Re: [PATCH] MIPS: Grant pte read permission, even if vma only have VM_WRITE permission.

["Maciej W. Rozycki" <macro@wdc.com>]

On Fri, 24 Jul 2020, Lichao Liu wrote:

> > IMHO it's exactly the point of RIXI enabled CPUs to support a
> > writeonly mapping even if most of other archs aren't able to
> > support it. So if there is no real good reason to change this,
> > I'm going to leave it this way.
[...]
> I think there are have two solutions to the problem:
> 1)modify fault_in_user_writeable(), 
>   must claim read permission when claiming write permission.
> 2)Grant pte read permission, even if vma only have VM_WRITE permission.
> 
> But not sure which one is more suitable.

 Well, the internal documentation is clear:

 * fault_in_user_writeable() - Fault in user address and verify RW access

so if it does only verify W rather than RW access, then it has to be fixed 
and verify both kinds of access at a time.  Presumably:

	mmap_read_lock(mm);
	ret = fixup_user_fault(current, mm, (unsigned long)uaddr, 0, NULL);
	if (!ret)
		ret = fixup_user_fault(current, mm, (unsigned long)uaddr,
				       FAULT_FLAG_WRITE, NULL);
        mmap_read_unlock(mm);

at the minimum or perhaps by expanding the interface of `fixup_user_fault' 
to also support FAULT_FLAG_RW so as to avoid the double call.

 As Thomas says silently expanding access permissions beyond what has been 
granted would be a security breach.

  Maciej