linux-kernel - Re: [PATCH] vfs: Fix possible chmod/truncate race condition.

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Message-ID: <m31vy2bhd0.fsf@dmon-lap.sw.ru>
Date:	Mon, 27 Oct 2008 10:57:15 +0300
From:	Dmitri Monakhov <dmonakhov@...nvz.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vfs: Fix possible chmod/truncate race condition.

Andrew Morton <akpm@...ux-foundation.org> writes:

> On Wed, 15 Oct 2008 23:35:27 +0400 Dmitri Monakhov <dmonakhov@...nvz.org> wrote:
>
>> 
>> Signed-off-by: Dmitri Monakhov <dmonakhov@...nvz.org>
>> ---
>>  fs/open.c |    3 +--
>>  1 files changed, 1 insertions(+), 2 deletions(-)
>> 
>> diff --git a/fs/open.c b/fs/open.c
>> index 07da935..3423b94 100644
>> --- a/fs/open.c
>> +++ b/fs/open.c
>> @@ -214,10 +214,9 @@ int do_truncate(struct dentry *dentry, loff_t length, unsigned int time_attrs,
>>  		newattrs.ia_valid |= ATTR_FILE;
>>  	}
>>  
>> +	mutex_lock(&dentry->d_inode->i_mutex);
>>  	/* Remove suid/sgid on truncate too */
>>  	newattrs.ia_valid |= should_remove_suid(dentry);
>> -
>> -	mutex_lock(&dentry->d_inode->i_mutex);
>>  	err = notify_change(dentry, &newattrs);
>>  	mutex_unlock(&dentry->d_inode->i_mutex);
>>  	return err;
>
> OK, I give up.  What race?
i_mode read not protected by i_mutex in should_remove_suid() and we
may have race condition with chown(). Sorry, seems I've been too crazy
about this. The truth is:
1: No one will call truncate() and chown() concurrently.
2: This race is still possible regardless to this fix.
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