Re: [PATCH v2 02/30] fuse: Clear setuid bit even in cache=never path

Vivek Goyal
2019-05-21T15:01:51Z

On Mon, May 20, 2019 at 04:41:37PM +0200, Miklos Szeredi wrote:
> On Wed, May 15, 2019 at 03:26:47PM -0400, Vivek Goyal wrote:
> > If fuse daemon is started with cache=never, fuse falls back to direct IO.
> > In that write path we don't call file_remove_privs() and that means setuid
> > bit is not cleared if unpriviliged user writes to a file with setuid bit set.
> >
> > pjdfstest chmod test 12.t tests this and fails.
>
> I think better sulution is to tell the server if the suid bit needs to be
> removed, so it can do so in a race free way.
>
> Here's the kernel patch, and I'll reply with the libfuse patch.

Hi Miklos,

I tested and it works for me.

Vivek

>
> ---
> fs/fuse2/file.c | 2 ++
> include/uapi/linux/fuse.h | 3 +++
> 2 files changed, 5 insertions(+)
>
> --- a/fs/fuse2/file.c
> +++ b/fs/fuse2/file.c
> @@ -363,6 +363,8 @@ static ssize_t fuse_send_write(struct fu
> inarg->flags |= O_DSYNC;
> if (iocb->ki_flags & IOCB_SYNC)
> inarg->flags |= O_SYNC;
> + if (!capable(CAP_FSETID))
> + inarg->write_flags |= FUSE_WRITE_KILL_PRIV;
> req->inh.opcode = FUSE_WRITE;
> req->inh.nodeid = ff->nodeid;
> req->inh.len = req->inline_inlen + count;
> --- a/include/uapi/linux/fuse.h
> +++ b/include/uapi/linux/fuse.h
> @@ -125,6 +125,7 @@
> *
> * 7.29
> * - add FUSE_NO_OPENDIR_SUPPORT flag
> + * - add FUSE_WRITE_KILL_PRIV flag
> */
>
> #ifndef _LINUX_FUSE_H
> @@ -318,9 +319,11 @@ struct fuse_file_lock {
> *
> * FUSE_WRITE_CACHE: delayed write from page cache, file handle is guessed
> * FUSE_WRITE_LOCKOWNER: lock_owner field is valid
> + * FUSE_WRITE_KILL_PRIV: kill suid and sgid bits
> */
> #define FUSE_WRITE_CACHE (1 << 0)
> #define FUSE_WRITE_LOCKOWNER (1 << 1)
> +#define FUSE_WRITE_KILL_PRIV (1 << 2)
>
> /**
> * Read flags
>
>