Re: [PATCH] fs: add an ioctl to get an owning userns for a superblock
From: Eric W. Biederman <hidden>
Date: 2017-05-10 00:40:27
Also in:
linux-fsdevel, lkml
Andrei Vagin [off-list ref] writes:
The introduced ioctl returns a file descriptor that refers to a owning user namespace for a superblock which is associated with a target file descriptor. EPERM is returned if the current process doesn't have CAP_SYS_ADMIN in the returned user namespace. This information is required to dump and restore mount namespaces. We need to know to which user namespace a superblock is belonged to. We already have the SIOCGSKNS ioctl for sockets to get a network namespace, so it looks reasonable to use the same interface for superblocks too. This functionality can be useful for users in order to understand a running system.
This will probably work. And the capability check eases any concerns I might have that this would be a trivial information leak. That said can we hold off just a little bit. If open_fs work actually turns into a real interface that would seem to be the perfect place to stick this functionality. Eric
quoted hunk
Cc: Alexander Viro <viro-RmSDqhL/yNMiFSDQTTA3OLVCufUGDwFn@public.gmane.org> Cc: Eric W. Biederman <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org> Signed-off-by: Andrei Vagin <redacted> --- fs/ioctl.c | 23 +++++++++++++++++++++++ include/uapi/linux/fs.h | 2 ++ 2 files changed, 25 insertions(+)diff --git a/fs/ioctl.c b/fs/ioctl.c index 569db68..22bbf37 100644 --- a/fs/ioctl.c +++ b/fs/ioctl.c@@ -16,6 +16,8 @@ #include <linux/buffer_head.h> #include <linux/falloc.h> #include <linux/sched/signal.h> +#include <linux/proc_fs.h> +#include <linux/user_namespace.h> #include "internal.h"@@ -614,6 +616,25 @@ static int ioctl_file_dedupe_range(struct file *file, void __user *arg) return ret; } +static struct ns_common *get_sb_userns(struct ns_common *ns_common) +{ + struct user_namespace *ns; + + ns = container_of(ns_common, struct user_namespace, ns); + + return &get_user_ns(ns)->ns; +} + +static int ioctl_fs_sb_userns(struct file *filp) +{ + struct super_block *sb = file_inode(filp)->i_sb; + + if (!ns_capable(sb->s_user_ns, CAP_SYS_ADMIN)) + return -EPERM; + + return open_related_ns(&sb->s_user_ns->ns, get_sb_userns); +} + /* * When you add any new common ioctls to the switches above and below * please update compat_sys_ioctl() too.@@ -677,6 +698,8 @@ int do_vfs_ioctl(struct file *filp, unsigned int fd, unsigned int cmd, case FIDEDUPERANGE: return ioctl_file_dedupe_range(filp, argp); + case FS_IOC_SB_USERNS: + return ioctl_fs_sb_userns(filp); default: if (S_ISREG(inode->i_mode))diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h index 33423aa..26ef2d5 100644 --- a/include/uapi/linux/fs.h +++ b/include/uapi/linux/fs.h@@ -246,6 +246,8 @@ struct fsxattr { #define FICLONE _IOW(0x94, 9, int) #define FICLONERANGE _IOW(0x94, 13, struct file_clone_range) #define FIDEDUPERANGE _IOWR(0x94, 54, struct file_dedupe_range) +/* Get a file descriptor to an owning userns for a superblock */ +#define FS_IOC_SB_USERNS _IOR('X', 55, int) #define FS_IOC_GETFLAGS _IOR('f', 1, long) #define FS_IOC_SETFLAGS _IOW('f', 2, long)