[PATCH] procfs: add smack subdir to attrs

From: Casey Schaufler
Date: Thu Jun 01 2017 - 15:49:18 EST

Subject: [PATCH] procfs: add smack subdir to attrs

Back in 2007 I made what turned out to be a rather serious
mistake in the implementation of the Smack security module.
The SELinux module used an interface in /proc to manipulate
the security context on processes. Rather than use a similar
interface, I used the same interface. The AppArmor team did
likewise. Now /proc/.../attr/current will tell you the
security "context" of the process, but it will be different
depending on the security module you're using.

This patch provides a subdirectory in /proc/.../attr for
Smack. Smack user space can use the "current" file in
this subdirectory and never have to worry about getting
SELinux attributes by mistake. Programs that use the
old interface will continue to work (or fail, as the case
may be) as before.

This patch does not include subdirectories for SELinux
or AppArmor. I do have a patch that provides those, and
will happily make it available should anyone see value
in it.

The original implementation is by Kees Cook.

Signed-off-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
---
Documentation/security/LSM.txt | 19 +++++++++----
fs/proc/base.c | 63 ++++++++++++++++++++++++++++++++++++------
fs/proc/internal.h | 1 +
include/linux/security.h | 15 ++++++----
security/security.c | 30 +++++++++++++++++---
5 files changed, 104 insertions(+), 24 deletions(-)

diff --git a/Documentation/security/LSM.txt b/Documentation/security/LSM.txt
index c2683f28ed36..57eafbc1d144 100644
--- a/Documentation/security/LSM.txt
+++ b/Documentation/security/LSM.txt
@@ -16,11 +16,10 @@ MAC extensions, other extensions can be built using the LSM to provide
specific changes to system operation when these tweaks are not available
in the core functionality of Linux itself.

-Without a specific LSM built into the kernel, the default LSM will be the
-Linux capabilities system. Most LSMs choose to extend the capabilities
-system, building their checks on top of the defined capability hooks.
-For more details on capabilities, see capabilities(7) in the Linux
-man-pages project.
+The Linux capabilities modules will always be included. For more details
+on capabilities, see capabilities(7) in the Linux man-pages project.
+This may be followed by any number of "minor" modules and at most one
+"major" module.

A list of the active security modules can be found by reading
/sys/kernel/security/lsm. This is a comma separated list, and
@@ -29,6 +28,14 @@ order in which checks are made. The capability module will always
be first, followed by any "minor" modules (e.g. Yama) and then
the one "major" module (e.g. SELinux) if there is one configured.

+Process attributes associated with "major" security modules should
+be accessed and maintained using the special files in /proc/.../attr.
+A security module may maintain a module specific subdirectory there,
+named after the module. /proc/.../attr/smack is provided by the Smack
+security module and contains all its special files. The files directly
+in /proc/.../attr remain as legacy interfaces for modules that provide
+subdirectories.
+
Based on https://lkml.org/lkml/2007/10/26/215,
a new LSM is accepted into the kernel when its intent (a description of
what it tries to protect against and in what cases one would expect to
@@ -38,4 +45,4 @@ that end users and distros can make a more informed decision about which
LSMs suit their requirements.

For extensive documentation on the available LSM hook interfaces, please
-see include/linux/security.h.
+see include/linux/lsm_hooks.h.
diff --git a/fs/proc/base.c b/fs/proc/base.c
index c87b6b9a8a76..2decd2fb5ef8 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -139,9 +139,13 @@ struct pid_entry {
#define REG(NAME, MODE, fops) \
NOD(NAME, (S_IFREG|(MODE)), NULL, &fops, {})
#define ONE(NAME, MODE, show) \
- NOD(NAME, (S_IFREG|(MODE)), \
+ NOD(NAME, (S_IFREG|(MODE)), \
NULL, &proc_single_file_operations, \
{ .proc_show = show } )
+#define ATTR(LSM, NAME, MODE) \
+ NOD(NAME, (S_IFREG|(MODE)), \
+ NULL, &proc_pid_attr_operations, \
+ { .lsm = LSM })

/*
* Count the number of hardlinks for the pid_entry table, excluding the .
@@ -2455,7 +2459,7 @@ static ssize_t proc_pid_attr_read(struct file * file, char __user * buf,
if (!task)
return -ESRCH;

- length = security_getprocattr(task,
+ length = security_getprocattr(task, PROC_I(inode)->op.lsm,
(char*)file->f_path.dentry->d_name.name,
&p);
put_task_struct(task);
@@ -2501,7 +2505,8 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
if (length < 0)
goto out_free;

- length = security_setprocattr(file->f_path.dentry->d_name.name,
+ length = security_setprocattr(PROC_I(inode)->op.lsm,
+ file->f_path.dentry->d_name.name,
page, count);
mutex_unlock(&current->signal->cred_guard_mutex);
out_free:
@@ -2518,13 +2523,53 @@ static const struct file_operations proc_pid_attr_operations = {
.llseek = generic_file_llseek,
};

+#define LSM_DIR_OPS(LSM) \
+static int proc_##LSM##_attr_dir_iterate(struct file *filp, \
+ struct dir_context *ctx) \
+{ \
+ return proc_pident_readdir(filp, ctx, \
+ LSM##_attr_dir_stuff, \
+ ARRAY_SIZE(LSM##_attr_dir_stuff)); \
+} \
+\
+static const struct file_operations proc_##LSM##_attr_dir_ops = { \
+ .read = generic_read_dir, \
+ .iterate = proc_##LSM##_attr_dir_iterate, \
+ .llseek = default_llseek, \
+}; \
+\
+static struct dentry *proc_##LSM##_attr_dir_lookup(struct inode *dir, \
+ struct dentry *dentry, unsigned int flags) \
+{ \
+ return proc_pident_lookup(dir, dentry, \
+ LSM##_attr_dir_stuff, \
+ ARRAY_SIZE(LSM##_attr_dir_stuff)); \
+} \
+\
+static const struct inode_operations proc_##LSM##_attr_dir_inode_ops = { \
+ .lookup = proc_##LSM##_attr_dir_lookup, \
+ .getattr = pid_getattr, \
+ .setattr = proc_setattr, \
+}
+
+#ifdef CONFIG_SECURITY_SMACK
+static const struct pid_entry smack_attr_dir_stuff[] = {
+ ATTR("smack", "current", 0666),
+};
+LSM_DIR_OPS(smack);
+#endif
+
static const struct pid_entry attr_dir_stuff[] = {
- REG("current", S_IRUGO|S_IWUGO, proc_pid_attr_operations),
- REG("prev", S_IRUGO, proc_pid_attr_operations),
- REG("exec", S_IRUGO|S_IWUGO, proc_pid_attr_operations),
- REG("fscreate", S_IRUGO|S_IWUGO, proc_pid_attr_operations),
- REG("keycreate", S_IRUGO|S_IWUGO, proc_pid_attr_operations),
- REG("sockcreate", S_IRUGO|S_IWUGO, proc_pid_attr_operations),
+ ATTR(NULL, "current", 0666),
+ ATTR(NULL, "prev", 0444),
+ ATTR(NULL, "exec", 0666),
+ ATTR(NULL, "fscreate", 0666),
+ ATTR(NULL, "keycreate", 0666),
+ ATTR(NULL, "sockcreate", 0666),
+#ifdef CONFIG_SECURITY_SMACK
+ DIR("smack", 0555,
+ proc_smack_attr_dir_inode_ops, proc_smack_attr_dir_ops),
+#endif
};

static int proc_attr_dir_readdir(struct file *file, struct dir_context *ctx)
diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index c5ae09b6c726..8b7d101c91ec 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -58,6 +58,7 @@ union proc_op {
int (*proc_show)(struct seq_file *m,
struct pid_namespace *ns, struct pid *pid,
struct task_struct *task);
+ const char *lsm;
};

struct proc_inode {
diff --git a/include/linux/security.h b/include/linux/security.h
index af675b576645..8ee9dbdfa1cb 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -365,8 +365,10 @@ int security_sem_semctl(struct sem_array *sma, int cmd);
int security_sem_semop(struct sem_array *sma, struct sembuf *sops,
unsigned nsops, int alter);
void security_d_instantiate(struct dentry *dentry, struct inode *inode);
-int security_getprocattr(struct task_struct *p, char *name, char **value);
-int security_setprocattr(const char *name, void *value, size_t size);
+int security_getprocattr(struct task_struct *p, const char *lsm, char *name,
+ char **value);
+int security_setprocattr(const char *lsm, const char *name, void *value,
+ size_t size);
int security_netlink_send(struct sock *sk, struct sk_buff *skb);
int security_ismaclabel(const char *name);
int security_secid_to_secctx(u32 secid, char **secdata, u32 *seclen);
@@ -1111,15 +1113,18 @@ static inline int security_sem_semop(struct sem_array *sma,
return 0;
}

-static inline void security_d_instantiate(struct dentry *dentry, struct inode *inode)
+static inline void security_d_instantiate(struct dentry *dentry,
+ struct inode *inode)
{ }

-static inline int security_getprocattr(struct task_struct *p, char *name, char **value)
+static inline int security_getprocattr(struct task_struct *p, const char *lsm,
+ char *name, char **value)
{
return -EINVAL;
}

-static inline int security_setprocattr(char *name, void *value, size_t size)
+static inline int security_setprocattr(const char *lsm, char *name,
+ void *value, size_t size)
{
return -EINVAL;
}
diff --git a/security/security.c b/security/security.c
index 549bddcc2116..e28bb987e26d 100644
--- a/security/security.c
+++ b/security/security.c
@@ -1216,14 +1216,36 @@ void security_d_instantiate(struct dentry *dentry, struct inode *inode)
}
EXPORT_SYMBOL(security_d_instantiate);

-int security_getprocattr(struct task_struct *p, char *name, char **value)
+int security_getprocattr(struct task_struct *p, const char *lsm, char *name,
+ char **value)
{
- return call_int_hook(getprocattr, -EINVAL, p, name, value);
+ struct security_hook_list *hp;
+ int rc;
+
+ list_for_each_entry(hp, &security_hook_heads.getprocattr, list) {
+ if (lsm != NULL && strcmp(lsm, hp->lsm))
+ continue;
+ rc = hp->hook.getprocattr(p, name, value);
+ if (rc != -ENOENT)
+ return rc;
+ }
+ return -EINVAL;
}

-int security_setprocattr(const char *name, void *value, size_t size)
+int security_setprocattr(const char *lsm, const char *name, void *value,
+ size_t size)
{
- return call_int_hook(setprocattr, -EINVAL, name, value, size);
+ struct security_hook_list *hp;
+ int rc;
+
+ list_for_each_entry(hp, &security_hook_heads.setprocattr, list) {
+ if (lsm != NULL && strcmp(lsm, hp->lsm))
+ continue;
+ rc = hp->hook.setprocattr(name, value, size);
+ if (rc != -ENOENT)
+ return rc;
+ }
+ return -EINVAL;
}

int security_netlink_send(struct sock *sk, struct sk_buff *skb)
--
2.11.0