linux-kernel - Re: [Patch] allow file truncations when both suid and write permissions set

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Message-ID: <4A4BB898.10909@redhat.com>
Date:	Wed, 01 Jul 2009 14:27:20 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Amerigo Wang <amwang@...hat.com>
CC:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	Eugene Teo <eteo@...hat.com>, viro@...iv.linux.org.uk,
	Eric Paris <eparis@...hat.com>
Subject: Re: [Patch] allow file truncations when both suid and write permissions
 set

Eric Sandeen wrote:
> Amerigo Wang wrote:
>> When suid is set and the non-owner user has write permission,
>> any writing into this file should be allowed and suid should be
>> removed after that.
>>
>> However, current kernel only allows writing without truncations,
>> when we do truncations on that file, we get EPERM. This is a bug.

...

> So I think the main problem here is simply that we didn't set
> ATTR_FORCE, right...
> 
> Seems a little odd to |= with ret, -then- check if it's non-0.  Maybe:
> 
>  	/* Remove suid/sgid on truncate too */
> -	newattrs.ia_valid |= should_remove_suid(dentry);
> +	ret = should_remove_suid(dentry);
> +	if (ret)
> +		newattrs.ia_valid |= (ret | ATTR_FORCE);
> 

On second thought, and after talking w/ eparis, I think this probably
needs a security_inode_killpriv() too... it seems like it might be best
to change file_remove_suid(*file) to dentry_remove_suid(*dentry) and
just call that from do_truncate()?

-Eric
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