Re: [Virtio-fs] [PATCH] virtiofsd: jail lo->proc_self_fd

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Apr 29, 2020 at 02:47:33PM +0200, Miklos Szeredi wrote:
> While it's not possible to escape the proc filesystem through
> lo->proc_self_fd, it is possible to escape to the root of the proc
> filesystem itself through "../..".

Hi Miklos,

So this attack will work with some form of *at(lo->proc_self_fd, "../..")
call?

> 
> Use a temporary mount for opening lo->proc_self_fd, that has it's root at
> /proc/self/fd/, preventing access to the ancestor directories.

Does this mean that now similar attack can happen using "../.." on tmpdir
fd instead and be able to look at peers of tmpdir. Or it is blocked
due to mount point or something else. 

Thanks
Vivek

> 
> Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
> ---
>  tools/virtiofsd/passthrough_ll.c | 27 +++++++++++++++++++++++++--
>  1 file changed, 25 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
> index 4c35c95b256c..bc9c44c760f4 100644
> --- a/tools/virtiofsd/passthrough_ll.c
> +++ b/tools/virtiofsd/passthrough_ll.c
> @@ -2536,6 +2536,8 @@ static void print_capabilities(void)
>  static void setup_namespaces(struct lo_data *lo, struct fuse_session *se)
>  {
>      pid_t child;
> +    char template[] = "virtiofsd-XXXXXX";
> +    char *tmpdir;
>  
>      /*
>       * Create a new pid namespace for *child* processes.  We'll have to
> @@ -2597,12 +2599,33 @@ static void setup_namespaces(struct lo_data *lo, struct fuse_session *se)
>          exit(1);
>      }
>  
> +    tmpdir = mkdtemp(template);
> +    if (!tmpdir) {
> +        fuse_log(FUSE_LOG_ERR, "tmpdir(%s): %m\n", template);
> +        exit(1);
> +    }
> +
> +    if (mount("/proc/self/fd", tmpdir, NULL, MS_BIND, NULL) < 0) {
> +        fuse_log(FUSE_LOG_ERR, "mount(/proc/self/fd, %s, MS_BIND): %m\n",
> +                 tmpdir);
> +        exit(1);
> +    }
> +
>      /* Now we can get our /proc/self/fd directory file descriptor */
> -    lo->proc_self_fd = open("/proc/self/fd", O_PATH);
> +    lo->proc_self_fd = open(tmpdir, O_PATH);
>      if (lo->proc_self_fd == -1) {
> -        fuse_log(FUSE_LOG_ERR, "open(/proc/self/fd, O_PATH): %m\n");
> +        fuse_log(FUSE_LOG_ERR, "open(%s, O_PATH): %m\n", tmpdir);
>          exit(1);
>      }
> +
> +    if (umount2(tmpdir, MNT_DETACH) < 0) {
> +        fuse_log(FUSE_LOG_ERR, "umount2(%s, MNT_DETACH): %m\n", tmpdir);
> +        exit(1);
> +    }
> +
> +    if (rmdir(tmpdir) < 0) {
> +        fuse_log(FUSE_LOG_ERR, "rmdir(%s): %m\n", tmpdir);
> +    }
>  }
>  
>  /*
> -- 
> 2.21.1
> 
> 
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