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

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