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

Kees Cook
2022-04-14T18:52:23Z

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, 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].

> Add in-kernel support for such feature as a DENY_WRITE_EXEC personality
> flag, inherited on fork() and execve(). The kernel tracks a previously
> writeable mapping via a new VM_WAS_WRITE flag (64-bit only
> architectures). I went for a personality flag by analogy with the
> READ_IMPLIES_EXEC one. However, I'm happy to change it to a prctl() if
> we don't want more personality flags. A minor downside with the
> personality flag is that there is no way for the user to query which
> flags are supported, so in patch 3 I added an AT_FLAGS bit to advertise
> this.

My instinct here is to use a prctl(), which maps to other kinds of modern
inherited state (like no_new_privs).

> Posting this as an RFC to start a discussion and cc'ing some of the
> systemd guys and those involved in the earlier thread around the glibc
> workaround for dynamic libraries [4]. Before thinking of upstreaming
> this we'd need the systemd folk to buy into replacing the MDWE SECCOMP
> BPF filter with the in-kernel one.
>
> Thanks,
>
> Catalin
>
> [1] https://www.freedesktop.org/software/systemd/man/systemd.exec.html#MemoryDenyWriteExecute=
> [2] https://bugzilla.redhat.com/show_bug.cgi?id=1888842
> [3] https://sourceware.org/bugzilla/show_bug.cgi?id=26831
> [3] https://lore.kernel.org/r/cover.1604393169.git.szabolcs.nagy@arm.com

So, yes, let's do it. It's long long overdue in the kernel. :)

-Kees

[1] https://github.com/KSPP/linux/issues/32
[2] https://github.com/KSPP/linux/issues/32#issuecomment-1084859611

>
> Catalin Marinas (4):
> mm: Track previously writeable vma permission
> mm, personality: Implement memory-deny-write-execute as a personality
> flag
> fs/binfmt_elf: Tell user-space about the DENY_WRITE_EXEC personality
> flag
> arm64: Select ARCH_ENABLE_DENY_WRITE_EXEC
>
> arch/arm64/Kconfig | 1 +
> fs/binfmt_elf.c | 2 ++
> include/linux/mm.h | 6 ++++++
> include/linux/mman.h | 18 +++++++++++++++++-
> include/uapi/linux/binfmts.h | 4 ++++
> include/uapi/linux/personality.h | 1 +
> mm/Kconfig | 4 ++++
> mm/mmap.c | 3 +++
> mm/mprotect.c | 5 +++++
> 9 files changed, 43 insertions(+), 1 deletion(-)
>

--
Kees Cook