Re: [PATCH] tmpfs: implement generic xattr support

[Serge E. Hallyn]

Quoting Miklos Szeredi (miklos@xxxxxxxxxx):
> Michal Suchanek <hramrach@xxxxxxxxxx> writes:
> > Applying this patch is not sufficient. Apparently more xattrs are
> > needed but adding them on top of this patch should be easy.
> >
> > The ones mentioned in the overlayfs doc are
> >
> > trusted.overlay.whiteout
> > trusted.overlay.opaque
> >
> > The patch implements security.*
> 
> Here's an updated patch.  It changes a number of things:
> 
>  - it implements shmem specific xattr ops instead of using the generic_*
>    functions.  Which means that there's no back and forth between the
>    VFS and the filesystem.  I basically copied the btrfs way of doing
>    things.
> 
>  - adds a new config option: CONFIG_TMPFS_XATTR and makes
>    CONFIG_TMPFS_POSIX_ACL depend on this.  This way xattr support can be
>    turned on without also adding ACL support.
> 
>  - now supports trusted.* namespace needed by overlayfs in addition to
>    security.*.  Doesn't yet support user.* since that needs more thought
>    wrt. kernel memory use.
> 
>  - supports xattrs on symlinks, again needed by overlayfs
> 
> Hugh, Eric, does this look acceptable?
> 
> Thanks,
> Miklos
> 
> ---
> From: Eric Paris <eparis@xxxxxxxxxx>
> Subject: tmpfs: implement generic xattr support
> 
> This patch implements generic xattrs for tmpfs filesystems.  The feodra
> project, while trying to replace suid apps with file capabilities,
> realized that tmpfs, which is used on the build systems, does not
> support file capabilities and thus cannot be used to build packages
> which use file capabilities.  Xattrs are also needed for overlayfs.
> 
> The xattr interface is a bit, odd.  If a filesystem does not implement any
> {get,set,list}xattr functions the VFS will call into some random LSM hooks and
> the running LSM can then implement some method for handling xattrs.  SELinux
> for example provides a method to support security.selinux but no other
> security.* xattrs.
> 
> As it stands today when one enables CONFIG_TMPFS_POSIX_ACL tmpfs will have
> xattr handler routines specifically to handle acls.  Because of this tmpfs
> would loose the VFS/LSM helpers to support the running LSM.  To make up for
> that tmpfs had stub functions that did nothing but call into the LSM hooks
> which implement the helpers.
> 
> This new patch does not use the LSM fallback functions and instead
> just implements a native get/set/list xattr feature for the full
> security.* and trusted.* namespace like a normal filesystem.  This
> means that tmpfs can now support both security.selinux and
> security.capability, which was not previously possible.
> 
> The basic implementation is that I attach a:
> 
> struct shmem_xattr {
> 	struct list_head list; /* anchored by shmem_inode_info->xattr_list */
> 	char *name;
> 	size_t size;
> 	char value[0];
> };
> 
> Into the struct shmem_inode_info for each xattr that is set.  This
> implementation could easily support the user.* namespace as well,
> except some care needs to be taken to prevent large amounts of
> unswappable memory being allocated for unprivileged users.
> 
> Signed-off-by: Eric Paris <eparis@xxxxxxxxxx>
> Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxx>

Looks good to me.

Acked-by: Serge Hallyn <serge.hallyn@xxxxxxxxxx>

thanks,
-serge

> ---
>  fs/Kconfig               |   18 ++
>  include/linux/shmem_fs.h |    1 
>  mm/shmem.c               |  302 +++++++++++++++++++++++++++++++++++++++--------
>  3 files changed, 273 insertions(+), 48 deletions(-)
> 
> Index: linux-2.6/fs/Kconfig
> ===================================================================
> --- linux-2.6.orig/fs/Kconfig	2011-04-19 21:09:33.000000000 +0200
> +++ linux-2.6/fs/Kconfig	2011-04-19 21:09:35.000000000 +0200
> @@ -121,9 +121,25 @@ config TMPFS
>  
>  	  See <file:Documentation/filesystems/tmpfs.txt> for details.
>  
> +config TMPFS_XATTR
> +	bool "Tmpfs extended attributes"
> +	depends on TMPFS
> +	default y
> +	help
> +	  Extended attributes are name:value pairs associated with inodes by
> +	  the kernel or by users (see the attr(5) manual page, or visit
> +	  <http://acl.bestbits.at/> for details).
> +
> +	  Currently this enables support for the trusted.* and
> +	  security.* namespaces.
> +
> +	  If unsure, say N.
> +
> +	  You need this for POSIX ACL support on tmpfs.
> +
>  config TMPFS_POSIX_ACL
>  	bool "Tmpfs POSIX Access Control Lists"
> -	depends on TMPFS
> +	depends on TMPFS_XATTR
>  	select GENERIC_ACL
>  	help
>  	  POSIX Access Control Lists (ACLs) support permissions for users and
> Index: linux-2.6/include/linux/shmem_fs.h
> ===================================================================
> --- linux-2.6.orig/include/linux/shmem_fs.h	2011-04-19 21:09:25.000000000 +0200
> +++ linux-2.6/include/linux/shmem_fs.h	2011-04-19 21:09:35.000000000 +0200
> @@ -19,6 +19,7 @@ struct shmem_inode_info {
>  	struct page		*i_indirect;	/* top indirect blocks page */
>  	swp_entry_t		i_direct[SHMEM_NR_DIRECT]; /* first blocks */
>  	struct list_head	swaplist;	/* chain of maybes on swap */
> +	struct list_head	xattr_list;	/* list of shmem_xattr */
>  	struct inode		vfs_inode;
>  };
>  
> Index: linux-2.6/mm/shmem.c
> ===================================================================
> --- linux-2.6.orig/mm/shmem.c	2011-04-19 21:09:25.000000000 +0200
> +++ linux-2.6/mm/shmem.c	2011-04-19 21:09:35.000000000 +0200
> @@ -99,6 +99,13 @@ static struct vfsmount *shm_mnt;
>  /* Pretend that each entry is of this size in directory's i_size */
>  #define BOGO_DIRENT_SIZE 20
>  
> +struct shmem_xattr {
> +	struct list_head list;	/* anchored by shmem_inode_info->xattr_list */
> +	char *name;		/* xattr name */
> +	size_t size;
> +	char value[0];
> +};
> +
>  /* Flag allocation requirements to shmem_getpage and shmem_swp_alloc */
>  enum sgp_type {
>  	SGP_READ,	/* don't exceed i_size, don't allocate page */
> @@ -822,6 +829,7 @@ static int shmem_notify_change(struct de
>  static void shmem_evict_inode(struct inode *inode)
>  {
>  	struct shmem_inode_info *info = SHMEM_I(inode);
> +	struct shmem_xattr *xattr, *nxattr;
>  
>  	if (inode->i_mapping->a_ops == &shmem_aops) {
>  		truncate_inode_pages(inode->i_mapping, 0);
> @@ -834,6 +842,11 @@ static void shmem_evict_inode(struct ino
>  			mutex_unlock(&shmem_swaplist_mutex);
>  		}
>  	}
> +
> +	list_for_each_entry_safe(xattr, nxattr, &info->xattr_list, list) {
> +		kfree(xattr->name);
> +		kfree(xattr);
> +	}
>  	BUG_ON(inode->i_blocks);
>  	shmem_free_inode(inode->i_sb);
>  	end_writeback(inode);
> @@ -1597,6 +1610,7 @@ static struct inode *shmem_get_inode(str
>  		spin_lock_init(&info->lock);
>  		info->flags = flags & VM_NORESERVE;
>  		INIT_LIST_HEAD(&info->swaplist);
> +		INIT_LIST_HEAD(&info->xattr_list);
>  		cache_no_acl(inode);
>  
>  		switch (mode & S_IFMT) {
> @@ -2048,62 +2062,225 @@ static void shmem_put_link(struct dentry
>  	}
>  }
>  
> -static const struct inode_operations shmem_symlink_inline_operations = {
> -	.readlink	= generic_readlink,
> -	.follow_link	= shmem_follow_link_inline,
> -};
> -
> -static const struct inode_operations shmem_symlink_inode_operations = {
> -	.readlink	= generic_readlink,
> -	.follow_link	= shmem_follow_link,
> -	.put_link	= shmem_put_link,
> -};
> -
> -#ifdef CONFIG_TMPFS_POSIX_ACL
> +#ifdef CONFIG_TMPFS_XATTR
>  /*
> - * Superblocks without xattr inode operations will get security.* xattr
> - * support from the VFS "for free". As soon as we have any other xattrs
> + * Superblocks without xattr inode operations may get some security.* xattr
> + * support from the LSM "for free". As soon as we have any other xattrs
>   * like ACLs, we also need to implement the security.* handlers at
>   * filesystem level, though.
>   */
>  
> -static size_t shmem_xattr_security_list(struct dentry *dentry, char *list,
> -					size_t list_len, const char *name,
> -					size_t name_len, int handler_flags)
> +static int shmem_xattr_get(struct dentry *dentry, const char *name,
> +			   void *buffer, size_t size)
>  {
> -	return security_inode_listsecurity(dentry->d_inode, list, list_len);
> -}
> +	struct shmem_inode_info *info;
> +	struct shmem_xattr *xattr;
> +	int ret = -ENODATA;
>  
> -static int shmem_xattr_security_get(struct dentry *dentry, const char *name,
> -		void *buffer, size_t size, int handler_flags)
> -{
> -	if (strcmp(name, "") == 0)
> -		return -EINVAL;
> -	return xattr_getsecurity(dentry->d_inode, name, buffer, size);
> +	info = SHMEM_I(dentry->d_inode);
> +
> +	spin_lock(&dentry->d_inode->i_lock);
> +	list_for_each_entry(xattr, &info->xattr_list, list) {
> +		if (strcmp(name, xattr->name))
> +			continue;
> +
> +		ret = xattr->size;
> +		if (buffer) {
> +			if (size < xattr->size)
> +				ret = -ERANGE;
> +			else
> +				memcpy(buffer, xattr->value, xattr->size);
> +		}
> +		break;
> +	}
> +	spin_unlock(&dentry->d_inode->i_lock);
> +	return ret;
>  }
>  
> -static int shmem_xattr_security_set(struct dentry *dentry, const char *name,
> -		const void *value, size_t size, int flags, int handler_flags)
> +static int shmem_xattr_set(struct dentry *dentry, const char *name,
> +			   const void *value, size_t size, int flags)
>  {
> -	if (strcmp(name, "") == 0)
> -		return -EINVAL;
> -	return security_inode_setsecurity(dentry->d_inode, name, value,
> -					  size, flags);
> +	struct inode *inode = dentry->d_inode;
> +	struct shmem_inode_info *info = SHMEM_I(inode);
> +	struct shmem_xattr *xattr;
> +	struct shmem_xattr *new_xattr = NULL;
> +	size_t len;
> +	int err = 0;
> +
> +	/* value == NULL means remove */
> +	if (value) {
> +		/* wrap around? */
> +		len = sizeof(*new_xattr) + size;
> +		if (len <= sizeof(*new_xattr))
> +			return -ENOMEM;
> +
> +		new_xattr = kmalloc(len, GFP_NOFS);
> +		if (!new_xattr)
> +		return -ENOMEM;
> +
> +		new_xattr->name = kstrdup(name, GFP_NOFS);
> +		if (!new_xattr->name) {
> +			kfree(new_xattr);
> +			return -ENOMEM;
> +		}
> +
> +		new_xattr->size = size;
> +		memcpy(new_xattr->value, value, size);
> +	}
> +
> +	spin_lock(&inode->i_lock);
> +	list_for_each_entry(xattr, &info->xattr_list, list) {
> +		if (!strcmp(name, xattr->name)) {
> +			if (flags & XATTR_CREATE) {
> +				xattr = new_xattr;
> +				err = -EEXIST;
> +			} else if (new_xattr) {
> +				list_replace(&xattr->list, &new_xattr->list);
> +			} else {
> +				list_del(&xattr->list);
> +			}
> +			goto out;
> +		}
> +	}
> +	if (flags & XATTR_REPLACE) {
> +		xattr = new_xattr;
> +		err = -ENODATA;
> +	} else {
> +		list_add(&new_xattr->list, &info->xattr_list);
> +		xattr = NULL;
> +	}
> +out:
> +	spin_unlock(&inode->i_lock);
> +	if (xattr)
> +		kfree(xattr->name);
> +	kfree(xattr);
> +	return 0;
>  }
>  
> -static const struct xattr_handler shmem_xattr_security_handler = {
> -	.prefix = XATTR_SECURITY_PREFIX,
> -	.list   = shmem_xattr_security_list,
> -	.get    = shmem_xattr_security_get,
> -	.set    = shmem_xattr_security_set,
> -};
>  
>  static const struct xattr_handler *shmem_xattr_handlers[] = {
> +#ifdef CONFIG_TMPFS_POSIX_ACL
>  	&generic_acl_access_handler,
>  	&generic_acl_default_handler,
> -	&shmem_xattr_security_handler,
> +#endif
>  	NULL
>  };
> +
> +static int shmem_xattr_validate(const char *name)
> +{
> +	struct { const char *prefix; size_t len; } arr[] = {
> +		{ XATTR_SECURITY_PREFIX, XATTR_SECURITY_PREFIX_LEN },
> +		{ XATTR_TRUSTED_PREFIX, XATTR_TRUSTED_PREFIX_LEN }};
> +	int i;
> +
> +	for (i = 0; i < ARRAY_SIZE(arr); i++) {
> +		size_t preflen = arr[i].len;
> +		if (strncmp(name, arr[i].prefix, preflen) == 0) {
> +			if (!name[preflen])
> +				return -EINVAL;
> +			return 0;
> +		}
> +	}
> +	return -EOPNOTSUPP;
> +}
> +
> +static ssize_t shmem_getxattr(struct dentry *dentry, const char *name,
> +			      void *buffer, size_t size)
> +{
> +	int err;
> +
> +	/*
> +	 * If this is a request for a synthetic attribute in the system.*
> +	 * namespace use the generic infrastructure to resolve a handler
> +	 * for it via sb->s_xattr.
> +	 */
> +	if (!strncmp(name, XATTR_SYSTEM_PREFIX, XATTR_SYSTEM_PREFIX_LEN))
> +		return generic_getxattr(dentry, name, buffer, size);
> +
> +	err = shmem_xattr_validate(name);
> +	if (err)
> +		return err;
> +
> +	return shmem_xattr_get(dentry, name, buffer, size);
> +}
> +
> +static int shmem_setxattr(struct dentry *dentry, const char *name,
> +			  const void *value, size_t size, int flags)
> +{
> +	int err;
> +
> +	/*
> +	 * If this is a request for a synthetic attribute in the system.*
> +	 * namespace use the generic infrastructure to resolve a handler
> +	 * for it via sb->s_xattr.
> +	 */
> +	if (!strncmp(name, XATTR_SYSTEM_PREFIX, XATTR_SYSTEM_PREFIX_LEN))
> +		return generic_setxattr(dentry, name, value, size, flags);
> +
> +	err = shmem_xattr_validate(name);
> +	if (err)
> +		return err;
> +
> +	if (size == 0)
> +		value = "";  /* empty EA, do not remove */
> +
> +	return shmem_xattr_set(dentry, name, value, size, flags);
> +
> +}
> +
> +static int shmem_removexattr(struct dentry *dentry, const char *name)
> +{
> +	int err;
> +
> +	/*
> +	 * If this is a request for a synthetic attribute in the system.*
> +	 * namespace use the generic infrastructure to resolve a handler
> +	 * for it via sb->s_xattr.
> +	 */
> +	if (!strncmp(name, XATTR_SYSTEM_PREFIX, XATTR_SYSTEM_PREFIX_LEN))
> +		return generic_removexattr(dentry, name);
> +
> +	err = shmem_xattr_validate(name);
> +	if (err)
> +		return err;
> +
> +	return shmem_xattr_set(dentry, name, NULL, 0, XATTR_REPLACE);
> +}
> +
> +static bool xattr_is_trusted(const char *name)
> +{
> +	return !strncmp(name, XATTR_TRUSTED_PREFIX, XATTR_TRUSTED_PREFIX_LEN);
> +}
> +
> +static ssize_t shmem_listxattr(struct dentry *dentry, char *buffer, size_t size)
> +{
> +	bool trusted = capable(CAP_SYS_ADMIN);
> +	struct shmem_xattr *xattr;
> +	struct shmem_inode_info *info;
> +	size_t used = 0;
> +
> +	info = SHMEM_I(dentry->d_inode);
> +
> +	spin_lock(&dentry->d_inode->i_lock);
> +	list_for_each_entry(xattr, &info->xattr_list, list) {
> +		/* skip "trusted." attributes for unprivileged callers */
> +		if (!trusted && xattr_is_trusted(xattr->name))
> +			continue;
> +
> +		used += strlen(xattr->name) + 1;
> +		if (buffer) {
> +			if (size < used) {
> +				used = -ERANGE;
> +				break;
> +			}
> +			strncpy(buffer, xattr->name, strlen(xattr->name) + 1);
> +			buffer += strlen(xattr->name) + 1;
> +		}
> +	}
> +	spin_unlock(&dentry->d_inode->i_lock);
> +
> +	return used;
> +}
>  #endif
>  
>  static struct dentry *shmem_get_parent(struct dentry *child)
> @@ -2384,8 +2561,10 @@ int shmem_fill_super(struct super_block
>  	sb->s_magic = TMPFS_MAGIC;
>  	sb->s_op = &shmem_ops;
>  	sb->s_time_gran = 1;
> -#ifdef CONFIG_TMPFS_POSIX_ACL
> +#ifdef CONFIG_TMPFS_XATTR
>  	sb->s_xattr = shmem_xattr_handlers;
> +#endif
> +#ifdef CONFIG_TMPFS_POSIX_ACL
>  	sb->s_flags |= MS_POSIXACL;
>  #endif
>  
> @@ -2483,16 +2662,41 @@ static const struct file_operations shme
>  static const struct inode_operations shmem_inode_operations = {
>  	.setattr	= shmem_notify_change,
>  	.truncate_range	= shmem_truncate_range,
> +#ifdef CONFIG_TMPFS_XATTR
> +	.setxattr	= shmem_setxattr,
> +	.getxattr	= shmem_getxattr,
> +	.listxattr	= shmem_listxattr,
> +	.removexattr	= shmem_removexattr,
> +#endif
>  #ifdef CONFIG_TMPFS_POSIX_ACL
> -	.setxattr	= generic_setxattr,
> -	.getxattr	= generic_getxattr,
> -	.listxattr	= generic_listxattr,
> -	.removexattr	= generic_removexattr,
>  	.check_acl	= generic_check_acl,
>  #endif
>  
>  };
>  
> +static const struct inode_operations shmem_symlink_inline_operations = {
> +	.readlink	= generic_readlink,
> +	.follow_link	= shmem_follow_link_inline,
> +#ifdef CONFIG_TMPFS_XATTR
> +	.setxattr	= shmem_setxattr,
> +	.getxattr	= shmem_getxattr,
> +	.listxattr	= shmem_listxattr,
> +	.removexattr	= shmem_removexattr,
> +#endif
> +};
> +
> +static const struct inode_operations shmem_symlink_inode_operations = {
> +	.readlink	= generic_readlink,
> +	.follow_link	= shmem_follow_link,
> +	.put_link	= shmem_put_link,
> +#ifdef CONFIG_TMPFS_XATTR
> +	.setxattr	= shmem_setxattr,
> +	.getxattr	= shmem_getxattr,
> +	.listxattr	= shmem_listxattr,
> +	.removexattr	= shmem_removexattr,
> +#endif
> +};
> +
>  static const struct inode_operations shmem_dir_inode_operations = {
>  #ifdef CONFIG_TMPFS
>  	.create		= shmem_create,
> @@ -2505,23 +2709,27 @@ static const struct inode_operations shm
>  	.mknod		= shmem_mknod,
>  	.rename		= shmem_rename,
>  #endif
> -#ifdef CONFIG_TMPFS_POSIX_ACL
> -	.setattr	= shmem_notify_change,
> +#ifdef CONFIG_TMPFS_XATTR
>  	.setxattr	= generic_setxattr,
>  	.getxattr	= generic_getxattr,
>  	.listxattr	= generic_listxattr,
>  	.removexattr	= generic_removexattr,
> +#endif
> +#ifdef CONFIG_TMPFS_POSIX_ACL
> +	.setattr	= shmem_notify_change,
>  	.check_acl	= generic_check_acl,
>  #endif
>  };
>  
>  static const struct inode_operations shmem_special_inode_operations = {
> -#ifdef CONFIG_TMPFS_POSIX_ACL
> -	.setattr	= shmem_notify_change,
> +#ifdef CONFIG_TMPFS_XATTR
>  	.setxattr	= generic_setxattr,
>  	.getxattr	= generic_getxattr,
>  	.listxattr	= generic_listxattr,
>  	.removexattr	= generic_removexattr,
> +#endif
> +#ifdef CONFIG_TMPFS_POSIX_ACL
> +	.setattr	= shmem_notify_change,
>  	.check_acl	= generic_check_acl,
>  #endif
>  };
> --
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