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Date: Tue, 27 Nov 2018 17:21:08 -0800
From: Nadav Amit <>
To: Rick Edgecombe <>
Cc: Andrew Morton <>,
 Andy Lutomirski <>,
 Will Deacon <>,
 linux-mm <>,
 LKML <>,
 Kernel Hardening <>,,,
 David Miller <>,
 Masami Hiramatsu <>,,,,,,,,,,,
Subject: Re: [PATCH 0/2] Don’t leave executable TLB entries to freed pages

> On Nov 27, 2018, at 5:06 PM, Nadav Amit <> wrote:
>> On Nov 27, 2018, at 4:07 PM, Rick Edgecombe <> 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. 
>> There are sort of three categories of this which show up across modules, bpf,
>> kprobes and ftrace:
>> 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.
> Interesting!
> Note that this may cause conflict with "x86: avoid W^X being broken during
> modules loading”, which I recently submitted.

I actually have not looked on the vmalloc() code too much recent, but it
seems … strange:

  void vm_unmap_aliases(void)

  	if (!__purge_vmap_area_lazy(start, end) && flush)
  		flush_tlb_kernel_range(start, end);

Since __purge_vmap_area_lazy() releases the memory, it seems there is a time
window between the release of the region and the TLB flush, in which the
area can be allocated for another purpose. This can result in a
(theoretical) correctness issue. No?

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