linux-kernel - Re: [PATCH 0/2] Don’t leave executable TLB entries to freed pages

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Date:   Tue, 27 Nov 2018 17:21:08 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Will Deacon <will.deacon@....com>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        naveen.n.rao@...ux.vnet.ibm.com, anil.s.keshavamurthy@...el.com,
        David Miller <davem@...emloft.net>,
        Masami Hiramatsu <mhiramat@...nel.org>, rostedt@...dmis.org,
        mingo@...hat.com, ast@...nel.org, daniel@...earbox.net,
        jeyu@...nel.org, netdev@...r.kernel.org, ard.biesheuvel@...aro.org,
        jannh@...gle.com, kristen@...ux.intel.com, dave.hansen@...el.com,
        deneen.t.dock@...el.com
Subject: Re: [PATCH 0/2] Don’t leave executable TLB entries to freed pages

> On Nov 27, 2018, at 5:06 PM, Nadav Amit <nadav.amit@...il.com> wrote:
> 
>> On Nov 27, 2018, at 4:07 PM, 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. 
>> 
>> 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)
  {       

  ...
  	mutex_lock(&vmap_purge_lock);
  	purge_fragmented_blocks_allcpus();
  	if (!__purge_vmap_area_lazy(start, end) && flush)
  		flush_tlb_kernel_range(start, end);
  	mutex_unlock(&vmap_purge_lock);
  }

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?