[PATCH v2 2/3] proc: query LSMs for introspective mem access (if PROC_MEM_FORCE_ALWAYS)

[Paul Moore]

On Fri, Aug 28, 2026 at 9:04 AM Jann Horn <jannh at google.com> wrote:
> On Fri, Aug 28, 2026 at 3:10 AM Paul Moore <paul at paul-moore.com> wrote:
> > On Aug 25, 2026 Jann Horn <jannh at google.com> 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>
> > > Acked-by: David Hildenbrand (Arm) <david at kernel.org>
> > > Acked-by: Lorenzo Stoakes (ARM) <ljs at kernel.org>
> > > ---
> > >  fs/proc/base.c                |  9 +++++++++
> > >  include/linux/lsm_hook_defs.h |  1 +
> > >  include/linux/security.h      |  6 ++++++
> > >  security/security.c           | 20 ++++++++++++++++++++
> > >  4 files changed, 36 insertions(+)
> > >
> > > diff --git a/fs/proc/base.c b/fs/proc/base.c
> > > index bec6197329dc..dc6fdcb47b79 100644
> > > --- a/fs/proc/base.c
> > > +++ b/fs/proc/base.c
> > > @@ -851,6 +851,11 @@ 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 on open bypassed because the opener used
> > > +      * the same MM (introspection)?
> > > +      */
> > > +     bool opened_by_owner;
> > >  };
> > >
> > >  static int mem_open(struct inode *inode, struct file *file)
> > > @@ -864,12 +869,14 @@ 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->opened_by_owner = 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 +893,8 @@ static bool proc_mem_foll_force(struct file *file, struct mm_struct *mm)
> > >               }
> > >               return ptrace_active;
> > >       default:
> > > +             if (priv->opened_by_owner)
> > > +                     return security_mem_foll_force_opened_by_owner(file->f_cred) == 0;
> > >               return true;
> > >       }
> > >  }
> >
> > First things first, we've got to shorten that hook name :)  What do you
> > think of security_proc_mem_foll_force()?
>
> If we move the "opened_by_owner" part into a flag then I guess that
> works... will do.
>
> > Beyond that, we really try to avoid making LSM hook calls conditional.  It
> > can limit what an LSM can enforce, it tends to be a bit more fragile, and
> > it adds some unnecessary work in the case where CONFIG_SECURITY is
> > disabled.  I would suggest passing 'opened_by_owner' flag as a second
> > parameter to the LSM hook and calling the hook unconditionally in the
> > default switch case as a replacement for the 'return true;' statement.  I
> > understand it may seem a bit odd, but we try to make the LSM interface as
> > generic as possible with respect to different models and this is one way
>
> I guess I can do that, but then the question becomes, what other modes
> of using the LSM hook that don't currently exist in the kernel should
> I be supporting with this? I can make this a parameter, but any LSM
> policy that actually uses the parameter in a different way would
> probably be buggy/inconsistent, unless other new LSM hooks are added.

Possibly.  People do all sorts of things with the LSM hooks, not all
of them relate to access control.

> If your intent is to make the hook work for any /proc/$pid/mem access,
> including when the caller is ptrace-attached, then I guess I have to
> move around the security hook call in proc_mem_foll_force() a little
> bit. I guess I'll do that in v3, though I really don't like trying to
> come up with a reasonable in-kernel API contract for a scenario that
> currently has zero users.

As I said, I don't really have a specific model/intent in mind other
than avoiding a conditional LSM hook call.

> > we do that.  It also ensures we don't have to process the 'opened_by_owner'
> > check in that case where the LSM is disabled.
>
> (I don't think that's an improvement - we have to do the comparison
> either way, but in the current version, we then use it to
> conditionally branch to a security hook, while the v3 patch will
> instead unconditionally call into the security hook.)

If we move the opened_by_owner check into the LSMs, in the
!CONFIG_SECURITY case we avoid the check entirely.  We try to do that
when we can to minimize impact of the LSM when it is disabled at build
time.

> > > +static inline int security_mem_foll_force_opened_by_owner(const struct cred *subject)
> > > +{
> > > +     return 0;
> > > +}
> >
> > With proc_mem_foll_force() currently returning true/1 in this case,
> > shouldn't the LSM hook return true/1 when disabled?
>
> No, I have proc_mem_foll_force() doing:
> "return security_mem_foll_force_opened_by_owner(file->f_cred) == 0;"

Ah, forgive me, my mind stopped reading once it hit the closing
parenthesis!  That's not a common pattern for calling LSM hooks so my
brain wasn't expecting that :)

-- 
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