linux-kernel - Re: [PATCH v3] mm/memfd: security hook for memfd

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Date:   Fri, 2 Dec 2022 15:23:37 -0800
From:   Jeff Xu <jeffxu@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     jeffxu@...omium.org, skhan@...uxfoundation.org,
        akpm@...ux-foundation.org, dmitry.torokhov@...il.com,
        dverkamp@...omium.org, hughd@...gle.com, jorgelo@...omium.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-mm@...ck.org, mnissler@...omium.org, jannh@...gle.com,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v3] mm/memfd: security hook for memfd_create

On Fri, Dec 2, 2022 at 2:58 PM Kees Cook <keescook@...omium.org> wrote:
>
> On Fri, Dec 02, 2022 at 01:34:03AM +0000, jeffxu@...omium.org wrote:
> > From: Jeff Xu <jeffxu@...omium.org>
> >
> > The new security_memfd_create allows lsm to check flags of
> > memfd_create.
> >
> > The security by default system (such as chromeos) can use this
> > to implement system wide lsm to allow only non-executable memfd
> > being created.
> >
> > Signed-off-by: Jeff Xu <jeffxu@...omium.org>
> > ---
> >  include/linux/lsm_hook_defs.h | 1 +
> >  include/linux/lsm_hooks.h     | 4 ++++
> >  include/linux/security.h      | 6 ++++++
> >  mm/memfd.c                    | 5 +++++
> >  4 files changed, 16 insertions(+)
> >
> > diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> > index ec119da1d89b..fd40840927c8 100644
> > --- a/include/linux/lsm_hook_defs.h
> > +++ b/include/linux/lsm_hook_defs.h
> > @@ -164,6 +164,7 @@ LSM_HOOK(int, 0, file_alloc_security, struct file *file)
> >  LSM_HOOK(void, LSM_RET_VOID, file_free_security, struct file *file)
> >  LSM_HOOK(int, 0, file_ioctl, struct file *file, unsigned int cmd,
> >        unsigned long arg)
> > +LSM_HOOK(int, 0, memfd_create, char *name, unsigned int flags)
> >  LSM_HOOK(int, 0, mmap_addr, unsigned long addr)
> >  LSM_HOOK(int, 0, mmap_file, struct file *file, unsigned long reqprot,
> >        unsigned long prot, unsigned long flags)
> > diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> > index 4ec80b96c22e..5a18a6552278 100644
> > --- a/include/linux/lsm_hooks.h
> > +++ b/include/linux/lsm_hooks.h
> > @@ -543,6 +543,10 @@
> >   *   simple integer value.  When @arg represents a user space pointer, it
> >   *   should never be used by the security module.
> >   *   Return 0 if permission is granted.
> > + * @memfd_create:
> > + *   @name is the name of memfd file.
> > + *   @flags is the flags used in memfd_create.
> > + *   Return 0 if permission is granted.
> >   * @mmap_addr :
> >   *   Check permissions for a mmap operation at @addr.
> >   *   @addr contains virtual address that will be used for the operation.
> > diff --git a/include/linux/security.h b/include/linux/security.h
> > index ca1b7109c0db..5b87a780822a 100644
> > --- a/include/linux/security.h
> > +++ b/include/linux/security.h
> > @@ -384,6 +384,7 @@ int security_file_permission(struct file *file, int mask);
> >  int security_file_alloc(struct file *file);
> >  void security_file_free(struct file *file);
> >  int security_file_ioctl(struct file *file, unsigned int cmd, unsigned long arg);
> > +int security_memfd_create(char *name, unsigned int flags);
> >  int security_mmap_file(struct file *file, unsigned long prot,
> >                       unsigned long flags);
> >  int security_mmap_addr(unsigned long addr);
> > @@ -963,6 +964,11 @@ static inline int security_file_ioctl(struct file *file, unsigned int cmd,
> >       return 0;
> >  }
> >
> > +static inline int security_memfd_create(char *name, unsigned int flags)
> > +{
> > +     return 0;
> > +}
>
> I think this is missing the security/security.c changes for the
> non-inline version?
>
Yes. I will add that in V4.

> -Kees
>
> > +
> >  static inline int security_mmap_file(struct file *file, unsigned long prot,
> >                                    unsigned long flags)
> >  {
> > diff --git a/mm/memfd.c b/mm/memfd.c
> > index 69e897dea6d5..96dcfbfed09e 100644
> > --- a/mm/memfd.c
> > +++ b/mm/memfd.c
> > @@ -346,6 +346,11 @@ SYSCALL_DEFINE2(memfd_create,
> >               goto err_name;
> >       }
> >
> > +     /* security hook for memfd_create */
> > +     error = security_memfd_create(name, flags);
> > +     if (error)
> > +             return error;
> > +
> >       if (flags & MFD_HUGETLB) {
> >               file = hugetlb_file_setup(name, 0, VM_NORESERVE,
> >                                       HUGETLB_ANONHUGE_INODE,
> > --
> > 2.39.0.rc0.267.gcb52ba06e7-goog
> >
>
> --
> Kees Cook