Re: [Patch v3] vfs: allow file truncations when both suid and write permissions set

[OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>]

Stephen Smalley <sds@tycho.nsa.gov> writes:

>> > SELinux shouldn't apply a permission check for the clearing of the suid
>> > bit on write or truncate.  It should only apply a permission check for
>> > the actual truncate or write operation, and then the clearing of the
>> > suid bit should always be forced if that check passed.
>> 
>> Ok. Yes. So, to do it efficiently without problem, I'm suggesting the
>> following or something (I don't know whether LSM should do this or not).
>> 
>> selinux_inode_setattr(),
>> 
>> 	ia_valid = iattr->ia_valid;
>> 	if (!(ia_valid & ATTR_FORCE) && (ia_valid & ATTR_FORCE_MASK)) {
>> 		err = dentry_has_perm(cred, NULL, dentry, FILE__SETATTR);
>> 		if (err)
>>                 	return err;
>> 		ia_valid &= ~ATTR_FORCE_MASK;
>> 	}
>> 	if (ia_valid & ATTR_NOT_FORCE_MASK)
>> 	 	err = dentry_has_perm(cred, NULL, dentry, FILE__WRITE);
>> 	return err;
>> 
>> I guess ATTR_FORCE_MASK would be (ATTR_MODE | ATTR_UID | ATTR_GID |
>> 			ATTR_ATIME_SET | ATTR_MTIME_SET) or something,
>> and ATTR_NOT_FORCE_MASK would be ATTR_SIZE or something.
>> 
>> I'm not sure this is the right code what selinux want to do though, but,
>> I hope it is clear what I want to say. (I'm assuming FILE__WRITE is for
>> check of ATTR_SIZE)
>
> The logic is supposed to map certain attribute changes (mode, owner,
> group, explicit setting of atime or mtime to a specific value rather
> than the current time) to the SELinux setattr permission, while mapping
> other attribute changes that occur naturally on a write (size, setting
> of mtime to current time) to the SELinux write permission.  That doesn't
> seem clear from using ATTR_FORCE_MASK vs ATTR_NOT_FORCE_MASK above - I'd
> use different naming conventions for clarity.

I see. Yes, the naming of this code doesn't matter at all. The code was
just intended to explain what I'm suggesting.

>> With this change, the caller can pass "(ATTR_SIZE | ATTR_MODE)" or
>> "(ATTR_SIZE | ATTR_MODE | ATTR_FORCE)" etc. for truncate().
>> 
>> [btw, "(ATTR_SIZE | ATTR_MODE)" is what do_truncate() does currently].
>
> That was a change in do_truncate(), commit
> 7b82dc0e64e93f430182f36b46b79fcee87d3532.
>
> It makes sense, but no one ever updated selinux_inode_setattr() to match
> that change.

I see. Yes, exactly. And for the user of non file owner case, I'm
thinking we would like to pass ATTR_FORCE too.

Thanks.
-- 
OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>