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Date: Fri, 7 Dec 2018 03:06:22 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "keescook@...omium.org" <keescook@...omium.org>, "luto@...nel.org"
	<luto@...nel.org>, "nadav.amit@...il.com" <nadav.amit@...il.com>
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	"Hansen, Dave" <dave.hansen@...el.com>
Subject: Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages

On Thu, 2018-12-06 at 15:08 -0800, Nadav Amit wrote:
> > On Dec 6, 2018, at 12:17 PM, Andy Lutomirski <luto@...nel.org> wrote:
> > 
> > On Thu, Dec 6, 2018 at 11:39 AM Nadav Amit <nadav.amit@...il.com> wrote:
> > > > On Dec 6, 2018, at 11:19 AM, Andy Lutomirski <luto@...nel.org> wrote:
> > > > 
> > > > On Thu, Dec 6, 2018 at 11:01 AM Tycho Andersen <tycho@...ho.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.
> > 
> > I am far from an expert on exploit techniques, but here's a
> > potentially useful model: let's assume there's an attacker who can
> > write controlled data to a controlled kernel address but cannot
> > directly modify control flow.  It would be nice for such an attacker
> > to have a very difficult time of modifying kernel text or of
> > compromising control flow.  So we're assuming a feature like kernel
> > CET or that the attacker finds it very difficult to do something like
> > modifying some thread's IRET frame.
> > 
> > Admittedly, for the kernel, this is an odd threat model, since an
> > attacker can presumably quite easily learn the kernel stack address of
> > one of their tasks, do some syscall, and then modify their kernel
> > thread's stack such that it will IRET right back to a fully controlled
> > register state with RSP pointing at an attacker-supplied kernel stack.
> > So this threat model gives very strong ROP powers. unless we have
> > either CET or some software technique to harden all the RET
> > instructions in the kernel.
> > 
> > I wonder if there's a better model to use.  Maybe with stack-protector
> > we get some degree of protection?  Or is all of this is rather weak
> > until we have CET or a RAP-like feature.
> 
> I believe that seeing the end-goal would make reasoning about patches
> easier, otherwise the complaint “but anyhow it’s all insecure” keeps popping
> up.
> 
> I’m not sure CET or other CFI would be enough even with this threat-model.
> The page-tables (the very least) need to be write-protected, as otherwise
> controlled data writes may just modify them. There are various possible
> solutions I presume: write_rare for page-tables, hypervisor-assisted
> security to obtain physical level NX/RO (a-la Microsoft VBS) or some sort of
> hardware enclave.
> 
> What do you think?

I am not sure which issue you are talking about. I think there are actually two
separate issues that are merged discussions from overlap of fix for the teardown
W^X window.

For the W^X stuff I had originally imagined the protection was for when an
attacker has a limited bug that could write to a location in the module space,
but not other locations due to only having the ability to overwrite part of a
pointer or some something like that. Then the module could execute the new code
as it ran normally after finishing loading. So that is why I was wondering about
the RW window during load. Still seems generally sensible to enforce W^X though.

I like your idea about something like text_poke to load modules. I think maybe
my modules KASLR patchset could help the above somewhat too since it loads at a
freshly randomized address.

Since the issue with the freed pages before flush (the original source of this
thread) doesn't require a write bug to insert the code, but does require a way
to jump to it, its kind of the opposite model of the above. So that's why I
think they are different.

I am still learning lots on kernel exploits though, maybe Kees can provide some
better insight here?

Thanks,

Rick




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