > > > implementation, it would be really helpful to start with a basic
> > > > proposal for the scope of changes needed to make it work to assess
> > > > whether these are manageable and acceptable cost.
> > >
> > > It's a matter of building with -mbranch-protection=standard
> > >
> > > Just the ASM labels need the first instruction to be a BTI. They're in
> > > the NOP space
> > > so they are backwards compatible, older hardware will just NOP it.
> >
> > I think it's a little more elaborate than that. Those asm instructions
> > need to be added (probably as .instr or .word or something, unless
> > there's a way to spell this particular nop that existing tooling will
> > understand).
>
> depends on your toolchain version. when we added this to bionic, the
> toolchain work was still happening. so you'll want to test against
> whatever your oldest-supported toolchain is.
>
You just use the hint <immediate> instructions, they are understood by old
toolchains. But you can only support a subset of the BTI/PAC instructions
but it's been enough for most projects that follow the normal ABI conventions
like OpenSSL/BoringSSL,etc, but not enough for libffi for example.
> > Or it could be made conditional, but that would require
> > converting any asm that's not already .S files to .S. Not bad, but not
as in inline asm? Unless it's a branch target, no need.
> > quite as trivial as adding something to CFLAGS.
That's not really what I said...
> >
> > I also wondered if [sig]setjmp/longjmp would be affected, but probably
> > not.
>
> bionic does use PAC, but i think glibc has its own "pointer mangling" thing?
You need it, as the first instruction from a branch (where longjmp returns to)
needs to be a BTI instruction.
>
> > > It's been done for many projects, glibc and bionic have it. The
> > > problem with BTI is that when one item in the link
> > > list doesn't support BTI the loader/linker turns it off. So when it's
> > > something like a libc that is fundamental in the link chain,
> > > it turns it off for everything.
> >
> > This presumably requires some kind of machinery for how dynamic
> > linking will work, and possibly turning it off if a library without it
> > is dlopened?
> >
> > My understanding doing some brief searches though was that you can
> > individually mprotect it off in certain regions. So maybe it's
> > possible to just enable only for DSOs that support it?
>
> correct.
>
> > > The initial scope of code changes would be what's reported when
> > > LDFLAGS=-Wl,-zforce-bti,--fatal-warnings
> >
> > Is there a way to disable these warnings so that every asm file does
> > not need to be cluttered with annotations?
>
> well, that's the ELF note stuff i was talking about, and if you don't
> have it you'll fall foul of the static linker saying "not all this
> code is BTI-enabled, therefore this .so isn't", and the dynamic linker
> doing nothing because the static linker effectively tells it not to.
Yep, well said ENH. It's been since Android since we crossed paths :-).
It's not that hard to annotate an asm file :-p I forget what project
(I think it was gnutls, but they just use openssl's code for the asm)
but I just put it in a header file and by virtue of #include'ing it you get the
notes added.
>
> > Rich
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