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

On Tue, Aug 25, 2026 at 3:19 PM Christian Brauner <[email protected]> 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".
> >
> > 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 <[email protected]>
> > ---
> >  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;
> >       file->private_data = no_free_ptr(priv);
> >       return 0;
> >  }
> >
> >  static bool proc_mem_foll_force(struct file *file, struct mm_struct *mm)
> >  {
> > +     struct mem_private *priv = file->private_data;
> >       struct task_struct *task;
> >       bool ptrace_active = false;
> >
> > @@ -886,6 +890,8 @@ static bool proc_mem_foll_force(struct file *file, struct mm_struct *mm)
> >               }
> >               return ptrace_active;
> >       default:
> > +             if (priv->introspection)
> > +                     return security_introspect_mem_foll_force(file->f_cred) == 0;
>
> Hm. Why not pass the reason to security_introspect_mem_foll_force() and
> call it unconditionally? Similarly it could also be called for the
> active ptracer case. Then you'd just need to pass a flag to the security
> hook and the LSM can decide based on that.

A process which is attached as a ptracer can modify memory with (for
example) PTRACE_POKETEXT and registers with (for example)
PTRACE_SETREGS. LSMs that want to prevent such debugging operations
are supposed to prevent ptrace attachment with the ptrace_access_check
hook.

I am just trying to plug the enforcement hole where ptrace-style
modification of process state is possible without going through
ptrace_access_check - which means just looking at these
"introspection" cases.

If I wanted to provide LSMs with a more granular ability to do some
PTRACE_MODE_ATTACH operations (like attaching via ptrace) while
blocking other operations (like PTRACE_POKETEXT or changing register
values), that would require more plumbing, so I'm not trying to do
that here.