Re: x86/paravirt: Detect over-sized patching bugs in paravirt_patch_call()

From: Juergen Gross
Date: Thu Apr 25 2019 - 07:30:51 EST

On 25/04/2019 12:57, Ingo Molnar wrote:
> * Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>> On Thu, Apr 25, 2019 at 11:50:39AM +0200, Ingo Molnar wrote:
>>> * Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>>>> On Thu, Apr 25, 2019 at 11:17:17AM +0200, Ingo Molnar wrote:
>>>>> It basically means that we silently won't do any patching and the kernel
>>>>> will crash later on in mysterious ways, because paravirt patching is
>>>>> usually relied on.
>>>> That's OK. The compiler emits an indirect CALL/JMP to the pv_ops
>>>> structure contents. That _should_ stay valid and function correctly at
>>>> all times.
>>> It might result in a correctly executing kernel in terms of code
>>> generation, but it doesn't result in a viable kernel: some of the places
>>> rely on the patching going through and don't know what to do when it
>>> doesn't and misbehave or crash in interesting ways.
>>> Guess how I know this. ;-)
>> What sites would that be? It really should work AFAIK.
> So for example I tried to increasing the size of one of the struct
> patch_xxl members:
> --- a/arch/x86/kernel/paravirt_patch.c
> +++ b/arch/x86/kernel/paravirt_patch.c
> @@ -28,7 +28,7 @@ struct patch_xxl {
> const unsigned char irq_restore_fl[2];
> # ifdef CONFIG_X86_64
> const unsigned char cpu_wbinvd[2];
> - const unsigned char cpu_usergs_sysret64[6];
> + const unsigned char cpu_usergs_sysret64[60];
> const unsigned char cpu_swapgs[3];
> const unsigned char mov64[3];
> # else
> Which with the vanilla kernel crashes on boot much, much later:
> [ 2.478026] PANIC: double fault, error_code: 0x0

Sure, there is no NOP padding applied. Pre-populating the area with
1 byte NOPs would avoid the crash.

> But in any case, even if many of the others will work if the patching
> fails silently, is there any case where we'd treat patching failure as an
> acceptable case?
> BUG_ON() in paravirt kernels is an easily debuggable condition and beats
> the above kinds of symptoms. But I can turn it into a WARN_ON_ONCE() if
> you think that's better?

I'd prefer the BUG_ON(). Its not as if those conditions will occur on
very few machines only. In case some patching isn't working we should
catch those issues early.