[PATCH v2 2/2] fuse: Send security context of inode on file creation

Casey Schaufler casey at schaufler-ca.com
Tue Oct 12 18:24:23 UTC 2021


On 10/12/2021 11:06 AM, Vivek Goyal wrote:
> When a new inode is created, send its security context to server along
> with creation request (FUSE_CREAT, FUSE_MKNOD, FUSE_MKDIR and FUSE_SYMLINK).
> This gives server an opportunity to create new file and set security
> context (possibly atomically). In all the configurations it might not
> be possible to set context atomically.
>
> Like nfs and ceph, use security_dentry_init_security() to dermine security
> context of inode and send it with create, mkdir, mknod, and symlink requests.
>
> Following is the information sent to server.
>
> - struct fuse_secctxs.
>   This contains total number of security contexts being sent.
>
> - struct fuse_secctx.
>   This contains total size of security context which follows this structure.
>   There is one fuse_secctx instance per security context.
>
> - xattr name string.
>   This string represents name of xattr which should be used while setting
>   security context. As of now it is hardcoded to "security.selinux".

Where is the name hardcoded? I looks as if you're getting the attribute
name along with the value from security_dentry_init_security().

>
> - security context.
>   This is the actual security context whose size is specified in fuse_secctx
>   struct.
>
> This patch is modified version of patch from
> Chirantan Ekbote <chirantan at chromium.org>
>
> v2:
> - Added "fuse_secctxs" structure where one can specify how many security
>   contexts are being sent. This can be useful down the line if we
>   have more than one security contexts being set.
>
> Signed-off-by: Vivek Goyal <vgoyal at redhat.com>
> ---
>  fs/fuse/dir.c             | 115 ++++++++++++++++++++++++++++++++++++--
>  fs/fuse/fuse_i.h          |   3 +
>  fs/fuse/inode.c           |   4 +-
>  include/uapi/linux/fuse.h |  20 +++++++
>  4 files changed, 136 insertions(+), 6 deletions(-)
>
> diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
> index d9b977c0f38d..ce62593a61f9 100644
> --- a/fs/fuse/dir.c
> +++ b/fs/fuse/dir.c
> @@ -17,6 +17,9 @@
>  #include <linux/xattr.h>
>  #include <linux/iversion.h>
>  #include <linux/posix_acl.h>
> +#include <linux/security.h>
> +#include <linux/types.h>
> +#include <linux/kernel.h>
>  
>  static void fuse_advise_use_readdirplus(struct inode *dir)
>  {
> @@ -456,6 +459,66 @@ static struct dentry *fuse_lookup(struct inode *dir, struct dentry *entry,
>  	return ERR_PTR(err);
>  }
>  
> +static int get_security_context(struct dentry *entry, umode_t mode,
> +				void **security_ctx, u32 *security_ctxlen)
> +{
> +	struct fuse_secctx *fsecctx;
> +	struct fuse_secctxs *fsecctxs;
> +	void *ctx, *full_ctx;
> +	u32 ctxlen, full_ctxlen;
> +	int err = 0;
> +	const char *name;
> +
> +	err = security_dentry_init_security(entry, mode, &entry->d_name,
> +					    &name, &ctx, &ctxlen);
> +	if (err) {
> +		if (err != -EOPNOTSUPP)
> +			goto out_err;
> +		/* No LSM is supporting this security hook. Ignore error */
> +		err = 0;
> +		ctxlen = 0;
> +	}
> +
> +	if (ctxlen > 0) {
> +		void *ptr;
> +
> +		full_ctxlen = sizeof(*fsecctxs) + sizeof(*fsecctx) +
> +			      strlen(name) + ctxlen + 1;
> +		full_ctx = kzalloc(full_ctxlen, GFP_KERNEL);
> +		if (!full_ctx) {
> +			err = -ENOMEM;
> +			kfree(ctx);
> +			goto out_err;
> +		}
> +
> +		ptr = full_ctx;
> +		fsecctxs = (struct fuse_secctxs*) ptr;
> +		fsecctxs->nr_secctx = 1;
> +		ptr += sizeof(*fsecctxs);
> +
> +		fsecctx = (struct fuse_secctx*) ptr;
> +		fsecctx->size = ctxlen;
> +		ptr += sizeof(*fsecctx);
> +
> +		strcpy(ptr, name);
> +		ptr += strlen(name) + 1;
> +		memcpy(ptr, ctx, ctxlen);
> +		kfree(ctx);
> +	} else {
> +		full_ctxlen = sizeof(*fsecctxs);
> +		full_ctx = kzalloc(full_ctxlen, GFP_KERNEL);
> +		if (!full_ctx) {
> +			err = -ENOMEM;
> +			goto out_err;
> +		}
> +	}
> +
> +	*security_ctxlen = full_ctxlen;
> +	*security_ctx = full_ctx;
> +out_err:
> +	return err;
> +}
> +
>  /*
>   * Atomic create+open operation
>   *
> @@ -476,6 +539,8 @@ static int fuse_create_open(struct inode *dir, struct dentry *entry,
>  	struct fuse_entry_out outentry;
>  	struct fuse_inode *fi;
>  	struct fuse_file *ff;
> +	void *security_ctx = NULL;
> +	u32 security_ctxlen;
>  
>  	/* Userspace expects S_IFREG in create mode */
>  	BUG_ON((mode & S_IFMT) != S_IFREG);
> @@ -517,6 +582,18 @@ static int fuse_create_open(struct inode *dir, struct dentry *entry,
>  	args.out_args[0].value = &outentry;
>  	args.out_args[1].size = sizeof(outopen);
>  	args.out_args[1].value = &outopen;
> +
> +	if (fm->fc->init_security) {
> +		err = get_security_context(entry, mode, &security_ctx,
> +					   &security_ctxlen);
> +		if (err)
> +			goto out_put_forget_req;
> +
> +		args.in_numargs = 3;
> +		args.in_args[2].size = security_ctxlen;
> +		args.in_args[2].value = security_ctx;
> +	}
> +
>  	err = fuse_simple_request(fm, &args);
>  	if (err)
>  		goto out_free_ff;
> @@ -554,6 +631,7 @@ static int fuse_create_open(struct inode *dir, struct dentry *entry,
>  
>  out_free_ff:
>  	fuse_file_free(ff);
> +	kfree(security_ctx);
>  out_put_forget_req:
>  	kfree(forget);
>  out_err:
> @@ -613,13 +691,15 @@ static int fuse_atomic_open(struct inode *dir, struct dentry *entry,
>   */
>  static int create_new_entry(struct fuse_mount *fm, struct fuse_args *args,
>  			    struct inode *dir, struct dentry *entry,
> -			    umode_t mode)
> +			    umode_t mode, bool init_security)
>  {
>  	struct fuse_entry_out outarg;
>  	struct inode *inode;
>  	struct dentry *d;
>  	int err;
>  	struct fuse_forget_link *forget;
> +	void *security_ctx = NULL;
> +	u32 security_ctxlen = 0;
>  
>  	if (fuse_is_bad(dir))
>  		return -EIO;
> @@ -633,7 +713,29 @@ static int create_new_entry(struct fuse_mount *fm, struct fuse_args *args,
>  	args->out_numargs = 1;
>  	args->out_args[0].size = sizeof(outarg);
>  	args->out_args[0].value = &outarg;
> +
> +	if (init_security) {
> +		unsigned short idx = args->in_numargs;
> +
> +		if ((size_t)idx >= ARRAY_SIZE(args->in_args)) {
> +			err = -ENOMEM;
> +			goto out_put_forget_req;
> +		}
> +
> +		err = get_security_context(entry, mode, &security_ctx,
> +					   &security_ctxlen);
> +		if (err)
> +			goto out_put_forget_req;
> +
> +		if (security_ctxlen > 0) {
> +			args->in_args[idx].size = security_ctxlen;
> +			args->in_args[idx].value = security_ctx;
> +			args->in_numargs++;
> +		}
> +	}
> +
>  	err = fuse_simple_request(fm, args);
> +	kfree(security_ctx);
>  	if (err)
>  		goto out_put_forget_req;
>  
> @@ -691,7 +793,7 @@ static int fuse_mknod(struct user_namespace *mnt_userns, struct inode *dir,
>  	args.in_args[0].value = &inarg;
>  	args.in_args[1].size = entry->d_name.len + 1;
>  	args.in_args[1].value = entry->d_name.name;
> -	return create_new_entry(fm, &args, dir, entry, mode);
> +	return create_new_entry(fm, &args, dir, entry, mode, fm->fc->init_security);
>  }
>  
>  static int fuse_create(struct user_namespace *mnt_userns, struct inode *dir,
> @@ -719,7 +821,8 @@ static int fuse_mkdir(struct user_namespace *mnt_userns, struct inode *dir,
>  	args.in_args[0].value = &inarg;
>  	args.in_args[1].size = entry->d_name.len + 1;
>  	args.in_args[1].value = entry->d_name.name;
> -	return create_new_entry(fm, &args, dir, entry, S_IFDIR);
> +	return create_new_entry(fm, &args, dir, entry, S_IFDIR,
> +				fm->fc->init_security);
>  }
>  
>  static int fuse_symlink(struct user_namespace *mnt_userns, struct inode *dir,
> @@ -735,7 +838,8 @@ static int fuse_symlink(struct user_namespace *mnt_userns, struct inode *dir,
>  	args.in_args[0].value = entry->d_name.name;
>  	args.in_args[1].size = len;
>  	args.in_args[1].value = link;
> -	return create_new_entry(fm, &args, dir, entry, S_IFLNK);
> +	return create_new_entry(fm, &args, dir, entry, S_IFLNK,
> +				fm->fc->init_security);
>  }
>  
>  void fuse_update_ctime(struct inode *inode)
> @@ -915,7 +1019,8 @@ static int fuse_link(struct dentry *entry, struct inode *newdir,
>  	args.in_args[0].value = &inarg;
>  	args.in_args[1].size = newent->d_name.len + 1;
>  	args.in_args[1].value = newent->d_name.name;
> -	err = create_new_entry(fm, &args, newdir, newent, inode->i_mode);
> +	err = create_new_entry(fm, &args, newdir, newent, inode->i_mode,
> +			       false);
>  	/* Contrary to "normal" filesystems it can happen that link
>  	   makes two "logical" inodes point to the same "physical"
>  	   inode.  We invalidate the attributes of the old one, so it
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index 319596df5dc6..885f34f9967f 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -765,6 +765,9 @@ struct fuse_conn {
>  	/* Propagate syncfs() to server */
>  	unsigned int sync_fs:1;
>  
> +	/* Initialize security xattrs when creating a new inode */
> +	unsigned int init_security:1;
> +
>  	/** The number of requests waiting for completion */
>  	atomic_t num_waiting;
>  
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index 36cd03114b6d..343bc9cfbd92 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -1152,6 +1152,8 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
>  			}
>  			if (arg->flags & FUSE_SETXATTR_EXT)
>  				fc->setxattr_ext = 1;
> +			if (arg->flags & FUSE_SECURITY_CTX)
> +				fc->init_security = 1;
>  		} else {
>  			ra_pages = fc->max_read / PAGE_SIZE;
>  			fc->no_lock = 1;
> @@ -1195,7 +1197,7 @@ void fuse_send_init(struct fuse_mount *fm)
>  		FUSE_PARALLEL_DIROPS | FUSE_HANDLE_KILLPRIV | FUSE_POSIX_ACL |
>  		FUSE_ABORT_ERROR | FUSE_MAX_PAGES | FUSE_CACHE_SYMLINKS |
>  		FUSE_NO_OPENDIR_SUPPORT | FUSE_EXPLICIT_INVAL_DATA |
> -		FUSE_HANDLE_KILLPRIV_V2 | FUSE_SETXATTR_EXT;
> +		FUSE_HANDLE_KILLPRIV_V2 | FUSE_SETXATTR_EXT | FUSE_SECURITY_CTX;
>  #ifdef CONFIG_FUSE_DAX
>  	if (fm->fc->dax)
>  		ia->in.flags |= FUSE_MAP_ALIGNMENT;
> diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
> index 2fe54c80051a..b31a0f79fde8 100644
> --- a/include/uapi/linux/fuse.h
> +++ b/include/uapi/linux/fuse.h
> @@ -986,4 +986,24 @@ struct fuse_syncfs_in {
>  	uint64_t	padding;
>  };
>  
> +/*
> + * For each security context, send fuse_secctx with size of security context
> + * fuse_secctx will be followed by security context name and this in turn
> + * will be followed by actual context label.
> + * fuse_secctx, name, context
> + * */
> +struct fuse_secctx {
> +	uint32_t	size;
> +	uint32_t	padding;
> +};
> +
> +/*
> + * Contains the information about how many fuse_secctx structures are being
> + * sent.
> + */
> +struct fuse_secctxs {
> +	uint32_t	nr_secctx;
> +	uint32_t	padding;
> +};
> +
>  #endif /* _LINUX_FUSE_H */



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