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Message-ID: <50f294e3-7d16-cc7b-e552-9001a65747e7@schaufler-ca.com>
Date: Tue, 31 Jul 2018 11:02:00 -0700
From: Casey Schaufler <casey@...aufler-ca.com>
To: Micah Morton <mortonm@...omium.org>,
linux-security-module@...r.kernel.org, serge@...lyn.com,
Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] security: Add LSM fixup hooks to set*gid syscalls.
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@...omium.org <mailto:mortonm@...omium.org>>
> Acked-by: Kees Cook <keescook@...omium.org <mailto:keescook@...omium.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
> *@...gs 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
>
> --
>
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