Re: [syzbot] upstream test error: KFENCE: use-after-free in kvm_fastop_exception

From: Dmitry Vyukov
Date: Tue Sep 28 2021 - 06:16:57 EST

On Tue, 28 Sept 2021 at 01:45, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> On Mon, Sep 27, 2021 at 04:07:51PM +0000, Sean Christopherson wrote:
> > I was asking about the exact location to confirm that the explosion is indeed
> > from exception fixup, which is the "unwinder scenario get confused" I was thinking
> > of. Based on the disassembly from syzbot, that does indeed appear to be the case
> > here, i.e. this
> >
> > 2a: 4c 8b 21 mov (%rcx),%r12
> >
> > is from exception fixup from somewhere in __d_lookup (can't tell exactly what
> > it's from, maybe KASAN?).
> >
> > > Is there more info on this "the unwinder gets confused"? Bug filed
> > > somewhere or an email thread? Is it on anybody's radar?
> >
> > I don't know if there's a bug report or if this is on anyone's radar. The issue
> > I've encountered in the past, and what I'm pretty sure is being hit here, is that
> > the ORC unwinder doesn't play nice with out-of-line fixup code, presumably because
> > there are no tables for the fixup. I believe kvm_fastop_exception() gets blamed
> > because it's the first label that's found when searching back through the tables.
> The ORC unwinder actually knows about .fixup, and unwinding through the
> .fixup code worked here, as evidenced by the entire stacktrace getting
> printed. Otherwise there would have been a bunch of question marks in
> the stack trace.
> The problem reported here -- falsely printing kvm_fastop_exception -- is
> actually in the arch-independent printing of symbol names, done by
> __sprint_symbol(). Most .fixup code fragments are anonymous, in the
> sense that they don't have symbols associated with them. For x86, here
> are the only defined symbols in .fixup:
> ffffffff81e02408 T kvm_fastop_exception
> ffffffff81e02728 t .E_read_words
> ffffffff81e0272b t .E_leading_bytes
> ffffffff81e0272d t .E_trailing_bytes
> ffffffff81e02734 t .E_write_words
> ffffffff81e02740 t .E_copy
> There's a lot of anonymous .fixup code which happens to be placed in the
> gap between "kvm_fastop_exception" and ".E_read_words". The kernel
> symbol printing code will go backwards from the given address and will
> print the first symbol it finds. So any anonymous code in that gap will
> falsely be reported as kvm_fastop_exception().
> I'm thinking the ideal way to fix this would be getting rid of the
> .fixup section altogether, and instead place a function's corresponding
> fixup code in a cold part of the original function, with the help of
> asm_goto and cold label attributes.
> That way, the original faulting function would be printed instead of an
> obscure reference to an anonymous .fixup code fragment. It would have
> other benefits as well. For example, not breaking livepatch...
> I'll try to play around with it.

Thanks for debugging this, Josh.
I think your solution can also help arm64 as it has the same issue.