Re: [RFC PATCH for 4.18 2/2] rseq: compat: clear high bits of rseq_cs fields

[Thomas Gleixner]

On Tue, 26 Jun 2018, Andy Lutomirski wrote:
> > On Jun 26, 2018, at 2:16 PM, Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> > 
> > Make the behavior rseq on compat tasks more robust by ensuring that
> > kernel/rseq.c:rseq_get_rseq_cs() clears the high bits of
> > rseq_cs->abort_ip, rseq_cs->start_ip and rseq_cs->post_commit_offset
> > when a 32-bit binary is run on a 64-bit kernel.
> > 
> > The intent here is that if user-space has garbage rather than zeroes
> > in its struct rseq_cs fields padding, the behavior will be the same
> > whether the binary is run on 32-bit or 64-bit kernels.
> > 
> > Use in_compat_syscall() when rseq_get_rseq_cs() is invoked from
> > system call context, and use is_compat_frame() when invoked from
> > signal delivery.
> > 
> 
> And when itâs invoked due to preemption unrelated to a syscall or signal,
> you malfunction?
>
> I think the only sane solution is to make these fields be u64, delete the
> LINUX_FIELD_ macros, and possibly teach the x86 slowpath return to inject
> a signal if itâs trying to return to a 32-bit context with garbage in the
> high bits of regs->ip so that we determistically fail if the user screws
> up.

Right. That's the only sane solution. Trying to play games with 32/64bit
for a dubious value is going to bite us within no time and just create ugly
workarounds left and right. Forcing a clear handling upfront avoids all of
that.

Thanks,

	tglx