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Date: Thu, 29 Nov 2018 18:49:26 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "mhiramat@...nel.org" <mhiramat@...nel.org>
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Subject: Re: [PATCH 0/2] Don’t leave executable TLB entries to freed pages

On Thu, 2018-11-29 at 23:06 +0900, Masami Hiramatsu wrote:
> On Tue, 27 Nov 2018 16:07:52 -0800
> Rick Edgecombe <rick.p.edgecombe@...el.com> wrote:
> 
> > Sometimes when memory is freed via the module subsystem, an executable
> > permissioned TLB entry can remain to a freed page. If the page is re-used to
> > back an address that will receive data from userspace, it can result in user
> > data being mapped as executable in the kernel. The root of this behavior is
> > vfree lazily flushing the TLB, but not lazily freeing the underlying pages. 
> 
> Good catch!
> 
> > 
> > There are sort of three categories of this which show up across modules,
> > bpf,
> > kprobes and ftrace:
> 
> For x86-64 kprobe, it sets the page NX and after that RW, and then release
> via module_memfree. So I'm not sure it really happens on kprobes. (Of course
> the default memory allocator is simpler so it may happen on other archs) But
> interesting fixes.
Yes, I think you are right, it should not leave an executable TLB entry in this
case. Ftrace actually does this on x86 as well.

Is there some other reason for calling set_memory_nx that should apply elsewhere
for module users? Or could it be removed in the case of this patch to centralize
the behavior?

Thanks,

Rick

> Thank you,
> 
> 
> > 
> > 1. When executable memory is touched and then immediatly freed
> > 
> >    This shows up in a couple error conditions in the module loader and BPF
> > JIT
> >    compiler.
> > 
> > 2. When executable memory is set to RW right before being freed
> > 
> >    In this case (on x86 and probably others) there will be a TLB flush when
> > its
> >    set to RW and so since the pages are not touched between setting the
> >    flush and the free, it should not be in the TLB in most cases. So this
> >    category is not as big of a concern. However, techinically there is still
> > a
> >    race where an attacker could try to keep it alive for a short window with
> > a
> >    well timed out-of-bound read or speculative read, so ideally this could
> > be
> >    blocked as well.
> > 
> > 3. When executable memory is freed in an interrupt
> > 
> >    At least one example of this is the freeing of init sections in the
> > module
> >    loader. Since vmalloc reuses the allocation for the work queue linked
> > list
> >    node for the deferred frees, the memory actually gets touched as part of
> > the
> >    vfree operation and so returns to the TLB even after the flush from
> > resetting
> >    the permissions.
> > 
> > I have only actually tested category 1, and identified 2 and 3 just from
> > reading
> > the code.
> > 
> > To catch all of these, module_alloc for x86 is changed to use a new flag
> > that
> > instructs the unmap operation to flush the TLB before freeing the pages.
> > 
> > If this solution seems good I can plug the flag in for other architectures
> > that
> > define PAGE_KERNEL_EXEC.
> > 
> > 
> > Rick Edgecombe (2):
> >   vmalloc: New flag for flush before releasing pages
> >   x86/modules: Make x86 allocs to flush when free
> > 
> >  arch/x86/kernel/module.c |  4 ++--
> >  include/linux/vmalloc.h  |  1 +
> >  mm/vmalloc.c             | 13 +++++++++++--
> >  3 files changed, 14 insertions(+), 4 deletions(-)
> > 
> > -- 
> > 2.17.1
> > 
> 
> 

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