Re: [PATCH v3 27/37] x86/shstk: Introduce routines modifying shstk

Peter Zijlstra
2022-11-17T14:18:48Z

On Wed, Nov 16, 2022 at 10:38:19PM +0000, Edgecombe, Rick P wrote:
> On Wed, 2022-11-16 at 11:18 +0100, Peter Zijlstra wrote:
> > > >
> > > > Should you write a 64bit value even if the task receiving a
> > > > signal is
> > > > 32bit ?
> > >
> > > 32 bit support was also dropped.
> >
> > How? Task could start life as 64bit, frob LDT to set up 32bit code
> > segment and jump into it and start doing 32bit syscalls, then what?
> >
> > AFAICT those 32bit syscalls will end up doing SA_IA32_ABI sigframes.
>
> Hmm, good point. This series used to support normal 32 bit apps via
> ia32 emulation which would have handled this. But I removed it (blocked
> in the enabling logic) because it didn't seem like it would get enough
> use to justify the extra code. That doesn't block this scenario here
> though.
>
> Pardon the possibly naive question, but is this 32/64 bit mixing
> something any normal, shstk-desiring, applications would actually do? O
> r more that they could do?

It is not something common, but it is something that things like Wine
do IIRC, and it would be a real shame if Wine could not use shadow
stacks or something, right ;-)

But more to the point; since the kernel cannot forbit this scenario
(aside from taking away the LDT entirely) it is something that needs
handling.