userns: Check euid no fsuid when establishing an unprivileged uid mapping

ebiederm · ebiederm · commit 80dd00a23784 · 2014-12-09T17:08:32.000-06:00
setresuid allows the euid to be set to any of uid, euid, suid, and fsuid. Therefor it is safe to allow an unprivileged user to map their euid and use CAP_SETUID privileged with exactly that uid, as no new credentials can be obtained. I can not find a combination of existing system calls that allows setting uid, euid, suid, and fsuid from the fsuid making the previous use of fsuid for allowing unprivileged mappings a bug. This is part of a fix for CVE-2014-8989. Cc: stable@vger.kernel.org Reviewed-by: Andy Lutomirski <luto@amacapital.net> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
@@ -819,7 +819,7 @@ static bool new_idmap_permitted(const struct file *file,
 		u32 id = new_map->extent[0].lower_first;
 		if (cap_setid == CAP_SETUID) {
 			kuid_t uid = make_kuid(ns->parent, id);
-			if (uid_eq(uid, file->f_cred->fsuid))
+			if (uid_eq(uid, file->f_cred->euid))
 				return true;
 		}
 	}

Original file line number	Diff line number	Diff line change
`@@ -819,7 +819,7 @@ static bool new_idmap_permitted(const struct file *file,`
`819`	`819`	`u32 id = new_map->extent[0].lower_first;`
`820`	`820`	`if (cap_setid == CAP_SETUID) {`
`821`	`821`	`kuid_t uid = make_kuid(ns->parent, id);`
`822`		`- if (uid_eq(uid, file->f_cred->fsuid))`
	`822`	`+ if (uid_eq(uid, file->f_cred->euid))`
`823`	`823`	`return true;`
`824`	`824`	`}`
`825`	`825`	`}`