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

[Ard Biesheuvel]

On 20 July 2017 at 09:36, James Morse <james.morse at arm.com> wrote:
> Hi Ard,
>
> On 20/07/17 06:35, Ard Biesheuvel wrote:
>> On 20 July 2017 at 00:32, Laura Abbott <labbott at 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 ...