linux-kernel - [PATCH v2 09/28] sys:__sys_setuid(): handle fsid mappings

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Date:   Fri, 14 Feb 2020 19:35:35 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Stéphane Graber <stgraber@...ntu.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Aleksa Sarai <cyphar@...har.com>, Jann Horn <jannh@...gle.com>
Cc:     smbarber@...omium.org, Seth Forshee <seth.forshee@...onical.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Serge Hallyn <serge@...lyn.com>,
        James Morris <jmorris@...ei.org>,
        Kees Cook <keescook@...omium.org>,
        Jonathan Corbet <corbet@....net>,
        Phil Estes <estesp@...il.com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        containers@...ts.linux-foundation.org,
        linux-security-module@...r.kernel.org, linux-api@...r.kernel.org,
        Christian Brauner <christian.brauner@...ntu.com>
Subject: [PATCH v2 09/28] sys:__sys_setuid(): handle fsid mappings

Switch setuid() to lookup fsids in the fsid mappings. If no fsid mappings are
setup the behavior is unchanged, i.e. fsids are looked up in the id mappings.
The kfsid to cleanly handle userns visible filesystem is set as before.

We require that a user must have a valid fsid mapping for the target id. This
is consistent with how the setid calls work today without fsid mappings.

Signed-off-by: Christian Brauner <christian.brauner@...ntu.com>
---
/* v2 */
- Christian Brauner <christian.brauner@...ntu.com>:
  - set kfsid which is used when dealing with proc permission checking
---
 kernel/sys.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index 864fa78f25a7..a8eefd748327 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -574,11 +574,16 @@ long __sys_setuid(uid_t uid)
 	struct cred *new;
 	int retval;
 	kuid_t kuid;
+	kuid_t kfsuid;
 
 	kuid = make_kuid(ns, uid);
 	if (!uid_valid(kuid))
 		return -EINVAL;
 
+	kfsuid = make_kfsuid(ns, uid);
+	if (!uid_valid(kfsuid))
+		return -EINVAL;
+
 	new = prepare_creds();
 	if (!new)
 		return -ENOMEM;
@@ -596,7 +601,8 @@ long __sys_setuid(uid_t uid)
 		goto error;
 	}
 
-	new->fsuid = new->euid = kuid;
+	new->kfsuid = new->euid = kuid;
+	new->fsuid = kfsuid;
 
 	retval = security_task_fix_setuid(new, old, LSM_SETID_ID);
 	if (retval < 0)
-- 
2.25.0