[PATCH RFC 0/4] mm, arm64: In-kernel support for memory-deny-write-execute (MDWE)

[Kees Cook]

On Wed, Apr 20, 2022 at 10:34:33PM +0300, Topi Miettinen wrote:
> On 20.4.2022 16.01, Catalin Marinas wrote:
> > On Thu, Apr 14, 2022 at 11:52:17AM -0700, Kees Cook wrote:
> > > On Wed, Apr 13, 2022 at 02:49:42PM +0100, Catalin Marinas wrote:
> > > > The background to this is that systemd has a configuration option called
> > > > MemoryDenyWriteExecute [1], implemented as a SECCOMP BPF filter. Its aim
> > > > is to prevent a user task from inadvertently creating an executable
> > > > mapping that is (or was) writeable. Since such BPF filter is stateless,
> > > > it cannot detect mappings that were previously writeable but
> > > > subsequently changed to read-only. Therefore the filter simply rejects
> > > > any mprotect(PROT_EXEC). The side-effect is that on arm64 with BTI
> > > > support (Branch Target Identification), the dynamic loader cannot change
> > > > an ELF section from PROT_EXEC to PROT_EXEC|PROT_BTI using mprotect().
> > > > For libraries, it can resort to unmapping and re-mapping but for the
> > > > main executable it does not have a file descriptor. The original bug
> > > > report in the Red Hat bugzilla - [2] - and subsequent glibc workaround
> > > > for libraries - [3].
> > > 
> > > Right, so, the systemd filter is a big hammer solution for the kernel
> > > not having a very easy way to provide W^X mapping protections to
> > > userspace. There's stuff in SELinux, and there have been several
> > > attempts[1] at other LSMs to do it too, but nothing stuck.
> > > 
> > > Given the filter, and the implementation of how to enable BTI, I see two
> > > solutions:
> > > 
> > > - provide a way to do W^X so systemd can implement the feature differently
> > > - provide a way to turn on BTI separate from mprotect to bypass the filter
> > > 
> > > I would agree, the latter seems like the greater hack,
> > 
> > We discussed such hacks in the past but they are just working around the
> > fundamental issue - systemd wants W^X but with BPF it can only achieve
> > it by preventing mprotect(PROT_EXEC) irrespective of whether the mapping
> > was already executable. If we find a better solution for W^X, we
> > wouldn't have to hack anything for mprotect(PROT_EXEC|PROT_BTI).
> > 
> > > so I welcome
> > > this RFC, though I think it might need to explore a bit of the feature
> > > space exposed by other solutions[1] (i.e. see SARA and NAX), otherwise
> > > it risks being too narrowly implemented. For example, playing well with
> > > JITs should be part of the design, and will likely need some kind of
> > > ELF flags and/or "sealing" mode, and to handle the vma alias case as
> > > Jann Horn pointed out[2].
> > 
> > I agree we should look at what we want to cover, though trying to avoid
> > re-inventing SELinux. With this patchset I went for the minimum that
> > systemd MDWE does with BPF.
> > 
> > I think JITs get around it using something like memfd with two separate
> > mappings to the same page. We could try to prevent such aliases but
> > allow it if an ELF note is detected (or get the JIT to issue a prctl()).
> > 
> > Anyway, with a prctl() we can allow finer-grained control starting with
> > anonymous and file mappings and later extending to vma aliases,
> > writeable files etc. On top we can add a seal mask so that a process
> > cannot disable a control was set. Something like (I'm not good at
> > names):
> > 
> > 	prctl(PR_MDWX_SET, flags, seal_mask);
> > 	prctl(PR_MDWX_GET);
> > 
> > with flags like:
> > 
> > 	PR_MDWX_MMAP - basics, should cover mmap() and mprotect()
> > 	PR_MDWX_ALIAS - vma aliases, allowed with an ELF note
> > 	PR_MDWX_WRITEABLE_FILE
> > 
> > (needs some more thinking)
> > 
> 
> For systemd, feature compatibility with the BPF version is important so that
> we could automatically switch to the kernel version once available without
> regressions. So I think PR_MDWX_MMAP (or maybe PR_MDWX_COMPAT) should match
> exactly what MemoryDenyWriteExecute=yes as implemented with BPF has: only
> forbid mmap(PROT_EXEC|PROT_WRITE) and mprotect(PROT_EXEC). Like BPF, once
> installed there should be no way to escape and ELF flags should be also
> ignored. ARM BTI should be allowed though (allow PROT_EXEC|PROT_BTI if the
> old flags had PROT_EXEC).
> 
> Then we could have improved versions (other PR_MDWX_ prctls) with lots more
> checks. This could be enabled with MemoryDenyWriteExecute=strict or so.
> 
> Perhaps also more relaxed versions (like SARA) could be interesting (system
> service running Python with FFI, or perhaps JVM etc), enabled with for
> example MemoryDenyWriteExecute=trampolines. That way even those programs
> would get some protection (though there would be a gap in the defences).

Yup, I think we're all on the same page. Catalin, can you respin with a
prctl for enabling MDWE? I propose just:

	prctl(PR_MDWX_SET, flags);
	prctl(PR_MDWX_GET);

	PR_MDWX_FLAG_MMAP
		disallows PROT_EXEC on any VMA that is or was PROT_WRITE,
		covering at least: mmap, mprotect, pkey_mprotect, and shmat.

I don't think anything should be allowed to be disabled once set.

-- 
Kees Cook