[PATCH 2/3] proc: query LSMs for introspective mem access (if PROC_MEM_FORCE_ALWAYS)

[Jann Horn]

On Tue, Aug 25, 2026 at 3:13 PM Christian Brauner <brauner at kernel.org> wrote:
> On Tue, Aug 18, 2026 at 09:51:06PM +0200, Jann Horn wrote:
> > If the system is running with PROC_MEM_FORCE_ALWAYS, LSMs currently have no
> > good opportunity to block a process from overwriting read-only code in its
> > own address space through FOLL_FORCE writes via /proc/self/mem.
> > The security_ptrace_access_check() LSM hook is bypassed when a process
> > opens /proc/self/mem because this is considered "introspection".
>
> Useful. I ran into this issue just yesterday in:
>
> https://github.com/systemd/systemd/pull/43511
>
> Btw, it's kinda annoying that PTRACE_POKE{TEXT,DATA} still allows foll
> force writes even if PROC_MEM_FORCE_NEVER is enabled. So ideally there'd
> also be a hook for that option.

You need either /proc/$pid/mem or PTRACE_POKETEXT for installing
software breakpoints. I think if you want to completely block
FOLL_FORCE through ptrace, you're probably in a scenario where ptrace
should either be blocked completely, or at least restricted to
mostly-read-only operations?

> The other thing - though not directly FOLL_FORCE related - is that
> process_vm_writev() isn't subject to any security restrictions other
> than seccomp filtering either.

It does go through mm_access() and ptrace_may_access() unless the
access is same-MM, so it can be blocked by LSMs like Yama.