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

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Date:   Tue, 21 May 2019 11:01:31 -0400
From:   Vivek Goyal <vgoyal@...hat.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, linux-nvdimm@...ts.01.org,
        stefanha@...hat.com, dgilbert@...hat.com, swhiteho@...hat.com
Subject: Re: [PATCH v2 02/30] fuse: Clear setuid bit even in cache=never path

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
> 
>