Re: [RFC 13/20] securityfs: Build securityfs_ns for namespacing support

Stefan Berger
2021-12-02T13:48:11Z

On 12/2/21 08:35, Christian Brauner wrote:
> On Tue, Nov 30, 2021 at 11:06:47AM -0500, Stefan Berger wrote:
>> Implement 'securityfs_ns' for support of IMA namespacing so that each
>> IMA (user) namespace can have its own front-end for showing the currently
>> active policy, the measurement list, number of violations and so on. This
>> filesystem shares much of the existing code of SecurityFS but requires a
>> new API call securityfs_ns_create_mount() for creating a new instance.
>>
>> The API calls of securityfs_ns have the prefix securityfs_ns_ and take
>> additional parameters struct vfsmount * and mount_count that allow for
>> multiple instances of this filesystem to exist.
>>
>> The filesystem can be mounted to the usual securityfs mount point like
>> this:
>>
>> mount -t securityfs_ns /sys/kernel/security /sys/kernel/security
>>
>> Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
>> ---
>> include/linux/security.h | 18 ++++
>> include/uapi/linux/magic.h | 1 +
>> security/inode.c | 197 +++++++++++++++++++++++++++++++++++--
>> 3 files changed, 210 insertions(+), 6 deletions(-)
>>
>> diff --git a/include/linux/security.h b/include/linux/security.h
>> index 7e0ba63b5dde..8e479266f544 100644
>> --- a/include/linux/security.h
>> +++ b/include/linux/security.h
>> @@ -1929,6 +1929,24 @@ struct dentry *securityfs_create_symlink(const char *name,
>> const struct inode_operations *iops);
>> extern void securityfs_remove(struct dentry *dentry);
>>
>> +extern struct dentry *securityfs_ns_create_file(const char *name, umode_t mode,
>> + struct dentry *parent, void *data,
>> + const struct file_operations *fops,
>> + const struct inode_operations *iops,
>> + struct vfsmount **mount, int *mount_count);
>> +extern struct dentry *securityfs_ns_create_dir(const char *name, struct dentry *parent,
>> + const struct inode_operations *iops,
>> + struct vfsmount **mount, int *mount_count);
>> +struct dentry *securityfs_ns_create_symlink(const char *name,
>> + struct dentry *parent,
>> + const char *target,
>> + const struct inode_operations *iops,
>> + struct vfsmount **mount, int *mount_count);
>> +extern void securityfs_ns_remove(struct dentry *dentry,
>> + struct vfsmount **mount, int *mount_count);
>> +struct vfsmount *securityfs_ns_create_mount(struct user_namespace *user_ns);
>> +extern struct vfsmount *securityfs_ns_mount;
>> +
>> #else /* CONFIG_SECURITYFS */
>>
>> static inline struct dentry *securityfs_create_dir(const char *name,
>> diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
>> index 35687dcb1a42..5c1cc6088dd2 100644
>> --- a/include/uapi/linux/magic.h
>> +++ b/include/uapi/linux/magic.h
>> @@ -11,6 +11,7 @@
>> #define CRAMFS_MAGIC_WEND 0x453dcd28 /* magic number with the wrong endianess */
>> #define DEBUGFS_MAGIC 0x64626720
>> #define SECURITYFS_MAGIC 0x73636673
>> +#define SECURITYFS_NS_MAGIC 0x73334473
>> #define SELINUX_MAGIC 0xf97cff8c
>> #define SMACK_MAGIC 0x43415d53 /* "SMAC" */
>> #define RAMFS_MAGIC 0x858458f6 /* some random number */
>> diff --git a/security/inode.c b/security/inode.c
>> index 429744ff4ab3..8077d1f31489 100644
>> --- a/security/inode.c
>> +++ b/security/inode.c
>> @@ -21,6 +21,7 @@
>> #include <linux/security.h>
>> #include <linux/lsm_hooks.h>
>> #include <linux/magic.h>
>> +#include <linux/user_namespace.h>
>>
>> static struct vfsmount *securityfs_mount;
>> static int securityfs_mount_count;
>> @@ -73,6 +74,61 @@ static struct file_system_type securityfs_type = {
>> .kill_sb = kill_litter_super,
>> };
>>
>> +static int securityfs_ns_fill_super(struct super_block *sb, struct fs_context *fc)
>> +{
>> + static const struct tree_descr files[] = {{""}};
>> + int error;
>> +
>> + error = simple_fill_super(sb, SECURITYFS_NS_MAGIC, files);
>> + if (error)
>> + return error;
>> +
>> + sb->s_op = &securityfs_super_operations;
>> +
>> + return 0;
>> +}
>> +
>> +static int securityfs_ns_get_tree(struct fs_context *fc)
>> +{
>> + return get_tree_keyed(fc, securityfs_ns_fill_super, fc->user_ns);
>> +}
>> +
>> +static const struct fs_context_operations securityfs_ns_context_ops = {
>> + .get_tree = securityfs_ns_get_tree,
>> +};
>> +
>> +static int securityfs_ns_init_fs_context(struct fs_context *fc)
>> +{
>> + fc->ops = &securityfs_ns_context_ops;
>> + return 0;
>> +}
>> +
>> +static struct file_system_type securityfs_ns_type = {
>> + .owner = THIS_MODULE,
>> + .name = "securityfs_ns",
>> + .init_fs_context = securityfs_ns_init_fs_context,
>> + .kill_sb = kill_litter_super,
>> + .fs_flags = FS_USERNS_MOUNT,
>> +};
>> +
>> +struct vfsmount *securityfs_ns_create_mount(struct user_namespace *user_ns)
>> +{
>> + struct fs_context *fc;
>> + struct vfsmount *mnt;
>> +
>> + fc = fs_context_for_mount(&securityfs_ns_type, SB_KERNMOUNT);
>> + if (IS_ERR(fc))
>> + return ERR_CAST(fc);
>> +
>> + put_user_ns(fc->user_ns);
>> + fc->user_ns = get_user_ns(user_ns);
>> +
>> + mnt = fc_mount(fc);
>> + put_fs_context(fc);
>> + return mnt;
>> +}
>> +
>> +
>> /**
>> * securityfs_create_dentry - create a dentry in the securityfs filesystem
>> *
>> @@ -155,8 +211,8 @@ static struct dentry *securityfs_create_dentry(const char *name, umode_t mode,
>> inode->i_atime = inode->i_mtime = inode->i_ctime = current_time(inode);
>> inode->i_private = data;
>> if (S_ISDIR(mode)) {
>> - inode->i_op = &simple_dir_inode_operations;
>> - inode->i_fop = &simple_dir_operations;
>> + inode->i_op = iops ? iops : &simple_dir_inode_operations;
>> + inode->i_fop = fops ? fops : &simple_dir_operations;
>> inc_nlink(inode);
>> inc_nlink(dir);
>> } else if (S_ISLNK(mode)) {
>> @@ -214,6 +270,41 @@ struct dentry *securityfs_create_file(const char *name, umode_t mode,
>> }
>> EXPORT_SYMBOL_GPL(securityfs_create_file);
>>
>> +/**
>> + * securityfs_ns_create_file - create a file in the securityfs_ns filesystem
>> + *
>> + * @name: a pointer to a string containing the name of the file to create.
>> + * @mode: the permission that the file should have
>> + * @parent: a pointer to the parent dentry for this file. This should be a
>> + * directory dentry if set. If this parameter is %NULL, then the
>> + * file will be created in the root of the securityfs_ns filesystem.
>> + * @data: a pointer to something that the caller will want to get to later
>> + * on. The inode.i_private pointer will point to this value on
>> + * the open() call.
>> + * @fops: a pointer to a struct file_operations that should be used for
>> + * this file.
>> + * @mount: Pointer to a pointer of a an existing vfsmount
>> + * @mount_count: The mount_count that goes along with the @mount
>> + *
>> + * This function creates a file in securityfs_ns with the given @name.
>> + *
>> + * This function returns a pointer to a dentry if it succeeds. This
>> + * pointer must be passed to the securityfs_ns_remove() function when the file
>> + * is to be removed (no automatic cleanup happens if your module is unloaded,
>> + * you are responsible here). If an error occurs, the function will return
>> + * the error value (via ERR_PTR).
>> + */
>> +struct dentry *securityfs_ns_create_file(const char *name, umode_t mode,
>> + struct dentry *parent, void *data,
>> + const struct file_operations *fops,
>> + const struct inode_operations *iops,
>> + struct vfsmount **mount, int *mount_count)
>> +{
>> + return securityfs_create_dentry(name, mode, parent, data, fops, iops,
>> + &securityfs_ns_type, mount, mount_count);
>> +}
>> +EXPORT_SYMBOL_GPL(securityfs_ns_create_file);
>> +
>> /**
>> * securityfs_create_dir - create a directory in the securityfs filesystem
>> *
>> @@ -240,6 +331,34 @@ struct dentry *securityfs_create_dir(const char *name, struct dentry *parent)
>> }
>> EXPORT_SYMBOL_GPL(securityfs_create_dir);
>>
>> +/**
>> + * securityfs_ns_create_dir - create a directory in the securityfs_ns filesystem
>> + *
>> + * @name: a pointer to a string containing the name of the directory to
>> + * create.
>> + * @parent: a pointer to the parent dentry for this file. This should be a
>> + * directory dentry if set. If this parameter is %NULL, then the
>> + * directory will be created in the root of the securityfs_ns filesystem.
>> + * @mount: Pointer to a pointer of a an existing vfsmount
>> + * @mount_count: The mount_count that goes along with the @mount
>> + *
>> + * This function creates a directory in securityfs_ns with the given @name.
>> + *
>> + * This function returns a pointer to a dentry if it succeeds. This
>> + * pointer must be passed to the securityfs_ns_remove() function when the file
>> + * is to be removed (no automatic cleanup happens if your module is unloaded,
>> + * you are responsible here). If an error occurs, the function will return
>> + * the error value (via ERR_PTR).
>> + */
>> +struct dentry *securityfs_ns_create_dir(const char *name, struct dentry *parent,
>> + const struct inode_operations *iops,
>> + struct vfsmount **mount, int *mount_count)
>> +{
>> + return securityfs_ns_create_file(name, S_IFDIR | 0755, parent, NULL, NULL,
>> + iops, mount, mount_count);
>> +}
>> +EXPORT_SYMBOL_GPL(securityfs_ns_create_dir);
>> +
>> struct dentry *_securityfs_create_symlink(const char *name,
>> struct dentry *parent,
>> const char *target,
>> @@ -263,6 +382,7 @@ struct dentry *_securityfs_create_symlink(const char *name,
>>
>> return dent;
>> }
>> +
>> /**
>> * securityfs_create_symlink - create a symlink in the securityfs filesystem
>> *
>> @@ -300,6 +420,42 @@ struct dentry *securityfs_create_symlink(const char *name,
>> }
>> EXPORT_SYMBOL_GPL(securityfs_create_symlink);
>>
>> +/**
>> + * securityfs_ns_create_symlink - create a symlink in the securityfs_ns filesystem
>> + *
>> + * @name: a pointer to a string containing the name of the symlink to
>> + * create.
>> + * @parent: a pointer to the parent dentry for the symlink. This should be a
>> + * directory dentry if set. If this parameter is %NULL, then the
>> + * directory will be created in the root of the securityfs_ns filesystem.
>> + * @target: a pointer to a string containing the name of the symlink's target.
>> + * If this parameter is %NULL, then the @iops parameter needs to be
>> + * setup to handle .readlink and .get_link inode_operations.
>> + * @iops: a pointer to the struct inode_operations to use for the symlink. If
>> + * this parameter is %NULL, then the default simple_symlink_inode
>> + * operations will be used.
>> + * @mount: Pointer to a pointer of a an existing vfsmount
>> + * @mount_count: The mount_count that goes along with the @mount
>> + *
>> + * This function creates a symlink in securityfs_ns with the given @name.
>> + *
>> + * This function returns a pointer to a dentry if it succeeds. This
>> + * pointer must be passed to the securityfs_ns_remove() function when the file
>> + * is to be removed (no automatic cleanup happens if your module is unloaded,
>> + * you are responsible here). If an error occurs, the function will return
>> + * the error value (via ERR_PTR).
>> + */
>> +struct dentry *securityfs_ns_create_symlink(const char *name,
>> + struct dentry *parent,
>> + const char *target,
>> + const struct inode_operations *iops,
>> + struct vfsmount **mount, int *mount_count)
>> +{
>> + return _securityfs_create_symlink(name, parent, target, iops,
>> + &securityfs_ns_type, mount, mount_count);
>> +}
>> +EXPORT_SYMBOL_GPL(securityfs_ns_create_symlink);
>> +
>> void _securityfs_remove(struct dentry *dentry, struct vfsmount **mount, int *mount_count)
>> {
>> struct inode *dir;
>> @@ -340,6 +496,27 @@ void securityfs_remove(struct dentry *dentry)
>>
>> EXPORT_SYMBOL_GPL(securityfs_remove);
>>
>> +/**
>> + * securityfs_ns_remove - removes a file or directory from the securityfs_ns filesystem
>> + *
>> + * @dentry: a pointer to a the dentry of the file or directory to be removed.
>> + * @mount: Pointer to a pointer of a an existing vfsmount
>> + * @mount_count: The mount_count that goes along with the @mount
>> + *
>> + * This function removes a file or directory in securityfs_ns that was previously
>> + * created with a call to another securityfs_ns function (like
>> + * securityfs_ns_create_file() or variants thereof.)
>> + *
>> + * This function is required to be called in order for the file to be
>> + * removed. No automatic cleanup of files will happen when a module is
>> + * removed; you are responsible here.
>> + */
>> +void securityfs_ns_remove(struct dentry *dentry, struct vfsmount **mount, int *mount_count)
>> +{
>> + _securityfs_remove(dentry, mount, mount_count);
>> +}
>> +EXPORT_SYMBOL_GPL(securityfs_ns_remove);
>> +
>> #ifdef CONFIG_SECURITY
>> static struct dentry *lsm_dentry;
>> static ssize_t lsm_read(struct file *filp, char __user *buf, size_t count,
>> @@ -364,14 +541,22 @@ static int __init securityfs_init(void)
>> return retval;
>>
>> retval = register_filesystem(&securityfs_type);
>> - if (retval) {
>> - sysfs_remove_mount_point(kernel_kobj, "security");
>> - return retval;
>> - }
>> + if (retval)
>> + goto remove_mount;
>> + retval = register_filesystem(&securityfs_ns_type);
>> + if (retval)
>> + goto unregister_filesystem;
> So you're introducing a new filesystem type securityfs_ns. Ithink that's
> simply wrong and feels like a hack. What issues did you run into when
> trying to convert the existing securityfs itself?

I primarily didn't want to touch the existing securityfs with its
existing users and it being a single instance filesystem. So I though
I'd create something with a new API just for namespaces that is
multi-instance capable.

>
> I see no immediate reason why a get_tree_keyed() conversion for
> securityfs wouldn't work even with the debugfs pin/unpin logic in there
> kept for the securityfs mounted in the initial userns.
Ok, let me try to convert securityfs then.