linux-kernel - [REVIEW][PATCH 4/6] fs: Allow superblock owner to access do_remount

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Date:   Wed, 23 May 2018 18:25:36 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Linux Containers <containers@...ts.linux-foundation.org>
Cc:     linux-fsdevel@...r.kernel.org,
        Seth Forshee <seth.forshee@...onical.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Christian Brauner <christian@...uner.io>,
        linux-kernel@...r.kernel.org,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: [REVIEW][PATCH 4/6] fs: Allow superblock owner to access do_remount_sb()

Superblock level remounts are currently restricted to global
CAP_SYS_ADMIN, as is the path for changing the root mount to
read only on umount. Loosen both of these permission checks to
also allow CAP_SYS_ADMIN in any namespace which is privileged
towards the userns which originally mounted the filesystem.

Signed-off-by: Seth Forshee <seth.forshee@...onical.com>
Acked-by: "Eric W. Biederman" <ebiederm@...ssion.com>
Acked-by: Serge Hallyn <serge.hallyn@...onical.com>
Signed-off-by: Eric W. Biederman <ebiederm@...ssion.com>
---
 fs/namespace.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 5f75969adff1..8ddd14806799 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1590,7 +1590,7 @@ static int do_umount(struct mount *mnt, int flags)
 		 * Special case for "unmounting" root ...
 		 * we just try to remount it readonly.
 		 */
-		if (!capable(CAP_SYS_ADMIN))
+		if (!ns_capable(sb->s_user_ns, CAP_SYS_ADMIN))
 			return -EPERM;
 		down_write(&sb->s_umount);
 		if (!sb_rdonly(sb))
@@ -2333,7 +2333,7 @@ static int do_remount(struct path *path, int ms_flags, int sb_flags,
 	down_write(&sb->s_umount);
 	if (ms_flags & MS_BIND)
 		err = change_mount_flags(path->mnt, ms_flags);
-	else if (!capable(CAP_SYS_ADMIN))
+	else if (!ns_capable(sb->s_user_ns, CAP_SYS_ADMIN))
 		err = -EPERM;
 	else
 		err = do_remount_sb(sb, sb_flags, data, 0);
-- 
2.14.1