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Date:   Wed, 5 Dec 2018 16:32:54 +0100
From:   Jan Kara <jack@suse.cz>
To:     Alexander Lochmann <alexander.lochmann@tu-dortmund.de>
Cc:     viro@zeniv.linux.org.uk, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Horst Schirmeier <horst.schirmeier@tu-dortmund.de>,
        Jan Kara <jack@suse.cz>, linux-block@vger.kernel.org,
        Jens Axboe <axboe@kernel.dk>
Subject: Re: [PATCH] Fix sync. in inode_has_no_xattr()
Message-ID: <20181205153254.GD30615@quack2.suse.cz>
References: <13eaa1c6-def5-afad-89eb-6f149db90684@tu-dortmund.de>
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On Wed 05-12-18 12:45:06, Alexander Lochmann wrote:
> 
> inode.i_flags might be altered without proper
> synchronisation when the inode belongs to devtmpfs.
> The following stacktrace shows how to get there:
> 13: entry_SYSENTER_32:460
> 12: do_fast_syscall_32:410
> 11: _static_cpu_has:146
> 10: do_syscall_32_irqs_on:322
> 09: SyS_pwrite64:636
> 08: SYSC_pwrite64:650
> 07: fdput:38
> 06: vfs_write:560
> 05: __vfs_write:512
> 04: new_sync_write:500
> 03: blkdev_write_iter:1977
> 02: __generic_file_write_iter:2897
> 01: file_remove_privs:1818
> 00: inode_has_no_xattr:3163
> 
> Found by LockDoc (Alexander Lochmann, Horst Schirmeier and Olaf
> Spinczyk)
> 
> Signed-off-by: Alexander Lochmann <alexander.lochmann@tu-dortmund.de>
> Signed-off-by: Horst Schirmeier <horst.schirmeier@tu-dortmund.de>
...
> +/*
> + * blkdev_write_iter() can call this without i_rwsem, need to be
> + * careful with i_flags update.
> + */
>  static inline void inode_has_no_xattr(struct inode *inode)
>  {
>  	if (!is_sxid(inode->i_mode) && (inode->i_sb->s_flags & SB_NOSEC))
> -		inode->i_flags |= S_NOSEC;
> +		inode_set_flags(inode, S_NOSEC, S_NOSEC);
>  }

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).

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