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

[Casey Schaufler]

On 7/31/2018 10:34 AM, Micah Morton wrote:
> 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 <mailto:mortonm at chromium.org>>
> Acked-by: Kees Cook <keescook at chromium.org <mailto:keescook at chromium.org>>

What security module is going to provide a hook for this?

> ---
> 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
>
> --
>