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

[Juergen Gross]

On 25/04/2019 13:30, Juergen Gross wrote:
> 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.

This is wrong, of course.

But the indirect jmp is failing as struct pv_ops isn't mapped by the
user page tables.


Juergen