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Date:   Fri, 2 Jul 2021 15:21:22 -0700
From:   Andrei Vagin <avagin@gmail.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
        linux-um@lists.infradead.org, criu@openvz.org, avagin@google.com,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Anton Ivanov <anton.ivanov@cambridgegreys.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Dmitry Safonov <0x7f454c46@gmail.com>,
        Ingo Molnar <mingo@redhat.com>, Jeff Dike <jdike@addtoit.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Richard Weinberger <richard@nod.at>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 2/4] arch/x86: implement the process_vm_exec syscall
Message-ID: <YN+RYi5honrgjFAw@gmail.com>
References: <20210414055217.543246-1-avagin@gmail.com>
 <20210414055217.543246-3-avagin@gmail.com>
 <YN7TgV9RDJTRaY8R@hirez.programming.kicks-ass.net>
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In-Reply-To: <YN7TgV9RDJTRaY8R@hirez.programming.kicks-ass.net>
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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;
> > +}