linux-kernel - Re: [PATCH] Fix sync. in blkdev_write_iter() acessing i

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Date:   Sat, 8 Dec 2018 00:49:44 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Alexander Lochmann <alexander.lochmann@...dortmund.de>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jan Kara <jack@...e.cz>,
        Horst Schirmeier <horst.schirmeier@...dortmund.de>
Subject: Re: [PATCH] Fix sync. in blkdev_write_iter() acessing i_flags

On Fri, Dec 07, 2018 at 08:49:16PM +0100, Alexander Lochmann wrote:

> > _What_ SUID bit?  We are talking about a write to block device, for fsck sake...
> > 
> That's the way I understood Jan's explanation:
> "
> Thinking more about this I'm not sure if this is actually the right
> solution. Because for example the write(2) can set S_NOSEC flag wrongly
> when it would race with chmod adding SUID bit. So probably we rather need
> to acquire i_rwsem in blkdev_write_iter() if file does not have S_NOSEC set
> (we don't want to acquire it unconditionally as that would heavily impact
> scalability of block device writes).

	IDGI.  We are talking about a block device here.  What business could
file_remove_privs() have doing _anything_ to it?  should_remove_suid() returns
to return 0 for those; what case do you have in mind?  Somebody setting
security.capabilities on a block device inode?

	IMO the bug here is file_remove_privs() not buggering off immediately
after having observed that we are dealing with a block device.  It really
has nothing useful to do.