Messages in this thread | | | Date | Thu, 21 Apr 2022 19:48:27 +0300 | Subject | Re: [PATCH RFC 0/4] mm, arm64: In-kernel support for memory-deny-write-execute (MDWE) | From | Topi Miettinen <> |
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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
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