Re: [PATCH 3/3] selinux: require EXECMEM or PTRACE for FOLL_FORCE introspection

On Fri, Aug 21, 2026 at 8:56 PM Lorenzo Stoakes (ARM) <[email protected]> wrote:
> On Tue, Aug 18, 2026 at 09:51:07PM +0200, Jann Horn wrote:
> > On systems configured with PROC_MEM_FORCE_ALWAYS, ensure that a process can
> > only create anonymous executable memory via /proc/self/mem if it has one
> > of:
> >
> >  - EXECMEM (like for other methods of creating anonymous executable pages)
> >  - PTRACE (like when using /proc/$pid/mem of another process)
> >
> > This closes a hole in EXECMEM enforcement that Project Zero has used in a
> > remote Android exploit chain:
> > It was possible to use a memory corruption bug in a service without EXECMEM
> > permission to overwrite executable code via /proc/self/mem, which made it
> > possible to load and run shellcode with a kernel exploit.
> >
> > Signed-off-by: Jann Horn <[email protected]>
> > ---
> >  security/selinux/hooks.c | 27 +++++++++++++++++++++++++++
> >  1 file changed, 27 insertions(+)
> >
> > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> > index 18dd28b2bb13..905137c47321 100644
> > --- a/security/selinux/hooks.c
> > +++ b/security/selinux/hooks.c
> > @@ -2157,6 +2157,32 @@ static int selinux_ptrace_traceme(struct task_struct *parent)
> >                           SECCLASS_PROCESS, PROCESS__PTRACE, NULL);
> >  }
> >
> > +/*
> > + * Decide whether it should be possible to read non-readable VMAs and write
> > + * non-writable VMAs via /proc/self/mem.
> > + * This only applies to systems configured with PROC_MEM_FORCE_ALWAYS, and only
> > + * triggers on accesses that are not visible to selinux_ptrace_access_check().
>
> It might be worth mentioning may_access_mm() here, and obviously propagate the
> suggested name change introspection -> opened_by_owner or fd_from_order maybe
> even?

Ack, makes sense to mention may_access_mm() here.

Ack, I'll rename this to selinux_mem_foll_force_opened_by_owner().

> > + *
> > + * This allows a process to overwrite read-only code in its own address space.
> > + *
> > + * Creating an audit record on denial doesn't make sense here, since we can't
> > + * tell whether FOLL_FORCE matters for the accessed VMAs.
> > + */
> > +static int selinux_introspect_mem_foll_force(const struct cred *subject)
> > +{
> > +     struct av_decision avd;
> > +     int rc;
> > +     u32 sid = cred_sid(subject);
> > +
> > +     /* Allow if the process is generally allowed to have executable anonymous memory. */
> > +     rc = avc_has_perm_noaudit(sid, sid, SECCLASS_PROCESS, PROCESS__EXECMEM, 0, &avd);
>
> But does it make sense for the shared-by-fd case? In that case you're now
> updating execmem for another process's memory right?

Since the existing check for PROCESS__PTRACE happens on open(), I
figured it would make sense to also check against the open()-time
credentials here.

(I will be changing this to check just for PROCESS__PTRACE based on
Stephen's feedback.)

> > +
> > +     /* Also allow if selinux_ptrace_access_check() would allow it. */
> > +     if (rc)
> > +             rc = avc_has_perm_noaudit(sid, sid, SECCLASS_PROCESS, PROCESS__PTRACE, 0, &avd);
> > +     return rc;
> > +}