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Subject: =?utf-8?Q?Re=3A_=5BPATCH_0/2=5D_Don=E2=80=99t_leave_executable_TL?=
 =?utf-8?Q?B_entries_to_freed_pages?=
From: Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <CDB8B7C1-FD55-44AD-9B71-B3A750BF5489@gmail.com>
Date: Tue, 27 Nov 2018 17:21:08 -0800
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Message-Id: <449E6648-5599-476D-8136-EE570101F930@gmail.com>
References: <20181128000754.18056-1-rick.p.edgecombe@intel.com>
 <CDB8B7C1-FD55-44AD-9B71-B3A750BF5489@gmail.com>
Sender: owner-linux-mm@kvack.org
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To: Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Andy Lutomirski <luto@kernel.org>, Will Deacon <will.deacon@arm.com>, linux-mm <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>, Kernel Hardening <kernel-hardening@lists.openwall.com>, naveen.n.rao@linux.vnet.ibm.com, anil.s.keshavamurthy@intel.com, David Miller <davem@davemloft.net>, Masami Hiramatsu <mhiramat@kernel.org>, rostedt@goodmis.org, mingo@redhat.com, ast@kernel.org, daniel@iogearbox.net, jeyu@kernel.org, netdev@vger.kernel.org, ard.biesheuvel@linaro.org, jannh@google.com, kristen@linux.intel.com, dave.hansen@intel.com, deneen.t.dock@intel.com

> On Nov 27, 2018, at 5:06 PM, Nadav Amit <nadav.amit@gmail.com> wrote:
>=20
>> On Nov 27, 2018, at 4:07 PM, Rick Edgecombe =
<rick.p.edgecombe@intel.com> wrote:
>>=20
>> 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.=20
>>=20
>> There are sort of three categories of this which show up across =
modules, bpf,
>> kprobes and ftrace:
>>=20
>> 1. When executable memory is touched and then immediatly freed
>>=20
>>  This shows up in a couple error conditions in the module loader and =
BPF JIT
>>  compiler.
>=20
> Interesting!
>=20
> Note that this may cause conflict with "x86: avoid W^X being broken =
during
> modules loading=E2=80=9D, which I recently submitted.

I actually have not looked on the vmalloc() code too much recent, but it
seems =E2=80=A6 strange:

  void vm_unmap_aliases(void)
  {      =20

  ...
  	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?