[PATCH 2/4] arch/x86: implement the process_vm_exec syscall

[Andrei Vagin]

On Fri, Jul 02, 2021 at 10:51:13AM +0200, Peter Zijlstra wrote:
> 
> I'm terrified of all of this...
> 
> On Tue, Apr 13, 2021 at 10:52:15PM -0700, Andrei Vagin wrote:
> 
> > +long swap_vm_exec_context(struct sigcontext __user *uctx)
> > +{
> > +	struct sigcontext ctx = {};
> > +	sigset_t set = {};
> > +
> > +
> > +	if (copy_from_user(&ctx, uctx, CONTEXT_COPY_SIZE))
> > +		return -EFAULT;
> > +	/* A floating point state is managed from user-space. */
> > +	if (ctx.fpstate != 0)
> > +		return -EINVAL;

Here, we check that ctx doesn't have an FPU state.

> > +	if (!user_access_begin(uctx, sizeof(*uctx)))
> > +		return -EFAULT;
> > +	unsafe_put_sigcontext(uctx, NULL, current_pt_regs(), (&set), Efault);
> > +	user_access_end();
> 
> But here you save the sigcontext without FPU state.
> 
> > +
> > +	if (__restore_sigcontext(current_pt_regs(), &ctx, 0))
> > +		goto badframe;
> 
> And here you restore sigcontext, *with* FPU state.  At which point your
> FPU state is irrecoverably lost.

process_vm_exec doesn't change a process FPU state. Unlike signals, here
we can control it from a user-space. A process can set an FPU state
before process_vm_exec and then retore its FPU state after the
call.

This version of patches has a bug that I fixed in my tree when I
implemented the user-space part for gVisor. I didn't take into account
that restore_sigcontext(ctx) clears a process fpu state if ctx->fpstate
is zero. I moved fpu__restore_sig out from __restore_sigcontext to fix
this issue:

https://github.com/avagin/linux-task-diag/commit/55b7194d00ff

> 
> Also, I'm not at all convinced this can ever do the right thing when the
> tasks don't agree on what the FPU state is. I suppose in the best case
> the save will EFAULT.
> 
> > +
> > +	return 0;
> > +Efault:
> > +	user_access_end();
> > +badframe:
> > +	signal_fault(current_pt_regs(), uctx, "swap_vm_exec_context");
> > +	return -EFAULT;
> > +}