[RFC PATCH 2/2] memfd: call security_inode_init_security_anon

["Thiébaud Weksteen" <tweek@google.com>]

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.

[1] https://crbug.com/1305267
[2] https://lore.kernel.org/lkml/20221215001205.51969-1-jeffxu@google.com/

Signed-off-by: Thiébaud Weksteen <tweek@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);
+	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) {
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);
 		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);
+		if (rc)
+			return rc;
 	} else {
 		isec->sclass = SECCLASS_ANON_INODE;
 		rc = security_transition_sid(
diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
index 5665aa5e7853..3ec85142771f 100644
--- a/security/selinux/include/classmap.h
+++ b/security/selinux/include/classmap.h
@@ -179,6 +179,8 @@ const struct security_class_mapping secclass_map[] = {
 	{ "anon_inode", { COMMON_FILE_PERMS, NULL } },
 	{ "io_uring", { "override_creds", "sqpoll", "cmd", "allowed", NULL } },
 	{ "user_namespace", { "create", NULL } },
+	{ "memfd_file",
+	  { COMMON_FILE_PERMS, "execute_no_trans", "entrypoint", NULL } },
 	/* last one */ { NULL, {} }
 };
 
-- 
2.50.1.703.g449372360f-goog