linux-kernel - Re: [RFC PATCH 14/18] KVM: Add asynchronous userfaults, KVM_READ

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Message-ID: <CALzav=d=LaVCFTLxzJD8C_=6+fxjsoLxdKOnxKBgn_QdNDOoXw@mail.gmail.com>
Date: Thu, 11 Jul 2024 16:52:38 -0700
From: David Matlack <dmatlack@...gle.com>
To: James Houghton <jthoughton@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>, 
	Oliver Upton <oliver.upton@...ux.dev>, James Morse <james.morse@....com>, 
	Suzuki K Poulose <suzuki.poulose@....com>, Zenghui Yu <yuzenghui@...wei.com>, 
	Sean Christopherson <seanjc@...gle.com>, Shuah Khan <shuah@...nel.org>, Peter Xu <peterx@...hat.org>, 
	Axel Rasmussen <axelrasmussen@...gle.com>, kvm@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	kvmarm@...ts.linux.dev
Subject: Re: [RFC PATCH 14/18] KVM: Add asynchronous userfaults, KVM_READ_USERFAULT

On Wed, Jul 10, 2024 at 4:42 PM James Houghton <jthoughton@...gle.com> wrote:
>
> +       case KVM_READ_USERFAULT: {
> +               struct kvm_fault fault;
> +               gfn_t gfn;
> +
> +               r = kvm_vm_ioctl_read_userfault(kvm, &gfn);
> +               if (r)
> +                       goto out;
> +
> +               fault.address = gfn;
> +
> +               /* TODO: if this fails, this gfn is lost. */
> +               r = -EFAULT;
> +               if (copy_to_user(&fault, argp, sizeof(fault)))

You could do the copy under the spin_lock() with
copy_to_user_nofault() to avoid losing gfn.