[PATCH review 02/13] mnt: Refactor fs_fully_visible into mount_too_revealing
Eric W. Biederman
ebiederm at xmission.com
Mon Jun 20 17:21:19 UTC 2016
Replace the call of fs_fully_visible in do_new_mount from before the
new superblock is allocated with a call of mount_too_revealing after
the superblock is allocated. This winds up being a much better location
for maintainability of the code.
The first change this enables is the replacement of FS_USERNS_VISIBLE
with SB_I_USERNS_VISIBLE. Moving the flag from struct filesystem_type
to sb_iflags on the superblock.
Unfortunately mount_too_revealing fundamentally needs to touch
mnt_flags adding several MNT_LOCKED_XXX flags at the appropriate
times. If the mnt_flags did not need to be touched the code
could be easily moved into the filesystem specific mount code.
Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>
---
fs/namespace.c | 38 +++++++++++++++++++++++++-------------
fs/proc/inode.c | 1 +
fs/proc/root.c | 2 +-
fs/sysfs/mount.c | 4 ++--
include/linux/fs.h | 4 +++-
5 files changed, 32 insertions(+), 17 deletions(-)
diff --git a/fs/namespace.c b/fs/namespace.c
index 783004af5707..1a69aa786975 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2375,7 +2375,7 @@ unlock:
return err;
}
-static bool fs_fully_visible(struct file_system_type *fs_type, int *new_mnt_flags);
+static bool mount_too_revealing(struct vfsmount *mnt, int *new_mnt_flags);
/*
* create a new mount for userspace and request it to be added into the
@@ -2408,12 +2408,6 @@ static int do_new_mount(struct path *path, const char *fstype, int flags,
flags |= MS_NODEV;
mnt_flags |= MNT_NODEV | MNT_LOCK_NODEV;
}
- if (type->fs_flags & FS_USERNS_VISIBLE) {
- if (!fs_fully_visible(type, &mnt_flags)) {
- put_filesystem(type);
- return -EPERM;
- }
- }
}
mnt = vfs_kern_mount(type, flags, name, data);
@@ -2425,6 +2419,11 @@ static int do_new_mount(struct path *path, const char *fstype, int flags,
if (IS_ERR(mnt))
return PTR_ERR(mnt);
+ if (mount_too_revealing(mnt, &mnt_flags)) {
+ mntput(mnt);
+ return -EPERM;
+ }
+
err = do_add_mount(real_mount(mnt), path, mnt_flags);
if (err)
mntput(mnt);
@@ -3216,22 +3215,19 @@ bool current_chrooted(void)
return chrooted;
}
-static bool fs_fully_visible(struct file_system_type *type, int *new_mnt_flags)
+static bool mnt_already_visible(struct mnt_namespace *ns, struct vfsmount *new,
+ int *new_mnt_flags)
{
- struct mnt_namespace *ns = current->nsproxy->mnt_ns;
int new_flags = *new_mnt_flags;
struct mount *mnt;
bool visible = false;
- if (unlikely(!ns))
- return false;
-
down_read(&namespace_sem);
list_for_each_entry(mnt, &ns->list, mnt_list) {
struct mount *child;
int mnt_flags;
- if (mnt->mnt.mnt_sb->s_type != type)
+ if (mnt->mnt.mnt_sb->s_type != new->mnt_sb->s_type)
continue;
/* This mount is not fully visible if it's root directory
@@ -3298,6 +3294,22 @@ found:
return visible;
}
+static bool mount_too_revealing(struct vfsmount *mnt, int *new_mnt_flags)
+{
+ struct mnt_namespace *ns = current->nsproxy->mnt_ns;
+ unsigned long s_iflags;
+
+ if (ns->user_ns == &init_user_ns)
+ return false;
+
+ /* Can this filesystem be too revealing? */
+ s_iflags = mnt->mnt_sb->s_iflags;
+ if (!(s_iflags & SB_I_USERNS_VISIBLE))
+ return false;
+
+ return !mnt_already_visible(ns, mnt, new_mnt_flags);
+}
+
static struct ns_common *mntns_get(struct task_struct *task)
{
struct ns_common *ns = NULL;
diff --git a/fs/proc/inode.c b/fs/proc/inode.c
index 42305ddcbaa0..78fa452d65ed 100644
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -462,6 +462,7 @@ int proc_fill_super(struct super_block *s)
struct inode *root_inode;
int ret;
+ s->s_iflags |= SB_I_USERNS_VISIBLE;
s->s_flags |= MS_NODIRATIME | MS_NOSUID | MS_NOEXEC;
s->s_blocksize = 1024;
s->s_blocksize_bits = 10;
diff --git a/fs/proc/root.c b/fs/proc/root.c
index 55bc7d6c8aac..a1b2860fec62 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -158,7 +158,7 @@ static struct file_system_type proc_fs_type = {
.name = "proc",
.mount = proc_mount,
.kill_sb = proc_kill_sb,
- .fs_flags = FS_USERNS_VISIBLE | FS_USERNS_MOUNT,
+ .fs_flags = FS_USERNS_MOUNT,
};
void __init proc_root_init(void)
diff --git a/fs/sysfs/mount.c b/fs/sysfs/mount.c
index f3db82071cfb..f31e36994dfb 100644
--- a/fs/sysfs/mount.c
+++ b/fs/sysfs/mount.c
@@ -42,7 +42,7 @@ static struct dentry *sysfs_mount(struct file_system_type *fs_type,
kobj_ns_drop(KOBJ_NS_TYPE_NET, ns);
else if (new_sb)
/* Userspace would break if executables appear on sysfs */
- root->d_sb->s_iflags |= SB_I_NOEXEC;
+ root->d_sb->s_iflags |= SB_I_USERNS_VISIBLE | SB_I_NOEXEC;
return root;
}
@@ -59,7 +59,7 @@ static struct file_system_type sysfs_fs_type = {
.name = "sysfs",
.mount = sysfs_mount,
.kill_sb = sysfs_kill_sb,
- .fs_flags = FS_USERNS_VISIBLE | FS_USERNS_MOUNT,
+ .fs_flags = FS_USERNS_MOUNT,
};
int __init sysfs_init(void)
diff --git a/include/linux/fs.h b/include/linux/fs.h
index dd288148a6b1..71988dd3af95 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1328,6 +1328,9 @@ struct mm_struct;
#define SB_I_CGROUPWB 0x00000001 /* cgroup-aware writeback enabled */
#define SB_I_NOEXEC 0x00000002 /* Ignore executables on this fs */
+/* sb->s_iflags to limit user namespace mounts */
+#define SB_I_USERNS_VISIBLE 0x00000010 /* fstype already mounted */
+
/* Possible states of 'frozen' field */
enum {
SB_UNFROZEN = 0, /* FS is unfrozen */
@@ -2011,7 +2014,6 @@ struct file_system_type {
#define FS_HAS_SUBTYPE 4
#define FS_USERNS_MOUNT 8 /* Can be mounted by userns root */
#define FS_USERNS_DEV_MOUNT 16 /* A userns mount does not imply MNT_NODEV */
-#define FS_USERNS_VISIBLE 32 /* FS must already be visible */
#define FS_RENAME_DOES_D_MOVE 32768 /* FS will handle d_move() during rename() internally. */
struct dentry *(*mount) (struct file_system_type *, int,
const char *, void *);
--
2.8.3
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