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

[Amerigo Wang]

OGAWA Hirofumi wrote:
> Stephen Smalley <sds@xxxxxxxxxxxxx> 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.
>
>   
Ok, I know what you are talking about now...

Maybe I should make another patch to fix this... :-/ I need some time to 
understand SELinux, before I finish it, keeping this patch is fine.

Thanks!

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