From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx2.suse.de ([195.135.220.15]:59434 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727103AbeLEPc5 (ORCPT ); Wed, 5 Dec 2018 10:32:57 -0500 Date: Wed, 5 Dec 2018 16:32:54 +0100 From: Jan Kara To: Alexander Lochmann Cc: viro@zeniv.linux.org.uk, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Horst Schirmeier , Jan Kara , linux-block@vger.kernel.org, Jens Axboe 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <13eaa1c6-def5-afad-89eb-6f149db90684@tu-dortmund.de> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: 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 > Signed-off-by: Horst Schirmeier ... > +/* > + * 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 SUSE Labs, CR