Re: [PATCH v5 7/9] virtiofsd: Create new file with fscreate set

[Vivek Goyal]

On Mon, Feb 07, 2022 at 11:38:12AM +0000, Dr. David Alan Gilbert wrote:
> * Vivek Goyal (vgoyal@redhat.com) wrote:
> > This patch adds support to set /proc/thread-self/attr/fscreate before
> > file creation. It is set to a value as sent by client. This will allow
> > for atomic creation of security context on files w.r.t file creation.
> > 
> > This is primarily useful when either there is no SELinux enabled on
> > host or host and guest policies are in sync and don't conflict.
> > 
> > Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> 
> Minor nit below, but I think this is right:
> 
> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> 
> I would however prefer if you could split this patch; it's a bit long to
> review.

Ok, I will look into splitting it.

> 
> 
> > ---
> >  tools/virtiofsd/passthrough_ll.c | 317 ++++++++++++++++++++++++++++---
> >  1 file changed, 290 insertions(+), 27 deletions(-)
> > 
> > diff --git a/tools/virtiofsd/passthrough_ll.c 
> > b/tools/virtiofsd/passthrough_ll.c
> > index 82023bf3d4..acb99aa2fc 100644
> > --- a/tools/virtiofsd/passthrough_ll.c
> > +++ b/tools/virtiofsd/passthrough_ll.c
> > @@ -173,10 +173,14 @@ struct lo_data {
> >  
> >      /* An O_PATH file descriptor to /proc/self/fd/ */
> >      int proc_self_fd;
> > +    /* An O_PATH file descriptor to /proc/self/task/ */
> > +    int proc_self_task;
> >      int user_killpriv_v2, killpriv_v2;
> >      /* If set, virtiofsd is responsible for setting umask during creation 
> > */
> >      bool change_umask;
> >      int user_posix_acl, posix_acl;
> > +    /* Keeps track if /proc/<pid>/attr/fscreate should be used or not */
> > +    bool use_fscreate;
> >  };
> >  
> >  static const struct fuse_opt lo_opts[] = {
> > @@ -230,6 +234,11 @@ static struct lo_inode *lo_find(struct lo_data *lo, 
> > struct stat *st,
> >  static int xattr_map_client(const struct lo_data *lo, const char 
> > *client_name,
> >                              char **out_name);
> >  
> > +#define FCHDIR_NOFAIL(fd) do {                         \
> > +        int fchdir_res = fchdir(fd);                   \
> > +        assert(fchdir_res == 0);                       \
> > +    } while (0)
> > +
> >  static bool is_dot_or_dotdot(const char *name)
> >  {
> >      return name[0] == '.' &&
> > @@ -256,6 +265,33 @@ static struct lo_data *lo_data(fuse_req_t req)
> >      return (struct lo_data *)fuse_req_userdata(req);
> >  }
> >  
> > +/*
> > + * Tries to figure out if /proc/<pid>/attr/fscrate is usable or not. With
> > + * selinux=0, read from fscreate returns -EINVAL.
> > + *
> > + * TODO: Link with libselinux and use is_selinux_enabled() instead down
> > + * the line. It probably will be more reliable indicator.
> > + */
> > +static bool is_fscreate_usable(struct lo_data *lo)
> > +{
> > +    char procname[64];
> > +    int fscreate_fd;
> > +    size_t bytes_read;
> > +
> > +    sprintf(procname, "%d/attr/fscreate", gettid());
> > +    fscreate_fd = openat(lo->proc_self_task, procname, O_RDWR);
> > +    if (fscreate_fd == -1) {
> > +        return false;
> > +    }
> > +
> > +    bytes_read = read(fscreate_fd, procname, 64);
> > +    close(fscreate_fd);
> > +    if (bytes_read == -1) {
> > +        return false;
> > +    }
> > +    return true;
> > +}
> > +
> >  /*
> >   * Load capng's state from our saved state if the current thread
> >   * hadn't previously been loaded.
> > @@ -1284,16 +1320,140 @@ static void lo_restore_cred_gain_cap(struct 
> > lo_cred *old, bool restore_umask,
> >      }
> >  }
> >  
> > +/* Helpers to set/reset fscreate */
> > +static int open_set_proc_fscreate(struct lo_data *lo, const void *ctx,
> > +                                  size_t ctxlen, int *fd)
> > +{
> > +    char procname[64];
> > +    int fscreate_fd, err = 0;
> > +    size_t written;
> > +
> > +    sprintf(procname, "%d/attr/fscreate", gettid());
> > +    fscreate_fd = openat(lo->proc_self_task, procname, O_WRONLY);
> > +    err = fscreate_fd == -1 ? errno : 0;
> > +    if (err) {
> > +        return err;
> > +    }
> > +
> > +    written = write(fscreate_fd, ctx, ctxlen);
> > +    err = written == -1 ? errno : 0;
> > +    if (err) {
> > +        goto out;
> > +    }
> > +
> > +    *fd = fscreate_fd;
> > +    return 0;
> > +out:
> > +    close(fscreate_fd);
> > +    return err;
> > +}
> > +
> > +static void close_reset_proc_fscreate(int fd)
> > +{
> > +    if ((write(fd, NULL, 0)) == -1) {
> > +        fuse_log(FUSE_LOG_WARNING, "Failed to reset fscreate. err=%d\n", 
> > errno);
> > +    }
> > +    close(fd);
> > +    return;
> > +}
> > +
> > +static int do_mknod_symlink_secctx(fuse_req_t req, struct lo_inode *dir,
> > +                                   const char *name, const char 
> > *secctx_name)
> > +{
> > +    int path_fd, err;
> > +    char procname[64];
> > +    struct lo_data *lo = lo_data(req);
> > +
> > +    if (!req->secctx.ctxlen) {
> > +        return 0;
> > +    }
> > +
> > +    /* Open newly created element with O_PATH */
> > +    path_fd = openat(dir->fd, name, O_PATH | O_NOFOLLOW);
> > +    err = path_fd == -1 ? errno : 0;
> > +    if (err) {
> > +        return err;
> > +    }
> > +    sprintf(procname, "%i", path_fd);
> > +    FCHDIR_NOFAIL(lo->proc_self_fd);
> > +    /* Set security context. This is not atomic w.r.t file creation */
> > +    err = setxattr(procname, secctx_name, req->secctx.ctx, 
> > req->secctx.ctxlen,
> > +                   0);
> > +    if (err) {
> > +        err = errno;
> > +    }
> > +    FCHDIR_NOFAIL(lo->root.fd);
> > +    close(path_fd);
> > +    return err;
> > +}
> > +
> > +static int do_mknod_symlink(fuse_req_t req, struct lo_inode *dir,
> > +                            const char *name, mode_t mode, dev_t rdev,
> > +                            const char *link)
> > +{
> > +    int err, fscreate_fd = -1;
> > +    const char *secctx_name = req->secctx.name;
> > +    struct lo_cred old = {};
> > +    struct lo_data *lo = lo_data(req);
> > +    char *mapped_name = NULL;
> > +    bool secctx_enabled = req->secctx.ctxlen;
> > +    bool do_fscreate = false;
> > +
> > +    if (secctx_enabled && lo->xattrmap) {
> > +        err = xattr_map_client(lo, req->secctx.name, &mapped_name);
> > +        if (err < 0) {
> > +            return -err;
> > +        }
> > +        secctx_name = mapped_name;
> > +    }
> > +
> > +    /*
> > +     * If security xattr has not been remapped and selinux is enabled on
> > +     * host, set fscreate and no need to do a setxattr() after file 
> > creation
> > +     */
> > +    if (secctx_enabled && !mapped_name && lo->use_fscreate) {
> > +        do_fscreate = true;
> > +        err = open_set_proc_fscreate(lo, req->secctx.ctx, 
> > req->secctx.ctxlen,
> > +                                     &fscreate_fd);
> > +        if (err) {
> > +            goto out;
> > +        }
> > +    }
> > +
> > +    err = lo_change_cred(req, &old, lo->change_umask && !S_ISLNK(mode));
> > +    if (err) {
> > +        goto out;
> > +    }
> > +
> > +    err = mknod_wrapper(dir->fd, name, link, mode, rdev);
> > +    err = err == -1 ? errno : 0;
> > +    lo_restore_cred(&old, lo->change_umask && !S_ISLNK(mode));
> > +    if (err) {
> > +        goto out;
> > +    }
> > +
> > +    if (!do_fscreate) {
> > +        err = do_mknod_symlink_secctx(req, dir, name, secctx_name);
> > +        if (err) {
> > +            unlinkat(dir->fd, name, S_ISDIR(mode) ? AT_REMOVEDIR : 0);
> > +        }
> > +    }
> > +out:
> > +    if (fscreate_fd != -1) {
> > +        close_reset_proc_fscreate(fscreate_fd);
> > +    }
> > +    g_free(mapped_name);
> > +    return err;
> > +}
> > +
> >  static void lo_mknod_symlink(fuse_req_t req, fuse_ino_t parent,
> >                               const char *name, mode_t mode, dev_t rdev,
> >                               const char *link)
> >  {
> > -    int res;
> >      int saverr;
> >      struct lo_data *lo = lo_data(req);
> >      struct lo_inode *dir;
> >      struct fuse_entry_param e;
> > -    struct lo_cred old = {};
> >  
> >      if (is_empty(name)) {
> >          fuse_reply_err(req, ENOENT);
> > @@ -1311,21 +1471,11 @@ static void lo_mknod_symlink(fuse_req_t req, 
> > fuse_ino_t parent,
> >          return;
> >      }
> >  
> > -    saverr = lo_change_cred(req, &old, lo->change_umask && !S_ISLNK(mode));
> > +    saverr = do_mknod_symlink(req, dir, name, mode, rdev, link);
> >      if (saverr) {
> >          goto out;
> >      }
> >  
> > -    res = mknod_wrapper(dir->fd, name, link, mode, rdev);
> > -
> > -    saverr = errno;
> > -
> > -    lo_restore_cred(&old, lo->change_umask && !S_ISLNK(mode));
> > -
> > -    if (res == -1) {
> > -        goto out;
> > -    }
> > -
> >      saverr = lo_do_lookup(req, parent, name, &e, NULL);
> >      if (saverr) {
> >          goto out;
> > @@ -2001,13 +2151,16 @@ static int lo_do_open(struct lo_data *lo, struct 
> > lo_inode *inode,
> >      return 0;
> >  }
> >  
> > -static int do_lo_create(fuse_req_t req, struct lo_inode *parent_inode,
> > -                        const char *name, mode_t mode,
> > -                        struct fuse_file_info *fi, int* open_fd)
> > +static int do_create_nosecctx(fuse_req_t req, struct lo_inode 
> > *parent_inode,
> > +                               const char *name, mode_t mode,
> > +                               struct fuse_file_info *fi, int *open_fd)
> >  {
> > -    int err = 0, fd;
> > +    int err, fd;
> >      struct lo_cred old = {};
> >      struct lo_data *lo = lo_data(req);
> > +    int flags;
> > +
> > +    flags = fi->flags | O_CREAT | O_EXCL;
> >  
> >      err = lo_change_cred(req, &old, lo->change_umask);
> >      if (err) {
> > @@ -2015,13 +2168,106 @@ static int do_lo_create(fuse_req_t req, struct 
> > lo_inode *parent_inode,
> >      }
> >  
> >      /* Try to create a new file but don't open existing files */
> > -    fd = openat(parent_inode->fd, name, fi->flags | O_CREAT | O_EXCL, 
> > mode);
> > -    if (fd == -1) {
> > -        err = errno;
> > -    } else {
> > +    fd = openat(parent_inode->fd, name, flags, mode);
> > +    err = fd == -1 ? errno : 0;
> > +    lo_restore_cred(&old, lo->change_umask);
> > +    if (!err) {
> >          *open_fd = fd;
> >      }
> > -    lo_restore_cred(&old, lo->change_umask);
> > +    return err;
> > +}
> > +
> > +static int do_create_secctx_fscreate(fuse_req_t req,
> > +                                     struct lo_inode *parent_inode,
> > +                                     const char *name, mode_t mode,
> > +                                     struct fuse_file_info *fi, int 
> > *open_fd)
> > +{
> > +    int err = 0, fd = -1, fscreate_fd = -1;
> > +    struct lo_data *lo = lo_data(req);
> > +
> > +    err = open_set_proc_fscreate(lo, req->secctx.ctx, req->secctx.ctxlen,
> > +                                 &fscreate_fd);
> > +    if (err) {
> > +        return err;
> > +    }
> > +
> > +    err = do_create_nosecctx(req, parent_inode, name, mode, fi, &fd);
> > +
> > +    close_reset_proc_fscreate(fscreate_fd);
> > +    if (!err) {
> > +        *open_fd = fd;
> > +    }
> > +    return err;
> > +}
> > +
> > +static int do_create_secctx_noatomic(fuse_req_t req,
> > +                                     struct lo_inode *parent_inode,
> > +                                     const char *name, mode_t mode,
> > +                                     struct fuse_file_info *fi,
> > +                                     const char *secctx_name, int *open_fd)
> > +{
> > +    int err = 0, fd = -1;
> > +
> > +    err = do_create_nosecctx(req, parent_inode, name, mode, fi, &fd);
> > +    if (err) {
> > +        goto out;
> > +    }
> > +
> > +    /* Set security context. This is not atomic w.r.t file creation */
> > +    err = fsetxattr(fd, secctx_name, req->secctx.ctx, req->secctx.ctxlen, 
> > 0);
> > +    err = err == -1 ? errno : 0;
> > +out:
> > +    if (!err) {
> > +        *open_fd = fd;
> > +    } else {
> > +        if (fd != -1) {
> > +            close(fd);
> > +            unlinkat(parent_inode->fd, name, 0);
> > +        }
> > +    }
> > +    return err;
> > +}
> > +
> > +static int do_lo_create(fuse_req_t req, struct lo_inode *parent_inode,
> > +                        const char *name, mode_t mode,
> > +                        struct fuse_file_info *fi, int *open_fd)
> > +{
> > +    struct lo_data *lo = lo_data(req);
> > +    char *mapped_name = NULL;
> > +    int err;
> > +    const char *ctxname = req->secctx.name;
> > +    bool secctx_enabled = req->secctx.ctxlen;
> > +
> > +    if (secctx_enabled && lo->xattrmap) {
> > +        err = xattr_map_client(lo, req->secctx.name, &mapped_name);
> > +        if (err < 0) {
> > +            return -err;
> > +        }
> > +
> > +        ctxname = mapped_name;
> > +    }
> > +
> > +    if (secctx_enabled) {
> > +        /*
> > +         * If security.selinux has not been remapped and selinux is 
> > enabled,
> > +         * use fscreate to set context before file creation.
> > +         * Otherwise fallback to non-atomic method of file creation
> > +         * and xattr settting.
> > +         */
> > +        if (!mapped_name && lo->use_fscreate) {
> > +            err = do_create_secctx_fscreate(req, parent_inode, name, mode, 
> > fi,
> > +                                            open_fd);
> > +            goto out;
> 
> This can turn into another nested } else { and you don't need the out:
> 

I guess I will have to put do_create_secctx_noatomic() into an else{} to
get rid of "goto out". The downside is that in second else loop, I
need to fallback non-atomic secctx creation. That means I will have
to call do_create_secctx_noatomic() one more time inside second else
block to fallback.

I feel I like current structure better until and unless you strongly
feel about it.

Thanks
Vivek

> Dave
> > +        }
> > +
> > +        err = do_create_secctx_noatomic(req, parent_inode, name, mode, fi,
> > +                                        ctxname, open_fd);
> > +    } else {
> > +        err = do_create_nosecctx(req, parent_inode, name, mode, fi, 
> > open_fd);
> > +    }
> > +
> > +out:
> > +    g_free(mapped_name);
> >      return err;
> >  }
> >  
> > @@ -2856,11 +3102,6 @@ static int xattr_map_server(const struct lo_data 
> > *lo, const char *server_name,
> >      return -ENODATA;
> >  }
> >  
> > -#define FCHDIR_NOFAIL(fd) do {                         \
> > -        int fchdir_res = fchdir(fd);                   \
> > -        assert(fchdir_res == 0);                       \
> > -    } while (0)
> > -
> >  static bool block_xattr(struct lo_data *lo, const char *name)
> >  {
> >      /*
> > @@ -3522,6 +3763,15 @@ static void setup_namespaces(struct lo_data *lo, 
> > struct fuse_session *se)
> >          exit(1);
> >      }
> >  
> > +    /* Get the /proc/self/task descriptor */
> > +    lo->proc_self_task = open("/proc/self/task/", O_PATH);
> > +    if (lo->proc_self_task == -1) {
> > +        fuse_log(FUSE_LOG_ERR, "open(/proc/self/task, O_PATH): %m\n");
> > +        exit(1);
> > +    }
> > +
> > +    lo->use_fscreate = is_fscreate_usable(lo);
> > +
> >      /*
> >       * We only need /proc/self/fd. Prevent ".." from accessing parent
> >       * directories of /proc/self/fd by bind-mounting it over /proc. Since 
> > / was
> > @@ -3738,6 +3988,14 @@ static void setup_chroot(struct lo_data *lo)
> >          exit(1);
> >      }
> >  
> > +    lo->proc_self_task = open("/proc/self/task", O_PATH);
> > +    if (lo->proc_self_fd == -1) {
> > +        fuse_log(FUSE_LOG_ERR, "open(\"/proc/self/task\", O_PATH): %m\n");
> > +        exit(1);
> > +    }
> > +
> > +    lo->use_fscreate = is_fscreate_usable(lo);
> > +
> >      /*
> >       * Make the shared directory the file system root so that FUSE_OPEN
> >       * (lo_open()) cannot escape the shared directory by opening a symlink.
> > @@ -3923,6 +4181,10 @@ static void fuse_lo_data_cleanup(struct lo_data *lo)
> >          close(lo->proc_self_fd);
> >      }
> >  
> > +    if (lo->proc_self_task >= 0) {
> > +        close(lo->proc_self_task);
> > +    }
> > +
> >      if (lo->root.fd >= 0) {
> >          close(lo->root.fd);
> >      }
> > @@ -3950,6 +4212,7 @@ int main(int argc, char *argv[])
> >          .posix_lock = 0,
> >          .allow_direct_io = 0,
> >          .proc_self_fd = -1,
> > +        .proc_self_task = -1,
> >          .user_killpriv_v2 = -1,
> >          .user_posix_acl = -1,
> >      };
> > -- 
> > 2.34.1
> > 
> -- 
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>