On 21.4.2022 18.35, Catalin Marinas wrote:
On Wed, Apr 20, 2022 at 04:21:45PM -0700, Kees Cook wrote:
On Wed, Apr 20, 2022 at 10:34:33PM +0300, Topi Miettinen wrote:
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).
I agree.
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.
Do we want the "was PROT_WRITE" or we just reject mprotect(PROT_EXEC) if
the vma is not already PROT_EXEC? The latter is closer to the current
systemd approach. The former allows an mprotect(PROT_EXEC) if the
mapping was PROT_READ only for example.
I'd drop the "was PROT_WRITE" for now if the aim is a drop-in
replacement for BPF MDWE.
I think we'd want existing installations with MemoryDenyWriteExecute=yes
not start failing when the implementation is changed to in-kernel
version. The implementation could be very simple and not even check
existing PROT_ flags (except for BTI case) to be maximally compatible to
BPF version. So I'd leave "was PROT_WRITE" and other checks to more
advanced versions, enabled with a different PR_MDWX_FLAG_.
-Topi
