On 12/07/2016 12:16 PM, Stephen Smalley wrote:
> commit aad82892af261b9903cc11c55be3ecf5f0b0b4f8 ("selinux: Add support for
> unprivileged mounts from user namespaces") prohibited any use of context
> mount options within non-init user namespaces. However, this breaks
> use of context mount options for tmpfs mounts within user namespaces,
> which are being used by Docker/runc. There is no reason to block such
> usage for tmpfs, ramfs or devpts. Exempt these filesystem types
> from this restriction.
>
> Before:
> sh$ userns_child_exec -p -m -U -M '0 1000 1' -G '0 1000 1' bash
> sh# mount -t tmpfs -o context=system_u:object_r:user_tmp_t:s0:c13 none /tmp
> mount: tmpfs is write-protected, mounting read-only
> mount: cannot mount tmpfs read-only
>
> After:
> sh$ userns_child_exec -p -m -U -M '0 1000 1' -G '0 1000 1' bash
> sh# mount -t tmpfs -o context=system_u:object_r:user_tmp_t:s0:c13 none /tmp
> sh# ls -Zd /tmp
> unconfined_u:object_r:user_tmp_t:s0 /tmp
>
> Note that this still isn't quite right, and I do not know why yet -
> the category (:c13) was dropped. This works correctly in the init
> namespace, and strace of mount shows that it is passing the context
> correctly to the kernel and returning 0.
Never mind, this patch is wrong.
>
> Signed-off-by: Stephen Smalley <sds@xxxxxxxxxxxxx>
> ---
> security/selinux/hooks.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 98a2e92..ef882a3 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -839,8 +839,12 @@ static int selinux_set_mnt_opts(struct super_block *sb,
> if (sb->s_user_ns != &init_user_ns) {
> if (context_sid || fscontext_sid || rootcontext_sid ||
> defcontext_sid) {
> - rc = -EACCES;
> - goto out;
> + if (strcmp(sb->s_type->name, "tmpfs") &&
> + strcmp(sb->s_type->name, "ramfs") &&
> + strcmp(sb->s_type->name, "devpts")) {
> + rc = -EACCES;
> + goto out;
> + }
> }
> if (sbsec->behavior == SECURITY_FS_USE_XATTR) {
> sbsec->behavior = SECURITY_FS_USE_MNTPOINT;
>
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