[PATCH v2] security: Add LSM fixup hooks to set*gid syscalls.

[Micah Morton]

The set*uid system calls all call an LSM fixup hook called
security_task_fix_setuid, which allows for altering the behavior of those
calls by a security module. Comments explaining the LSM_SETID_* constants
in /include/linux/security.h imply that the constants are to be used for
both the set*uid and set*gid syscalls, but the set*gid syscalls do not
have the relevant hooks, meaning a security module can only alter syscalls
that change user identity attributes but not ones that change group
identity attributes. This patch adds the necessary LSM hook, called
security_task_fix_setgid, and calls the hook from the appropriate places
in the set*gid syscalls.Tested by putting a print statement in the hook and
seeing it triggered from the various set*gid syscalls.

Signed-off-by: Micah Morton <mortonm at chromium.org>
Acked-by: Kees Cook <keescook at chromium.org>
---
NOTE: the security_task_fix_setgid line in sys_setfsgid is over 80
characters, but I figured I'd just follow how it was done in sys_setfsuid
rather than trying to wrap the line, since the functions are nearly
identical.

diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 8f1131c8dd54..a2166c812a97 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -599,6 +599,15 @@
  * @old is the set of credentials that are being replaces
  * @flags contains one of the LSM_SETID_* values.
  * Return 0 on success.
+ * @task_fix_setgid:
+ *     Update the module's state after setting one or more of the group
+ *     identity attributes of the current process.  The @flags parameter
+ *     indicates which of the set*gid system calls invoked this hook.
+ *     @new is the set of credentials that will be installed.
Modifications
+ *     should be made to this rather than to @current->cred.
+ *     @old is the set of credentials that are being replaced
+ *     @flags contains one of the LSM_SETID_* values.
+ *     Return 0 on success.
  * @task_setpgid:
  * Check permission before setting the process group identifier of the
  * process @p to @pgid.
@@ -1587,6 +1596,8 @@ union security_list_options {
       enum kernel_read_file_id id);
  int (*task_fix_setuid)(struct cred *new, const struct cred *old,
  int flags);
+ int (*task_fix_setgid)(struct cred *new, const struct cred *old,
+ int flags);
  int (*task_setpgid)(struct task_struct *p, pid_t pgid);
  int (*task_getpgid)(struct task_struct *p);
  int (*task_getsid)(struct task_struct *p);
@@ -1876,6 +1887,7 @@ struct security_hook_heads {
  struct hlist_head kernel_post_read_file;
  struct hlist_head kernel_module_request;
  struct hlist_head task_fix_setuid;
+ struct hlist_head task_fix_setgid;
  struct hlist_head task_setpgid;
  struct hlist_head task_getpgid;
  struct hlist_head task_getsid;
diff --git a/include/linux/security.h b/include/linux/security.h
index 63030c85ee19..a82d97cf13ab 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -325,6 +325,8 @@ int security_kernel_post_read_file(struct file *file,
char *buf, loff_t size,
     enum kernel_read_file_id id);
 int security_task_fix_setuid(struct cred *new, const struct cred *old,
       int flags);
+int security_task_fix_setgid(struct cred *new, const struct cred *old,
+      int flags);
 int security_task_setpgid(struct task_struct *p, pid_t pgid);
 int security_task_getpgid(struct task_struct *p);
 int security_task_getsid(struct task_struct *p);
@@ -929,6 +931,13 @@ static inline int security_task_fix_setuid(struct cred
*new,
  return cap_task_fix_setuid(new, old, flags);
 }

+static inline int security_task_fix_setgid(struct cred *new,
+    const struct cred *old,
+    int flags)
+{
+ return 0;
+}
+
 static inline int security_task_setpgid(struct task_struct *p, pid_t pgid)
 {
  return 0;
diff --git a/kernel/sys.c b/kernel/sys.c
index 38509dc1f77b..f6ef922c6815 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -392,6 +392,10 @@ long __sys_setregid(gid_t rgid, gid_t egid)
  new->sgid = new->egid;
  new->fsgid = new->egid;

+ retval = security_task_fix_setgid(new, old, LSM_SETID_RE);
+ if (retval < 0)
+ goto error;
+
  return commit_creds(new);

 error:
@@ -434,6 +438,10 @@ long __sys_setgid(gid_t gid)
  else
  goto error;

+ retval = security_task_fix_setgid(new, old, LSM_SETID_ID);
+ if (retval < 0)
+ goto error;
+
  return commit_creds(new);

 error:
@@ -755,6 +763,10 @@ long __sys_setresgid(gid_t rgid, gid_t egid, gid_t
sgid)
  new->sgid = ksgid;
  new->fsgid = new->egid;

+ retval = security_task_fix_setgid(new, old, LSM_SETID_RES);
+ if (retval < 0)
+ goto error;
+
  return commit_creds(new);

 error:
@@ -861,7 +873,8 @@ long __sys_setfsgid(gid_t gid)
      ns_capable(old->user_ns, CAP_SETGID)) {
  if (!gid_eq(kgid, old->fsgid)) {
  new->fsgid = kgid;
- goto change_okay;
+ if (security_task_fix_setgid(new, old, LSM_SETID_FS) == 0)
+ goto change_okay;
  }
  }

diff --git a/security/security.c b/security/security.c
index 68f46d849abe..587786fc0aaa 100644
--- a/security/security.c
+++ b/security/security.c
@@ -1062,6 +1062,12 @@ int security_task_fix_setuid(struct cred *new, const
struct cred *old,
  return call_int_hook(task_fix_setuid, 0, new, old, flags);
 }

+int security_task_fix_setgid(struct cred *new, const struct cred *old,
+      int flags)
+{
+ return call_int_hook(task_fix_setgid, 0, new, old, flags);
+}
+
 int security_task_setpgid(struct task_struct *p, pid_t pgid)
 {
  return call_int_hook(task_setpgid, 0, p, pgid);

--
4.18-rc2

--