[PATCH 05/11] fs: Allow superblock owner to access do_remount_sb()

[Dongsu Park]

From: Seth Forshee <seth.forshee at canonical.com>

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.

Patch v4 is available: https://patchwork.kernel.org/patch/8944631/

Cc: linux-fsdevel at vger.kernel.org
Cc: linux-kernel at vger.kernel.org
Cc: Alexander Viro <viro at zeniv.linux.org.uk>
Cc: "Eric W. Biederman" <ebiederm at xmission.com>
Cc: Serge Hallyn <serge at hallyn.com>
Signed-off-by: Seth Forshee <seth.forshee at canonical.com>
Signed-off-by: Dongsu Park <dongsu at kinvolk.io>
---
 fs/namespace.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index e158ec6b..830040d7 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1589,7 +1589,7 @@ 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))
@@ -2327,7 +2327,7 @@ 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);
-- 
2.13.6