[PATCH 1/2] fs: Provide function that allocates a secure anonymous inode

[Paul Moore]

On Wed, Jun 4, 2025 at 3:59 AM Mike Rapoport <rppt at kernel.org> wrote:
>
> (added Paul Moore for selinux bits)

Thanks Mike.

I'm adding the LSM and SELinux lists too since there are others that
will be interested as well.

> On Mon, Jun 02, 2025 at 12:17:54PM -0700, Ackerley Tng wrote:
> > The new function, alloc_anon_secure_inode(), returns an inode after
> > running checks in security_inode_init_security_anon().
> >
> > Also refactor secretmem's file creation process to use the new
> > function.
> >
> > Suggested-by: David Hildenbrand <david at redhat.com>
> > Signed-off-by: Ackerley Tng <ackerleytng at google.com>
> > ---
> >  fs/anon_inodes.c   | 22 ++++++++++++++++------
> >  include/linux/fs.h |  1 +
> >  mm/secretmem.c     |  9 +--------
> >  3 files changed, 18 insertions(+), 14 deletions(-)
> >
> > diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
> > index 583ac81669c2..4c3110378647 100644
> > --- a/fs/anon_inodes.c
> > +++ b/fs/anon_inodes.c
> > @@ -55,17 +55,20 @@ static struct file_system_type anon_inode_fs_type = {
> >       .kill_sb        = kill_anon_super,
> >  };
> >
> > -static struct inode *anon_inode_make_secure_inode(
> > -     const char *name,
> > -     const struct inode *context_inode)
> > +static struct inode *anon_inode_make_secure_inode(struct super_block *s,
> > +             const char *name, const struct inode *context_inode,
> > +             bool fs_internal)
> >  {
> >       struct inode *inode;
> >       int error;
> >
> > -     inode = alloc_anon_inode(anon_inode_mnt->mnt_sb);
> > +     inode = alloc_anon_inode(s);
> >       if (IS_ERR(inode))
> >               return inode;
> > -     inode->i_flags &= ~S_PRIVATE;
> > +
> > +     if (!fs_internal)
> > +             inode->i_flags &= ~S_PRIVATE;
> > +
> >       error = security_inode_init_security_anon(inode, &QSTR(name),
> >                                                 context_inode);
> >       if (error) {
> > @@ -75,6 +78,12 @@ static struct inode *anon_inode_make_secure_inode(
> >       return inode;
> >  }
> >
> > +struct inode *alloc_anon_secure_inode(struct super_block *s, const char *name)
> > +{
> > +     return anon_inode_make_secure_inode(s, name, NULL, true);
> > +}
> > +EXPORT_SYMBOL_GPL(alloc_anon_secure_inode);
> > +
> >  static struct file *__anon_inode_getfile(const char *name,
> >                                        const struct file_operations *fops,
> >                                        void *priv, int flags,
> > @@ -88,7 +97,8 @@ static struct file *__anon_inode_getfile(const char *name,
> >               return ERR_PTR(-ENOENT);
> >
> >       if (make_inode) {
> > -             inode = anon_inode_make_secure_inode(name, context_inode);
> > +             inode = anon_inode_make_secure_inode(anon_inode_mnt->mnt_sb,
> > +                                                  name, context_inode, false);
> >               if (IS_ERR(inode)) {
> >                       file = ERR_CAST(inode);
> >                       goto err;
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index 016b0fe1536e..0fded2e3c661 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -3550,6 +3550,7 @@ extern int simple_write_begin(struct file *file, struct address_space *mapping,
> >  extern const struct address_space_operations ram_aops;
> >  extern int always_delete_dentry(const struct dentry *);
> >  extern struct inode *alloc_anon_inode(struct super_block *);
> > +extern struct inode *alloc_anon_secure_inode(struct super_block *, const char *);
> >  extern int simple_nosetlease(struct file *, int, struct file_lease **, void **);
> >  extern const struct dentry_operations simple_dentry_operations;
> >
> > diff --git a/mm/secretmem.c b/mm/secretmem.c
> > index 1b0a214ee558..c0e459e58cb6 100644
> > --- a/mm/secretmem.c
> > +++ b/mm/secretmem.c
> > @@ -195,18 +195,11 @@ static struct file *secretmem_file_create(unsigned long flags)
> >       struct file *file;
> >       struct inode *inode;
> >       const char *anon_name = "[secretmem]";
> > -     int err;
> >
> > -     inode = alloc_anon_inode(secretmem_mnt->mnt_sb);
> > +     inode = alloc_anon_secure_inode(secretmem_mnt->mnt_sb, anon_name);
> >       if (IS_ERR(inode))
> >               return ERR_CAST(inode);
>
> I don't think we should not hide secretmem and guest_memfd inodes from
> selinux, so clearing S_PRIVATE for them is not needed and you can just drop
> fs_internal parameter in anon_inode_make_secure_inode()

It's especially odd since I don't see any comments or descriptions
about why this is being done.  The secretmem change is concerning as
this is user accessible and marking the inode with S_PRIVATE will
bypass a number of LSM/SELinux access controls, possibly resulting in
a security regression (one would need to dig a bit deeper to see what
is possible with secretmem and which LSM/SELinux code paths would be
affected).

I'm less familiar with guest_memfd, but generally speaking if
userspace can act on the inode/fd then we likely don't want the
S_PRIVATE flag stripped from the anon_inode.

Ackerley can you provide an explanation about why the change in
S_PRIVATE was necessary?

> > -     err = security_inode_init_security_anon(inode, &QSTR(anon_name), NULL);
> > -     if (err) {
> > -             file = ERR_PTR(err);
> > -             goto err_free_inode;
> > -     }
> > -
> >       file = alloc_file_pseudo(inode, secretmem_mnt, "secretmem",
> >                                O_RDWR, &secretmem_fops);
> >       if (IS_ERR(file))
> > --
> > 2.49.0.1204.g71687c7c1d-goog

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