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

[Randy Dunlap]

On 07/31/2018 02:47 PM, Micah Morton wrote:
> The ChromiumOS LSM used by ChromeOS will provide a hook for this, in
> order to enforce ChromeOS-specific policies regarding which UIDs/GIDs a
> process with CAP_SET{UID/GID} can transition to. The
> security_task_fix_setuid LSM hook is very helpful in enabling such a feature
> for ChromeOS that governs UID transitions, but unfortunately for us it looks
> like the equivalent hook that would allow us to govern GID transitions from an
> LSM never got added.
> 
> Resending with plain text mode enabled.
> 
> ---
> 
> 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@xxxxxxxxxxxx>
> Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> 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
> 
> --
> 
> On Tue, Jul 31, 2018 at 1:09 PM James Morris <jmorris@xxxxxxxxx> wrote:
>>
>> On Tue, 31 Jul 2018, Micah Morton wrote:
>>
>>> +static inline int security_task_fix_setgid(struct cred *new,
>>> +    const struct cred *old,
>>> +    int flags)
>>> +{
>>> + return 0;
>>> +}
>>> +
>>
>> This looks whitespace-damaged.  Please send patches as plain text.

Still is ...

-- 
~Randy