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
