[RFC PATCH 2/2] memfd: call security_inode_init_security_anon

[Stephen Smalley]

On Thu, Aug 7, 2025 at 3:57 AM Thiébaud Weksteen <tweek at google.com> wrote:
>
> Prior to this change, no security hooks were called at the creation of a
> memfd file. It means that, for SELinux as an example, it will receive
> the default type of the filesystem that backs the in-memory inode. In
> most cases, that would be tmpfs, but if MFD_HUGETLB is passed, it will
> be hugetlbfs. Both can be considered implementation details of memfd.
>
> It also means that it is not possible to differentiate between a file
> coming from memfd_create and a file coming from a standard tmpfs mount
> point.
>
> Additionally, no permission is validated at creation, which differs from
> the similar memfd_secret syscall.
>
> Call security_inode_init_security_anon during creation. This ensures
> that the file is setup similarly to other anonymous inodes. On SELinux,
> it means that the file will receive the security context of its task.
>
> The ability to limit fexecve on memfd has been of interest to avoid
> potential pitfalls where /proc/self/exe or similar would be executed
> [1][2]. Reuse the "execute_no_trans" and "entrypoint" access vectors,
> similarly to the file class. These access vectors may not make sense for
> the existing "anon_inode" class. Therefore, define and assign a new
> class "memfd_file" to support such access vectors.

To provide backward compatibility, I would anticipate that you will
need to define a new SELinux policy capability and make this change
conditional on it being enabled, see:
https://github.com/SELinuxProject/selinux-kernel/wiki/Getting-Started#adding-a-new-selinux-policy-capability
for instructions and links to examples.

Otherwise, see below.

>
> [1] https://crbug.com/1305267
> [2] https://lore.kernel.org/lkml/20221215001205.51969-1-jeffxu@google.com/
>
> Signed-off-by: Thiébaud Weksteen <tweek at google.com>
> ---
>  mm/memfd.c                          | 16 ++++++++++++++--
>  security/selinux/hooks.c            | 15 +++++++++++----
>  security/selinux/include/classmap.h |  2 ++
>  3 files changed, 27 insertions(+), 6 deletions(-)
>
> diff --git a/mm/memfd.c b/mm/memfd.c
> index bbe679895ef6..13bff0e91816 100644
> --- a/mm/memfd.c
> +++ b/mm/memfd.c
> @@ -433,6 +433,9 @@ static struct file *alloc_file(const char *name, unsigned int flags)
>  {
>         unsigned int *file_seals;
>         struct file *file;
> +       struct inode *inode;
> +       int err = 0;
> +       const char *anon_name = "[memfd]";
>
>         if (flags & MFD_HUGETLB) {
>                 file = hugetlb_file_setup(name, 0, VM_NORESERVE,
> @@ -444,12 +447,21 @@ static struct file *alloc_file(const char *name, unsigned int flags)
>         }
>         if (IS_ERR(file))
>                 return file;
> +
> +       inode = file_inode(file);
> +       err = security_inode_init_security_anon(inode,
> +                       LSM_ANON_INODE_MEMFD,
> +                       &QSTR(anon_name), NULL);

Since the anon_name already indicates that this is a memfd, so can't
you already distinguish these via name-based type_transition rules?
Why do we need the enum argument?

> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 8d36d5ebb6e5..49742930e706 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -2367,8 +2367,8 @@ static int selinux_bprm_creds_for_exec(struct linux_binprm *bprm)
>         ad.u.file = bprm->file;
>
>         if (new_tsec->sid == old_tsec->sid) {
> -               rc = avc_has_perm(old_tsec->sid, isec->sid,
> -                                 SECCLASS_FILE, FILE__EXECUTE_NO_TRANS, &ad);
> +               rc = avc_has_perm(old_tsec->sid, isec->sid, isec->sclass,
> +                                 FILE__EXECUTE_NO_TRANS, &ad);

Here and below I am a little concerned that we could end up reaching
this code on an inode with an isec->sclass that does not define the
execute_no_trans and entrypoint permissions. We should do something to
make that never happens, or check for it and always deny in that case.

>                 if (rc)
>                         return rc;
>         } else {
> @@ -2378,8 +2378,8 @@ static int selinux_bprm_creds_for_exec(struct linux_binprm *bprm)
>                 if (rc)
>                         return rc;
>
> -               rc = avc_has_perm(new_tsec->sid, isec->sid,
> -                                 SECCLASS_FILE, FILE__ENTRYPOINT, &ad);
> +               rc = avc_has_perm(new_tsec->sid, isec->sid, isec->sclass,
> +                                 FILE__ENTRYPOINT, &ad);
>                 if (rc)
>                         return rc;
>
> @@ -2997,6 +2997,13 @@ static int selinux_inode_init_security_anon(struct inode *inode,
>
>                 isec->sclass = context_isec->sclass;
>                 isec->sid = context_isec->sid;
> +       } else if (type == LSM_ANON_INODE_MEMFD) {
> +               isec->sclass = SECCLASS_MEMFD_FILE;
> +               rc = security_transition_sid(
> +                       sid, sid,
> +                       isec->sclass, name, &isec->sid);

Again, name-based type_transitions ought to be able to distinguish
memfd based on the name argument IIUC.