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Message-ID: <6afc91a9f5caef96b2ca335b6d143670@paul-moore.com>
Date: Wed, 03 Sep 2025 17:26:21 -0400
From: Paul Moore <paul@...l-moore.com>
To: "Thiébaud Weksteen" <tweek@...gle.com>, James Morris <jmorris@...ei.org>, Stephen Smalley <stephen.smalley.work@...il.com>, Hugh Dickins <hughd@...gle.com>, Jeff Vander Stoep <jeffv@...gle.com>, Nick Kralevich <nnk@...gle.com>, Jeff Xu <jeffxu@...gle.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: "Thiébaud Weksteen" <tweek@...gle.com>, linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org, selinux@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] memfd,selinux: call security_inode_init_security_anon
On Aug 25, 2025 "=?UTF-8?q?Thi=C3=A9baud=20Weksteen?=" <tweek@...gle.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.
>
> Guard these changes behind a new policy capability named "memfd_class".
>
> [1] https://crbug.com/1305267
> [2] https://lore.kernel.org/lkml/20221215001205.51969-1-jeffxu@google.com/
>
> Signed-off-by: Thiébaud Weksteen <tweek@...gle.com>
> Acked-by: Stephen Smalley <stephen.smalley.work@...il.com>
> Tested-by: Stephen Smalley <stephen.smalley.work@...il.com>
> ---
> Changes since RFC:
> - Remove enum argument, simply compare the anon inode name
> - Introduce a policy capability for compatility
> - Add validation of class in selinux_bprm_creds_for_exec
>
> include/linux/memfd.h | 2 ++
> mm/memfd.c | 14 +++++++++--
> security/selinux/hooks.c | 27 ++++++++++++++++++----
> security/selinux/include/classmap.h | 2 ++
> security/selinux/include/policycap.h | 1 +
> security/selinux/include/policycap_names.h | 1 +
> security/selinux/include/security.h | 5 ++++
> 7 files changed, 46 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/memfd.h b/include/linux/memfd.h
> index 6f606d9573c3..cc74de3dbcfe 100644
> --- a/include/linux/memfd.h
> +++ b/include/linux/memfd.h
> @@ -4,6 +4,8 @@
>
> #include <linux/file.h>
>
> +#define MEMFD_ANON_NAME "[memfd]"
> +
> #ifdef CONFIG_MEMFD_CREATE
> extern long memfd_fcntl(struct file *file, unsigned int cmd, unsigned int arg);
> struct folio *memfd_alloc_folio(struct file *memfd, pgoff_t idx);
> diff --git a/mm/memfd.c b/mm/memfd.c
> index bbe679895ef6..63b439eb402a 100644
> --- a/mm/memfd.c
> +++ b/mm/memfd.c
> @@ -433,6 +433,8 @@ static struct file *alloc_file(const char *name, unsigned int flags)
> {
> unsigned int *file_seals;
> struct file *file;
> + struct inode *inode;
> + int err = 0;
>
> if (flags & MFD_HUGETLB) {
> file = hugetlb_file_setup(name, 0, VM_NORESERVE,
> @@ -444,12 +446,20 @@ 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,
> + &QSTR(MEMFD_ANON_NAME), NULL);
> + if (err) {
> + fput(file);
> + file = ERR_PTR(err);
> + return file;
> + }
> +
> file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE;
> file->f_flags |= O_LARGEFILE;
>
> if (flags & MFD_NOEXEC_SEAL) {
> - struct inode *inode = file_inode(file);
> -
> inode->i_mode &= ~0111;
> file_seals = memfd_file_seals_ptr(file);
> if (file_seals) {
Hugh, Baolin, and shmem/mm folks, are you okay with the changes above? If
so it would be nice to get an ACK from one of you.
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index c95a5874bf7d..429b2269b35a 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -93,6 +93,7 @@
> #include <linux/fanotify.h>
> #include <linux/io_uring/cmd.h>
> #include <uapi/linux/lsm.h>
> +#include <linux/memfd.h>
>
> #include "avc.h"
> #include "objsec.h"
> @@ -2366,9 +2367,12 @@ static int selinux_bprm_creds_for_exec(struct linux_binprm *bprm)
> ad.type = LSM_AUDIT_DATA_FILE;
> ad.u.file = bprm->file;
>
> + if (isec->sclass != SECCLASS_FILE && isec->sclass != SECCLASS_MEMFD_FILE)
> + return -EPERM;
In the interest of failing fast, this should probably be moved up in the
function to just after where @isec is set. There are also a number of
checks that happen prior to this placement, but after the isec assignment.
While I don't think any of those checks should be an issue, I'd rather
not to have to worry about those and just fail the non-FILE/MEMFD_FILE
case as soon as we can in selinux_bprm_creds_for_exec().
> 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);
> if (rc)
> return rc;
> } else {
> @@ -2378,8 +2382,8 @@ 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;
>
> @@ -2974,10 +2978,18 @@ static int selinux_inode_init_security_anon(struct inode *inode,
> struct common_audit_data ad;
> struct inode_security_struct *isec;
> int rc;
> + bool is_memfd = false;
>
> if (unlikely(!selinux_initialized()))
> return 0;
>
> + if (name != NULL && name->name != NULL &&
> + !strcmp(name->name, MEMFD_ANON_NAME)) {
> + if (!selinux_policycap_memfd_class())
> + return 0;
> + is_memfd = true;
> + }
> +
> isec = selinux_inode(inode);
>
> /*
> @@ -2996,6 +3008,13 @@ static int selinux_inode_init_security_anon(struct inode *inode,
>
> isec->sclass = context_isec->sclass;
> isec->sid = context_isec->sid;
> + } else if (is_memfd) {
> + isec->sclass = SECCLASS_MEMFD_FILE;
> + rc = security_transition_sid(
> + sid, sid,
> + isec->sclass, name, &isec->sid);
> + if (rc)
> + return rc;
> } else {
> isec->sclass = SECCLASS_ANON_INODE;
> rc = security_transition_sid(
We're duplicating the security_transition_sid() call which seems less
than ideal, how about something like this?
if (context_inode) {
/* ... existing stuff ... */
} else {
if (is_memfd)
isec->sclass = SECCLASS_MEMFD_FILE;
else
isec->sclass = SECCLASS_ANON_INODE;
rc = security_transition_sid(...);
if (rc)
return rc;
}
--
paul-moore.com
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