Re: [PATCH v7 0/4] arm64: Enable BTI for the executable as well as the interpreter

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, Nov 15, 2021 at 03:27:10PM +0000, Mark Brown wrote:
> Deployments of BTI on arm64 have run into issues interacting with
> systemd's MemoryDenyWriteExecute feature.  Currently for dynamically
> linked executables the kernel will only handle architecture specific
> properties like BTI for the interpreter, the expectation is that the
> interpreter will then handle any properties on the main executable.
> For BTI this means remapping the executable segments PROT_EXEC |
> PROT_BTI.
> 
> This interacts poorly with MemoryDenyWriteExecute since that is
> implemented using a seccomp filter which prevents setting PROT_EXEC on
> already mapped memory and lacks the context to be able to detect that
> memory is already mapped with PROT_EXEC.  This series resolves this by
> handling the BTI property for both the interpreter and the main
> executable.
> 
> This does mean that we may get more code with BTI enabled if running on
> a system without BTI support in the dynamic linker, this is expected to
> be a safe configuration and testing seems to confirm that. It also
> reduces the flexibility userspace has to disable BTI but it is expected
> that for cases where there are problems which require BTI to be disabled
> it is more likely that it will need to be disabled on a system level.

Given the silence on this series over the past months, I propose we drop
it. It's a bit unfortunate that systemd's MemoryDenyWriteExecute cannot
work with BTI but I also think the former is a pretty blunt hardening
mechanism (rejecting any mprotect(PROT_EXEC) regardless of the previous
attributes).

I'm not a security expert to assess whether MDWX is more important than
BTI (hardware availability also influences the distros decision). My
suggestion would be to look at a better way to support the MDWX on the
long run that does not interfere with BTI.

-- 
Catalin