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

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Message-Id: <FB747B26-5C1A-4417-9DE9-8C356AB1C593@gmail.com>
Date:   Wed, 28 Nov 2018 10:29:52 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Will Deacon <will.deacon@....com>
Cc:     Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        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>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, ast@...nel.org,
        Daniel Borkmann <daniel@...earbox.net>, jeyu@...nel.org,
        netdev@...r.kernel.org, Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Jann Horn <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 28, 2018, at 1:57 AM, Will Deacon <will.deacon@....com> wrote:
> 
> On Tue, Nov 27, 2018 at 05:21:08PM -0800, Nadav Amit wrote:
>>> 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?
> 
> If __purge_vmap_area_lazy() returns false, then it hasn't freed the memory,
> so we only invalidate the TLB if 'flush' is true in that case. If
> __purge_vmap_area_lazy() returns true instead, then it takes care of the TLB
> invalidation before the freeing.

Right. Sorry for my misunderstanding.

Thanks,
Nadav