[kernel-hardening] Re: [RFC PATCH 6/6] arm64: add VMAP_STACK and detect out-of-bounds SP

[labbott@redhat.com (Laura Abbott)]

On 07/20/2017 10:30 AM, Ard Biesheuvel wrote:
> On 20 July 2017 at 09:56, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
>> On 20 July 2017 at 09:36, James Morse <james.morse@arm.com> wrote:
>>> Hi Ard,
>>>
>>> On 20/07/17 06:35, Ard Biesheuvel wrote:
>>>> On 20 July 2017 at 00:32, Laura Abbott <labbott@redhat.com> wrote:
>>>>> I didn't notice any performance impact but I also wasn't trying that
>>>>> hard. I did try this with a different configuration and ran into
>>>>> stackspace errors almost immediately:
>>>>>
>>>>> [ 0.358026] smp: Brought up 1 node, 8 CPUs
>>>>> [ 0.359359] SMP: Total of 8 processors activated.
>>>>> [ 0.359542] CPU features: detected feature: 32-bit EL0 Support
>>>>> [    0.361781] Insufficient stack space to handle exception!
>>>
>>> [...]
>>>
>>>>> [    0.367382] Task stack: [0xffffff8008e80000..0xffffff8008e84000]
>>>>> [    0.367519] IRQ stack:  [0xffffffc03bf62000..0xffffffc03bf66000]
>>>>
>>>> The IRQ stack is not 16K aligned ...
>>>
>>>>> [    0.367687] ESR: 0x00000000 -- Unknown/Uncategorized
>>>>> [    0.367868] FAR: 0x0000000000000000
>>>>> [    0.368059] Kernel panic - not syncing: kernel stack overflow
>>>>> [    0.368252] CPU: 4 PID: 0 Comm: swapper/4 Not tainted 4.12.0-00018-ge9cf49d604ef-dirty #23
>>>>> [    0.368427] Hardware name: linux,dummy-virt (DT)
>>>>> [    0.368612] Call trace:
>>>>> [    0.368774] [<ffffff8008087fd8>] dump_backtrace+0x0/0x228
>>>>> [    0.368979] [<ffffff80080882c8>] show_stack+0x10/0x20
>>>>> [    0.369270] [<ffffff80084602dc>] dump_stack+0x88/0xac
>>>>> [    0.369459] [<ffffff800816328c>] panic+0x120/0x278
>>>>> [    0.369582] [<ffffff8008088b40>] handle_bad_stack+0xd0/0xd8
>>>>> [    0.369799] [<ffffff80080bfb94>] __do_softirq+0x74/0x210
>>>>> [    0.370560] SMP: stopping secondary CPUs
>>>>> [    0.384269] Rebooting in 5 seconds..
>>>>>
>>>>> The config is based on what I use for booting my Hikey android
>>>>> board. I haven't been able to narrow down exactly which
>>>>> set of configs set this off.
>>>>>
>>>>
>>>> ... so for some reason, the percpu atom size change fails to take effect here.
>>>
>>> I'm not completely up to speed with these series, so this may be noise:
>>>
>>> When we added the IRQ stack Jungseok Lee discovered that alignment greater than
>>> PAGE_SIZE only applies to CPU0. Secondary CPUs read the per-cpu init data into a
>>> page-aligned area, but any greater alignment requirement is lost.
>>>
>>> Because of this the irqstack was only 16byte aligned, and struct thread_info had
>>> to be be discovered without depending on stack alignment.
>>>
>>
>> We [attempted to] address that by increasing the per-CPU atom size to
>> THREAD_ALIGN if CONFIG_VMAP_STACK=y, but as I am typing this, I wonder
>> if that percolates all the way down to the actual vmap() calls. I will
>> investigate ...
> 
> The issue is easily reproducible in QEMU as well, when building from
> the same config. I tracked it down to CONFIG_NUMA=y, which sets
> CONFIG_NEED_PER_CPU_EMBED_FIRST_CHUNK=y, affecting the placement of
> the static per-CPU data (including the IRQ stack).
> 
> However, what I hadn't realised is that the first chunk is referenced
> via the linear mapping, so we will need to [vm]allocate the per-CPU
> IRQ stacks explicitly, and record the address in a per-CPU pointer
> variable instead.
> 
> I have updated my branch accordingly.
> 
Yep, this version works, both in QEMU and booting Android.

Thanks,
Laura