On 2012/09/14 17:33, Eric W. Biederman wrote: > Zhao Hongjiang <zhaohongjiang37@xxxxxxxxx> writes: > >> On 2012-9-14 5:26, Eric W. Biederman wrote: >>> Zhao Hongjiang <zhaohongjiang37@xxxxxxxxx> writes: >>> >>>> From: Zhao Hongjiang <zhaohongjiang@xxxxxxxxxx> >>>> >>>> Relax the permission checks to allow unprivileged users that have >>>> CAP_SYS_ADMIN permissions in the user namespace referred to by the >>>> current mount namespace to be allowed to remount filesystems. >>> >>> Remount in general make filesystem configuration changes not mount level >>> changes. >>> >>> In general remount is not safe for unprivielged users. >>> >>> Do you have a use case where you need to remount a filesystem? >> >> As we can do a umount+mount,I don't see why remount operation is not allowed. >> Shouldn't we add checks in remount path in the specific filesystem to ensure >> safety instead when we enable unprivilleged mount? > > But the thing is remount != mount+umount. Remount is change lowlevel > filesystem options. > > The basic danger is if someone in the primary user namespace mounted a > filesystem, and then we cloned that filesystem. > > umounting filesystems is ok. There reference count will drop or they > will just unmount if the ref count goes to zero. > > However mount -o remount -r /home could very easily remount everyone's > home directory in all mount namespaces read-only by making the > filesystem itself readonly. > > That danger applies even to some extent even if the options are safe for > us to perform at the filesystem level. > > Now that doesn't mean remount is a hopeless operation. What it does > mean is that we need to be very carefully with enabling remounting > of a filesystem. > Hi Eric what's you idea about the patch below. Maybe it better to add a new fs_flags FS_USERNS_REMOUNT? It's not a good experience that remount is disabled in container. Thanks!
>From 8c5a01c007d72c748018665d3bd27cd2bde52c0f Mon Sep 17 00:00:00 2001 From: Gao feng <gaofeng@xxxxxxxxxxxxxx> Date: Thu, 17 Jan 2013 14:41:00 +0800 Subject: [PATCH] userns: allow remount filesystem in un-init userns The proc and sysfs filesystem already enable userns support, remounting these filesystems in un-init userns do no harm to the host. Signed-off-by: Gao feng <gaofeng@xxxxxxxxxxxxxx> --- fs/namespace.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/fs/namespace.c b/fs/namespace.c index 55605c5..b9d83fb 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -1748,7 +1748,10 @@ static int do_remount(struct path *path, int flags, int mnt_flags, struct super_block *sb = path->mnt->mnt_sb; struct mount *mnt = real_mount(path->mnt); - if (!capable(CAP_SYS_ADMIN)) + if (sb->s_type->fs_flags & FS_USERNS_MOUNT) { + if (!ns_capable(mnt->mnt_ns->user_ns, CAP_SYS_ADMIN)) + return -EPERM; + } else if (!capable(CAP_SYS_ADMIN)) return -EPERM; if (!check_mnt(mnt)) -- 1.7.11.7
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