Re: [PATCH v2 14/39] commoncap: handle idmapped mounts

Paul Moore
2020-11-23T03:24:36Z

On Sun, Nov 15, 2020 at 5:39 AM Christian Brauner
<christian.brauner@ubuntu.com> wrote:
> When interacting with user namespace and non-user namespace aware
> filesystem capabilities the vfs will perform various security checks to
> determine whether or not the filesystem capabilities can be used by the
> caller (e.g. during exec), or even whether they need to be removed. The
> main infrastructure for this resides in the capability codepaths but they
> are called through the LSM security infrastructure even though they are not
> technically an LSM or optional. This extends the existing security hooks
> security_inode_removexattr(), security_inode_killpriv(),
> security_inode_getsecurity() to pass down the mount's user namespace and
> makes them aware of idmapped mounts.
> In order to actually get filesystem capabilities from disk the capability
> infrastructure exposes the get_vfs_caps_from_disk() helper. For user
> namespace aware filesystem capabilities a root uid is stored alongside the
> capabilities.
> In order to determine whether the caller can make use of the filesystem
> capability or whether it needs to be ignored it is translated according to
> the superblock's user namespace. If it can be translated to uid 0 according
> to that id mapping the caller can use the filesystem capabilities stored on
> disk. If we are accessing the inode that holds the filesystem capabilities
> through an idmapped mount we need to map the root uid according to the
> mount's user namespace.
> Afterwards the checks are identical to non-idmapped mounts. Reading
> filesystem caps from disk enforces that the root uid associated with the
> filesystem capability must have a mapping in the superblock's user
> namespace and that the caller is either in the same user namespace or is a
> descendant of the superblock's user namespace. For filesystems that are
> mountable inside user namespace the container can just mount the filesystem
> and won't usually need to idmap it. If it does create an idmapped mount it
> can mark it with a user namespace it has created and which is therefore a
> descendant of the s_user_ns. For filesystems that are not mountable inside
> user namespaces the descendant rule is trivially true because the s_user_ns
> will be the initial user namespace.
>
> If the initial user namespace is passed all operations are a nop so
> non-idmapped mounts will not see a change in behavior and will also not see
> any performance impact.
>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: David Howells <dhowells@redhat.com>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: linux-fsdevel@vger.kernel.org
> Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

...

> diff --git a/kernel/auditsc.c b/kernel/auditsc.c
> index 8dba8f0983b5..ddb9213a3e81 100644
> --- a/kernel/auditsc.c
> +++ b/kernel/auditsc.c
> @@ -1944,7 +1944,7 @@ static inline int audit_copy_fcaps(struct audit_names *name,
> if (!dentry)
> return 0;
>
> - rc = get_vfs_caps_from_disk(dentry, &caps);
> + rc = get_vfs_caps_from_disk(&init_user_ns, dentry, &caps);
> if (rc)
> return rc;
>
> @@ -2495,7 +2495,8 @@ int __audit_log_bprm_fcaps(struct linux_binprm *bprm,
> ax->d.next = context->aux;
> context->aux = (void *)ax;
>
> - get_vfs_caps_from_disk(bprm->file->f_path.dentry, &vcaps);
> + get_vfs_caps_from_disk(mnt_user_ns(bprm->file->f_path.mnt),
> + bprm->file->f_path.dentry, &vcaps);

As audit currently records information in the context of the
initial/host namespace I'm guessing we don't want the mnt_user_ns()
call above; it seems like &init_user_ns would be the right choice
(similar to audit_copy_fcaps()), yes?

> ax->fcap.permitted = vcaps.permitted;
> ax->fcap.inheritable = vcaps.inheritable;

--
paul moore