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 14:30:14 +0200 |
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On 25/04/2019 13:30, Juergen Gross wrote: > 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.
This is wrong, of course.
But the indirect jmp is failing as struct pv_ops isn't mapped by the user page tables.
Juergen
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