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

[Jann Horn]

On Thu, Aug 20, 2026 at 7:22 PM David Hildenbrand (Arm)
<david at kernel.org> wrote:
> On 8/18/26 21:51, 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".
> >
> > This causes a hole in SELinux EXECMEM enforcement, which tries to ensure
> > that a process cannot create executable anonymous pages.
> >
> > PROC_MEM_FORCE_PTRACE prevents that and ensures that such FOLL_FORCE
> > accesses are only possible when the LSM allows ptrace() attachment; but it
> > is unclear how quickly PROC_MEM_FORCE_PTRACE can be deployed in
> > environments running lots of third-party code, such as Android.
> >
> > So, introduce a new LSM hook that can forbid FOLL_FORCE specifically for
> > such "introspective" accesses.
> >
> > Signed-off-by: Jann Horn <jannh at google.com>
> > ---
> >  fs/proc/base.c                |  6 ++++++
> >  include/linux/lsm_hook_defs.h |  1 +
> >  include/linux/security.h      |  6 ++++++
> >  security/security.c           | 15 +++++++++++++++
> >  4 files changed, 28 insertions(+)
> >
> > diff --git a/fs/proc/base.c b/fs/proc/base.c
> > index bec6197329dc..3dfaef49bb70 100644
> > --- a/fs/proc/base.c
> > +++ b/fs/proc/base.c
> > @@ -851,6 +851,8 @@ static int __mem_open(struct inode *inode, struct file *file, unsigned int mode)
> >  /* private_data for proc_mem_operations */
> >  struct mem_private {
> >       struct mm_struct *mm;
> > +     /* Was the ptrace access check bypassed due to introspection? */
> > +     bool introspection;
> >  };
> >
> >  static int mem_open(struct inode *inode, struct file *file)
> > @@ -864,12 +866,14 @@ static int mem_open(struct inode *inode, struct file *file)
> >       priv->mm = proc_mem_open(inode, PTRACE_MODE_ATTACH);
> >       if (IS_ERR_OR_NULL(priv->mm))
> >               return priv->mm ? PTR_ERR(priv->mm) : -ESRCH;
> > +     priv->introspection = priv->mm == current->mm;
>
> Is the feat that the fd could be passed to someone else that would then not be
> detected as introspection?

Yes, exactly, that's the primary reason why I did it this way.