[RFC PATCH 1/2] lsm: introduce new hook security_vm_execstack

[Christian Göttsche]

On Fri, 15 Mar 2024 at 19:22, Casey Schaufler <casey at schaufler-ca.com> wrote:
>
> On 3/15/2024 11:08 AM, Christian Göttsche wrote:
> > Add a new hook guarding instantiations of programs with executable
> > stack.  They are being warned about since commit 47a2ebb7f505 ("execve:
> > warn if process starts with executable stack").  Lets give LSMs the
> > ability to control their presence on a per application basis.
>
> This seems like a hideously expensive way to implement a flag
> disallowing execution of programs with executable stacks. What's
> wrong with adding a flag VM_NO_EXECUTABLE_STACK?

That would be global and not on a per application basis.
One might want to exempt known legacy programs.
Also is performance a concern for this today's rare occurrence?

> >
> > Signed-off-by: Christian Göttsche <cgzones at googlemail.com>
> > ---
> >  fs/exec.c                     |  4 ++++
> >  include/linux/lsm_hook_defs.h |  1 +
> >  include/linux/security.h      |  6 ++++++
> >  security/security.c           | 13 +++++++++++++
> >  4 files changed, 24 insertions(+)
> >
> > diff --git a/fs/exec.c b/fs/exec.c
> > index 8cdd5b2dd09c..e6f9e980c6b1 100644
> > --- a/fs/exec.c
> > +++ b/fs/exec.c
> > @@ -829,6 +829,10 @@ int setup_arg_pages(struct linux_binprm *bprm,
> >       BUG_ON(prev != vma);
> >
> >       if (unlikely(vm_flags & VM_EXEC)) {
> > +             ret = security_vm_execstack();
> > +             if (ret)
> > +                     goto out_unlock;
> > +
> >               pr_warn_once("process '%pD4' started with executable stack\n",
> >                            bprm->file);
> >       }
> > diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> > index 185924c56378..b31d0744e7e7 100644
> > --- a/include/linux/lsm_hook_defs.h
> > +++ b/include/linux/lsm_hook_defs.h
> > @@ -49,6 +49,7 @@ LSM_HOOK(int, 0, syslog, int type)
> >  LSM_HOOK(int, 0, settime, const struct timespec64 *ts,
> >        const struct timezone *tz)
> >  LSM_HOOK(int, 1, vm_enough_memory, struct mm_struct *mm, long pages)
> > +LSM_HOOK(int, 0, vm_execstack, void)
> >  LSM_HOOK(int, 0, bprm_creds_for_exec, struct linux_binprm *bprm)
> >  LSM_HOOK(int, 0, bprm_creds_from_file, struct linux_binprm *bprm, const struct file *file)
> >  LSM_HOOK(int, 0, bprm_check_security, struct linux_binprm *bprm)
> > diff --git a/include/linux/security.h b/include/linux/security.h
> > index d0eb20f90b26..084b96814970 100644
> > --- a/include/linux/security.h
> > +++ b/include/linux/security.h
> > @@ -294,6 +294,7 @@ int security_quota_on(struct dentry *dentry);
> >  int security_syslog(int type);
> >  int security_settime64(const struct timespec64 *ts, const struct timezone *tz);
> >  int security_vm_enough_memory_mm(struct mm_struct *mm, long pages);
> > +int security_vm_execstack(void);
> >  int security_bprm_creds_for_exec(struct linux_binprm *bprm);
> >  int security_bprm_creds_from_file(struct linux_binprm *bprm, const struct file *file);
> >  int security_bprm_check(struct linux_binprm *bprm);
> > @@ -624,6 +625,11 @@ static inline int security_vm_enough_memory_mm(struct mm_struct *mm, long pages)
> >       return __vm_enough_memory(mm, pages, cap_vm_enough_memory(mm, pages));
> >  }
> >
> > +static inline int security_vm_execstack(void)
> > +{
> > +     return 0;
> > +}
> > +
> >  static inline int security_bprm_creds_for_exec(struct linux_binprm *bprm)
> >  {
> >       return 0;
> > diff --git a/security/security.c b/security/security.c
> > index 0144a98d3712..f75240d0d99d 100644
> > --- a/security/security.c
> > +++ b/security/security.c
> > @@ -1125,6 +1125,19 @@ int security_vm_enough_memory_mm(struct mm_struct *mm, long pages)
> >       return __vm_enough_memory(mm, pages, cap_sys_admin);
> >  }
> >
> > +/**
> > + * security_vm_execstack() - Check if starting a program with executable stack
> > + * is allowed
> > + *
> > + * Check whether starting a program with an executable stack is allowed.
> > + *
> > + * Return: Returns 0 if permission is granted.
> > + */
> > +int security_vm_execstack(void)
> > +{
> > +     return call_int_hook(vm_execstack);
> > +}
> > +
> >  /**
> >   * security_bprm_creds_for_exec() - Prepare the credentials for exec()
> >   * @bprm: binary program information