[CFT][PATCH 4/7] userns: Check euid no fsuid when establishing an unprivileged uid mapping

[ebiederm@xmission.com (Eric W. Biederman)]

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
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
 kernel/user_namespace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 8e7c87162171..da1eeb927b21 100644
--- 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;
 		}
 	}
-- 
1.9.1