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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: <20181128095734.GA23467@arm.com>
Date:   Wed, 28 Nov 2018 10:29:52 -0800
Cc:     Rick Edgecombe <rick.p.edgecombe@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        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>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>, ast@kernel.org,
        Daniel Borkmann <daniel@iogearbox.net>, jeyu@kernel.org,
        netdev@vger.kernel.org, Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Jann Horn <jannh@google.com>, kristen@linux.intel.com,
        dave.hansen@intel.com, deneen.t.dock@intel.com
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Message-Id: <FB747B26-5C1A-4417-9DE9-8C356AB1C593@gmail.com>
References: <20181128000754.18056-1-rick.p.edgecombe@intel.com>
 <CDB8B7C1-FD55-44AD-9B71-B3A750BF5489@gmail.com>
 <449E6648-5599-476D-8136-EE570101F930@gmail.com>
 <20181128095734.GA23467@arm.com>
To:     Will Deacon <will.deacon@arm.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Nov 28, 2018, at 1:57 AM, Will Deacon <will.deacon@arm.com> wrote:
>=20
> 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@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.
>>=20
>> I actually have not looked on the vmalloc() code too much recent, but =
it
>> seems =E2=80=A6 strange:
>>=20
>>  void vm_unmap_aliases(void)
>>  {      =20
>>=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);
>>  }
>>=20
>> 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?
>=20
> 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