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Date:   Mon, 10 Dec 2018 10:47:22 +0100
From:   Jan Kara <jack@suse.cz>
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Alexander Lochmann <alexander.lochmann@tu-dortmund.de>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Jan Kara <jack@suse.cz>,
        Horst Schirmeier <horst.schirmeier@tu-dortmund.de>
Subject: Re: [PATCH] Fix sync. in blkdev_write_iter() acessing i_flags
Message-ID: <20181210094722.GB29289@quack2.suse.cz>
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 <20181208004944.GA2217@ZenIV.linux.org.uk>
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On Sat 08-12-18 00:49:44, Al Viro wrote:
> 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.

I didn't notice that S_ISREG() check in should_remove_suid(). My bad. And I
wasn't quite sure whether some security module does not rely on
inode_need_killpriv security hook. But now when I grep I see that
cap_inode_need_killpriv() is really the only user and security.capabilities
probably don't make sense for it since block devices cannot be executed
anyway.

So yes, the easiest fix is to just bail from file_remove_privs(). Probably
for anything that is not a regular file, right? Directories cannot be
written anyway and for pipes and character devices same logic applies as
for block devices.

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR