Subject: Re: [PATCH v1 2/2] arm64: Enable BTI for main executable as well as the interpreter

From: Dave Martin <[email-masked]>
Date: Mon, 7 Jun 2021 12:25:38 +0100

On Thu, Jun 03, 2021 at 07:04:31PM +0100, Catalin Marinas via Libc-alpha wrote:
> On Thu, Jun 03, 2021 at 05:51:34PM +0100, Mark Brown wrote:
> > On Thu, Jun 03, 2021 at 04:40:35PM +0100, Dave Martin wrote:
> > > Do we know how libcs will detect that they don't need to do the
> > > mprotect() calls?  Do we need a detection mechanism at all?
> > > 
> > > Ignoring certain errors from mprotect() when ld.so is trying to set
> > > PROT_BTI on the main executable's code pages is probably a reasonable,
> > > backwards-compatible compromise here, but it seems a bit wasteful.
> > 
> > I think the theory was that they would just do the mprotect() calls and
> > ignore any errors as they currently do, or declare that they depend on a
> > new enough kernel version I guess (not an option for glibc but might be
> > for others which didn't do BTI yet).
> 
> I think we discussed the possibility of an AT_FLAGS bit. Until recently,
> this field was 0 but it gained a new bit now. If we are to expose this
> to arch-specific things, it may need some reservations. Anyway, that's
> an optimisation that can be added subsequently.

I suppose so, but AT_FLAGS doesn't seem appropriate somehow.

I wonder why we suddenly start considering adding a flag to AT_FLAGS
every few months, when it had sat empty for decades.  This may say
something about the current health of the kernel ABI, but I'm not sure
exactly what.

I think having mprotect() fail in a predictable way may be preferable
for now: glibc still only needs to probe with a single call and could
cache the knowledge after that.  Code outside libc / ld.so seems quite
unlikely to care about this.

Since only the executable segment(s) of the main binary need to be
protected, this should require only a very small number of mprotect()
calls in normal situations.  Although it feels a bit cruddy as a design,
cost-wise I think that extra overhead would be swamped by other noise in
realistic scenarios.  Often, there is just a single executable segment,
so the common case would probably require just one mprotect() call.  I
don't know if it ever gets much more complicated when using the
standard linker scripts.

Any ideas on how we would document this behaviour?  The kernel and libc
behaviour are 100% clear: you _are_ allowed to twiddle PROT_BTI on
executable mappings, and there is no legitimate (or even useful) reason
to disallow this.  It's only systemd deliberately breaking the API that
causes the behaviour seem by "userspace" to vary.

Cheers
---Dave

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