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Date:   Thu, 6 Dec 2018 20:19:35 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "luto@...nel.org" <luto@...nel.org>,
        "tycho@...ho.ws" <tycho@...ho.ws>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
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        "ast@...nel.org" <ast@...nel.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
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        "jannh@...gle.com" <jannh@...gle.com>,
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        "Dock, Deneen T" <deneen.t.dock@...el.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 11:19 -0800, Andy Lutomirski 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?
Hi,

Sorry, I've been having email troubles today.

I found some cases where vmap with PAGE_KERNEL_RO happens, which would not set
NP/RO in the directmap, so it would be sort of inconsistent whether the
directmap of vmalloc range allocations were readable or not. I couldn't see any
places where it would cause problems today though.

I was ready to assume that all TLBs don't cache NP, because I don't know how
usages where a page fault is used to load something could work without lots of
flushes. If that's the case, then all archs with directmap permissions could
share a single vmalloc special permission flush implementation that works like
Andy described originally. It could be controlled with an
ARCH_HAS_DIRECT_MAP_PERMS. We would just need something like set_pages_np and
set_pages_rw on any archs with directmap permissions. So seems simpler to me
(and what I have been doing) unless I'm missing the problem.

If you all think so I can indeed take a shot at it, I just don't see what the
problem was with the original solution, that seems less likely to break
anything.

Thanks,

Rick

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