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

[Jann Horn]

On Fri, Aug 21, 2026 at 4:19 PM David Hildenbrand (Arm)
<david at kernel.org> wrote:
> On 8/20/26 20:44, Jann Horn wrote:
> > 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.
>
> Okay, would "opened_by_owner" or something like that be clearer? At least
> "introspection" is less intuitive for me.

Ack, I'll rename it to something like that for the next version.