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

[Ard Biesheuvel]

On 13 July 2017 at 18:55, Mark Rutland <mark.rutland at arm.com> wrote:
> On Thu, Jul 13, 2017 at 05:10:50PM +0100, Mark Rutland wrote:
>> On Thu, Jul 13, 2017 at 12:49:48PM +0100, Ard Biesheuvel wrote:
>> > On 13 July 2017 at 11:49, Mark Rutland <mark.rutland at arm.com> wrote:
>> > > On Thu, Jul 13, 2017 at 07:58:50AM +0100, Ard Biesheuvel wrote:
>> > >> On 12 July 2017 at 23:33, Mark Rutland <mark.rutland at arm.com> wrote:
>
>> > Given that the very first stp in kernel_entry will fault if we have
>> > less than S_FRAME_SIZE bytes of stack left, I think we should check
>> > that we have at least that much space available.
>>
>> I was going to reply saying that I didn't agree, but in writing up
>> examples, I mostly convinced myself that this is the right thing to do.
>> So I mostly agree!
>>
>> This would mean we treat the first impossible-to-handle exception as
>> that fatal case, which is similar to x86's double-fault, triggered when
>> the HW can't stack the regs. All other cases are just arbitrary faults.
>>
>> However, to provide that consistently, we'll need to perform this check
>> at every exception boundary, or some of those cases will result in a
>> recursive fault first.
>>
>> So I think there are three choices:
>>
>> 1) In el1_sync, only check SP bounds, and live with the recursive
>>    faults.
>>
>> 2) in el1_sync, check there's room for the regs, and live with the
>>    recursive faults for overflow on other exceptions.
>>
>> 3) In all EL1 entry paths, check there's room for the regs.
>
> FWIW, for the moment I've applied (2), as you suggested, to my
> arm64/vmap-stack branch, adding an additional:
>
>         sub     x0, x0, #S_FRAME_SIZE
>
> ... to the entry path.
>
> I think it's worth trying (3) so that we consistently report these
> cases, benchmarks permitting.
>

OK, so here's a crazy idea: what if we
a) carve out a dedicated range in the VMALLOC area for stacks
b) for each stack, allocate a naturally aligned window of 2x the stack
size, and map the stack inside it, leaving the remaining space
unmapped

That way, we can compare SP (minus S_FRAME_SIZE) against a mask that
is a build time constant, to decide whether its value points into a
stack or not. Of course, it may be pointing into the wrong stack, but
that should not prevent us from taking the exception, and we can deal
with that later. It would give us a very cheap way to perform this
test on the hot paths.

>> I believe that determining whether the exception was caused by a stack
>> overflow is not something we can do robustly or efficiently.
>>

Actually, if the stack pointer is within S_FRAME_SIZE of the base, and
the faulting address points into the guard page, that is a pretty
strong indicator that the stack overflowed. That shouldn't be too
costly?

> It's probably worth putting the fast-path check directly into the
> vectors, where we currently only use 1/32 of the instruction slots
> available to us.
>
>> As above, I think it's helpful to think of this as something closer to a
>> double-fault handler (i.e. aiming to catch when we can't take the
>> exception safely), rather than something that's trying to catch logical
>> stack overflows.
>
> Does this make sense to you?
>
> I've tried to reword the log output, as below, to give this impression.
>
> [   49.288232] Insufficient stack space to handle exception!

This could be a separate warning, if we find out that the actual
exception was caused by something else.

> [   49.288245] CPU: 5 PID: 2208 Comm: bash Not tainted 4.12.0-00005-ga781af2 #81
> [   49.300680] Hardware name: ARM Juno development board (r1) (DT)
> [   49.306549] task: ffff800974955100 task.stack: ffff00000d6f0000
> [   49.312426] PC is at recursive_loop+0x10/0x50
> [   49.316747] LR is at recursive_loop+0x34/0x50
> [   49.321066] pc : [<ffff000008588aa0>] lr : [<ffff000008588ac4>] pstate: 40000145
> [   49.328398] sp : ffff00000d6eff30
> [   49.331682] x29: ffff00000d6f0350 x28: ffff800974955100
> [   49.336953] x27: ffff000008942000 x26: ffff000008f0d758
> [   49.342223] x25: ffff00000d6f3eb8 x24: ffff00000d6f3eb8
> [   49.347493] x23: ffff000008f0d490 x22: 0000000000000009
> [   49.352764] x21: ffff800974a57000 x20: ffff000008f0d4e0
> [   49.358034] x19: 0000000000000013 x18: 0000ffffe7e2e4f0
> [   49.363304] x17: 0000ffff9c1256a4 x16: ffff0000081f8b88
> [   49.368574] x15: 00002a81b8000000 x14: 00000000fffffff0
> [   49.373845] x13: ffff000008f6278a x12: ffff000008e62818
> [   49.379115] x11: 0000000000000000 x10: 000000000000019e
> [   49.384385] x9 : 0000000000000004 x8 : ffff00000d6f0770
> [   49.389656] x7 : 1313131313131313 x6 : 000000000000019e
> [   49.394925] x5 : 0000000000000000 x4 : 0000000000000000
> [   49.400205] x3 : 0000000000000000 x2 : 0000000000000400
> [   49.405484] x1 : 0000000000000013 x0 : 0000000000000012
> [   49.410764] Task stack: [0xffff00000d6f0000..0xffff00000d6f4000]
> [   49.416728] IRQ stack:  [0xffff80097ffb90a0..0xffff80097ffbd0a0]
> [   49.422692] ESR: 0x96000047 -- DABT (current EL)
> [   49.427277] FAR: 0xffff00000d6eff30
> [   49.430742] Kernel panic - not syncing: kernel stack overflow
> [   49.436451] CPU: 5 PID: 2208 Comm: bash Not tainted 4.12.0-00005-ga781af2 #81
> [   49.443534] Hardware name: ARM Juno development board (r1) (DT)
> [   49.449412] Call trace:
> [   49.451852] [<ffff0000080885f0>] dump_backtrace+0x0/0x230
> [   49.457218] [<ffff0000080888e4>] show_stack+0x14/0x20
> [   49.462240] [<ffff00000839be0c>] dump_stack+0x9c/0xc0
> [   49.467261] [<ffff000008175218>] panic+0x11c/0x294
> [   49.472024] [<ffff000008089184>] handle_bad_stack+0xe4/0xe8
> [   49.477561] [<ffff000008588ac4>] recursive_loop+0x34/0x50
> [   49.482926] SMP: stopping secondary CPUs
> [   49.487145] Kernel Offset: disabled
> [   49.490609] Memory Limit: none
> [   49.493649] ---[ end Kernel panic - not syncing: kernel stack overflow
>

Yes, this looks nice.

> ... I still need to attack the backtracing to walk across stacks.
>

Yup