Messages in this thread | | | Subject | Re: x86/paravirt: Detect over-sized patching bugs in paravirt_patch_call() | From | Juergen Gross <> | Date | Thu, 25 Apr 2019 13:30:46 +0200 |
| |
On 25/04/2019 12:57, Ingo Molnar wrote: > > * Peter Zijlstra <peterz@infradead.org> wrote: > >> On Thu, Apr 25, 2019 at 11:50:39AM +0200, Ingo Molnar wrote: >>> >>> * Peter Zijlstra <peterz@infradead.org> 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.
Juergen
| |