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:

quoted

+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.

quoted

+	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.

quoted

+
+	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.

quoted

+
+	return 0;
+Efault:
+	user_access_end();
+badframe:
+	signal_fault(current_pt_regs(), uctx, "swap_vm_exec_context");
+	return -EFAULT;
+}