[PATCH v3 14/25] security: Introduce file_post_open hook

Roberto Sassu
2023-09-04T13:38:55Z

From: Roberto Sassu <roberto.sassu@huawei.com>

In preparation to move IMA and EVM to the LSM infrastructure, introduce the
file_post_open hook. Also, export security_file_post_open() for NFS.

It is useful for IMA to calculate the digest of the file content, and to
decide based on that digest whether the file should be made accessible to
the requesting process.

LSMs should use this hook instead of file_open, if they need to make their
decision based on an opened file (for example by inspecting the file
content). The file is not open yet in the file_open hook. The new hook can
return an error and can cause the open to be aborted.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
---
fs/namei.c | 2 ++
fs/nfsd/vfs.c | 6 ++++++
include/linux/lsm_hook_defs.h | 1 +
include/linux/security.h | 6 ++++++
security/security.c | 17 +++++++++++++++++
5 files changed, 32 insertions(+)

diff --git a/fs/namei.c b/fs/namei.c
index 1f5ec71360de..7dc4626859f0 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3634,6 +3634,8 @@ static int do_open(struct nameidata *nd,
error = may_open(idmap, &nd->path, acc_mode, open_flag);
if (!error && !(file->f_mode & FMODE_OPENED))
error = vfs_open(&nd->path, file);
+ if (!error)
+ error = security_file_post_open(file, op->acc_mode);
if (!error)
error = ima_file_check(file, op->acc_mode);
if (!error && do_truncate)
diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 8a2321d19194..3450bb1c8a18 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -862,6 +862,12 @@ __nfsd_open(struct svc_rqst *rqstp, struct svc_fh *fhp, umode_t type,
goto out_nfserr;
}

+ host_err = security_file_post_open(file, may_flags);
+ if (host_err) {
+ fput(file);
+ goto out_nfserr;
+ }
+
host_err = ima_file_check(file, may_flags);
if (host_err) {
fput(file);
diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index 1153e7163b8b..60ed33f0c80d 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -188,6 +188,7 @@ LSM_HOOK(int, 0, file_send_sigiotask, struct task_struct *tsk,
struct fown_struct *fown, int sig)
LSM_HOOK(int, 0, file_receive, struct file *file)
LSM_HOOK(int, 0, file_open, struct file *file)
+LSM_HOOK(int, 0, file_post_open, struct file *file, int mask)
LSM_HOOK(int, 0, file_truncate, struct file *file)
LSM_HOOK(int, 0, task_alloc, struct task_struct *task,
unsigned long clone_flags)
diff --git a/include/linux/security.h b/include/linux/security.h
index 665bba3e0081..a0f16511c059 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -403,6 +403,7 @@ int security_file_send_sigiotask(struct task_struct *tsk,
struct fown_struct *fown, int sig);
int security_file_receive(struct file *file);
int security_file_open(struct file *file);
+int security_file_post_open(struct file *file, int mask);
int security_file_truncate(struct file *file);
int security_task_alloc(struct task_struct *task, unsigned long clone_flags);
void security_task_free(struct task_struct *task);
@@ -1044,6 +1045,11 @@ static inline int security_file_open(struct file *file)
return 0;
}

+static inline int security_file_post_open(struct file *file, int mask)
+{
+ return 0;
+}
+
static inline int security_file_truncate(struct file *file)
{
return 0;
diff --git a/security/security.c b/security/security.c
index 3947159ba5e9..3e0078b51e46 100644
--- a/security/security.c
+++ b/security/security.c
@@ -2856,6 +2856,23 @@ int security_file_open(struct file *file)
return fsnotify_perm(file, MAY_OPEN);
}

+/**
+ * security_file_post_open() - Recheck access to a file after it has been opened
+ * @file: the file
+ * @mask: access mask
+ *
+ * Recheck access with mask after the file has been opened. The hook is useful
+ * for LSMs that require the file content to be available in order to make
+ * decisions.
+ *
+ * Return: Returns 0 if permission is granted.
+ */
+int security_file_post_open(struct file *file, int mask)
+{
+ return call_int_hook(file_post_open, 0, file, mask);
+}
+EXPORT_SYMBOL_GPL(security_file_post_open);
+
/**
* security_file_truncate() - Check if truncating a file is allowed
* @file: file
--
2.34.1