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Date:   Fri, 14 Feb 2020 19:35:40 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     St├ęphane 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 14/28] sys:__sys_setresgid(): handle fsid mappings

Switch setresgid() 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.

During setresgid() the kfsgid is set to the kegid corresponding the egid that is
requested by userspace. If the requested egid is -1 the kfsgid is reset to the
current kegid. For the latter case this means we need to lookup the
corresponding userspace egid corresponding to the current kegid in the id
mappings and translate this egid into the corresponding kfsgid in the fsid
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 | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index 54e072145146..78592deee2d8 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -756,7 +756,7 @@ long __sys_setresgid(gid_t rgid, gid_t egid, gid_t sgid)
 	const struct cred *old;
 	struct cred *new;
 	int retval;
-	kgid_t krgid, kegid, ksgid;
+	kgid_t krgid, kegid, ksgid, kfsgid;
 
 	krgid = make_kgid(ns, rgid);
 	kegid = make_kgid(ns, egid);
@@ -789,11 +789,21 @@ long __sys_setresgid(gid_t rgid, gid_t egid, gid_t sgid)
 
 	if (rgid != (gid_t) -1)
 		new->gid = krgid;
-	if (egid != (gid_t) -1)
+	if (egid != (gid_t) -1) {
 		new->egid = kegid;
+		kfsgid = make_kfsgid(ns, egid);
+	} else {
+		kfsgid = kgid_to_kfsgid(new->user_ns, new->egid);
+	}
+	if (!gid_valid(kfsgid)) {
+		retval = -EINVAL;
+		goto error;
+	}
+
 	if (sgid != (gid_t) -1)
 		new->sgid = ksgid;
-	new->fsgid = new->egid;
+	new->kfsgid = new->egid;
+	new->fsgid = kfsgid;
 
 	return commit_creds(new);
 
-- 
2.25.0

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