linux-kernel - Re: [PATCH] kvm: add proper frame pointer logic for vmx

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Date:   Tue, 15 Jan 2019 16:54:38 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Qian Cai <cai@....pw>, Paolo Bonzini <pbonzini@...hat.com>,
        rkrcmar@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, hpa@...or.com, x86@...nel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm: add proper frame pointer logic for vmx

On Tue, Jan 15, 2019 at 04:38:49PM -0600, Josh Poimboeuf wrote:
> On Tue, Jan 15, 2019 at 11:06:17AM -0800, Sean Christopherson wrote:
> > > I can see there are five options to solve it.
> > > 
> > > 1) always inline vmx_vcpu_run()
> > > 2) always noinline vmx_vcpu_run()
> > > 3) add -fdiable-ipa-fnsplit option to Makefile for vmx.o
> > > 4) let STACK_FRAME_NON_STANDARD support part.* syntax.
> > > 5) trim-down vmx_vcpu_run() even more to not causing splitting by GCC.
> > > 
> > > Option 1) and 2) seems give away the decision for user with
> > > CONFIG_CC_OPTIMIZE_FOR_(PERFORMANCE/SIZE).
> > > 
> > > Option 3) prevents other functions there for splitting for optimization.
> > > 
> > > Option 4) and 5) seems tricky to implement.
> > > 
> > > I am not more leaning to 3) as only other fuction will miss splitting is
> > > vmx_segment_access_rights().
> > 
> > Option 4) is the most correct, but "tricky" is an understatement.  Unless
> > Josh is willing to pick up the task it'll likely have to wait.
> > 
> > There's actually a few more options:
> > 
> >  6) Replace "pop %rbp" in the vmx_vmenter() asm blob with an open-coded
> >     equivalent, e.g. "mov [%rsp], %rbp; add $8, %rsp".  This runs an end-
> >     around on objtool since objtool explicitly keys off "pop %rbp" and NOT
> >     "mov ..., %rbp" (which is probably an objtool checking flaw?").
> > 
> >  7) Move the vmx_vmenter() asm blob and a few other lines of code into a
> >     separate helper, e.g. __vmx_vcpu_run(), and mark that as having a
> >     non-standard stack frame.
> 
> Do you mean moving the asm blob to a .S file instead of inline asm?  If
> so, I think that's definitely a good idea.  It would be a nice cleanup,
> regardless of the objtool false positive.

No, just moving the inline asm to a separate function.  Moving the entire
inline asm blob is annoying because it references a large number of struct
offsets and doesn't solve the fundamental problem (more on this later).

The VMLAUNCH and VMRESUME invocations themselves, i.e. the really nasty
bits, have already been moved to a .S file (by the commit that exposed
this warning).  That approach eliminates the worst of the conflicts with
compiler optimizations without having to deal with exposing the struct
offsets to asm.

> That would allow vmx_vcpu_run() to be a "normal" C function which
> objtool can validate (and also create ORC data for).  It would also
> prevent future nasty GCC optimizations (which was why the __noclone was
> needed in the first place).

Moving the inline asm to a separate function (on top of having a separate
.S file for VMLAUNCH/VMRESUME) accomplishes sort of the same thing, i.e.
vmx_vcpu_run() gets to be a normal function.  It's not as bulletproof
from a tooling perspective, but odds are pretty good that all usage of
STACK_FRAME_NON_STANDARD will break if the compiler manages to muck up
the inline asm wrapper function.

And having a dedicated function for VMLAUNCH/VMRESUME is actually nice
from a stack trace perspective.  The transition to/from the guest is by
far the most likely source of faults, i.e. a stack trace that originates
in vmx_vmenter() all but guarantees that a fault occurred on VM-Enter or
immediately after VM-Exit.

> And also, I *think* objtool would no longer warn in that case, because
> there would no longer be any calls in the function after popping %rbp.
> Though if I'm wrong about that, I'd be glad to help fix the warning one
> way or another.

In the vmx_vcpu_run() case, warning on calls after "pop %rbp" is actually
a false positive.  The POP restores the host's RBP, i.e. the stack frame,
meaning all calls after the POP are ok.  The window where stack traces
will go awry is between loading RBP with the guest's value and the POP to
restore the host's stack frame, i.e. in this case "mov <guest_rbp>, %rbp"
should trigger a warning irrespective of any calls.

I'm not saying it's actually worth updating objtool, rather that "fixing"
the KVM issue by moving the inline asm to a dedicated .S file doesn't
solve the fundamental problem that VM-Enter/VM-Exit needs to temporarily
corrupt RBP.