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
