Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages

[Nadav Amit <nadav.amit@gmail.com>]

> On Dec 6, 2018, at 11:19 AM, Andy Lutomirski <luto@kernel.org> wrote:
> 
> On Thu, Dec 6, 2018 at 11:01 AM Tycho Andersen <tycho@tycho.ws> wrote:
>> On Thu, Dec 06, 2018 at 10:53:50AM -0800, Andy Lutomirski wrote:
>>>> If we are going to unmap the linear alias, why not do it at vmalloc()
>>>> time rather than vfree() time?
>>> 
>>> That’s not totally nuts. Do we ever have code that expects __va() to
>>> work on module data?  Perhaps crypto code trying to encrypt static
>>> data because our APIs don’t understand virtual addresses.  I guess if
>>> highmem is ever used for modules, then we should be fine.
>>> 
>>> RO instead of not present might be safer.  But I do like the idea of
>>> renaming Rick's flag to something like VM_XPFO or VM_NO_DIRECT_MAP and
>>> making it do all of this.
>> 
>> Yeah, doing it for everything automatically seemed like it was/is
>> going to be a lot of work to debug all the corner cases where things
>> expect memory to be mapped but don't explicitly say it. And in
>> particular, the XPFO series only does it for user memory, whereas an
>> additional flag like this would work for extra paranoid allocations
>> of kernel memory too.
> 
> I just read the code, and I looks like vmalloc() is already using
> highmem (__GFP_HIGH) if available, so, on big x86_32 systems, for
> example, we already don't have modules in the direct map.
> 
> So I say we go for it.  This should be quite simple to implement --
> the pageattr code already has almost all the needed logic on x86.  The
> only arch support we should need is a pair of functions to remove a
> vmalloc address range from the address map (if it was present in the
> first place) and a function to put it back.  On x86, this should only
> be a few lines of code.
> 
> What do you all think?  This should solve most of the problems we have.
> 
> If we really wanted to optimize this, we'd make it so that
> module_alloc() allocates memory the normal way, then, later on, we
> call some function that, all at once, removes the memory from the
> direct map and applies the right permissions to the vmalloc alias (or
> just makes the vmalloc alias not-present so we can add permissions
> later without flushing), and flushes the TLB.  And we arrange for
> vunmap to zap the vmalloc range, then put the memory back into the
> direct map, then free the pages back to the page allocator, with the
> flush in the appropriate place.
> 
> I don't see why the page allocator needs to know about any of this.
> It's already okay with the permissions being changed out from under it
> on x86, and it seems fine.  Rick, do you want to give some variant of
> this a try?

Setting it as read-only may work (and already happens for the read-only
module data). I am not sure about setting it as non-present.

At some point, a discussion about a threat-model, as Rick indicated, would
be required. I presume ROP attacks can easily call set_all_modules_text_rw()
and override all the protections.