linux-kernel - Re: [PATCH] KVM: guest_memfd: Elaborate on how release() vs. get

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Message-ID: <aRsIstNXu0Bl0oID@yzhao56-desk.sh.intel.com>
Date: Mon, 17 Nov 2025 19:36:18 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Paolo Bonzini <pbonzini@...hat.com>, <kvm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Vishal Annapurve <vannapurve@...gle.com>
Subject: Re: [PATCH] KVM: guest_memfd: Elaborate on how release() vs.
 get_pfn() is safe against UAF

On Thu, Nov 13, 2025 at 03:22:29PM -0800, Sean Christopherson wrote:
> Add more context and information to the comment in kvm_gmem_release() that
> explains why there's no synchronization on RCU _or_ kvm->srcu.  Point (b)
> from commit 67b43038ce14 ("KVM: guest_memfd: Remove RCU-protected attribute
> from slot->gmem.file")
> 
>       b) kvm->srcu ensures that kvm_gmem_unbind() and freeing of a memslot
>          occur after the memslot is no longer visible to kvm_gmem_get_pfn().
> 
> is especially difficult to fully grok, particularly in light of commit
> ae431059e75d ("KVM: guest_memfd: Remove bindings on memslot deletion when
> gmem is dying"), which addressed a race between unbind() and release().
As mentioned in commit ae431059e75d ("KVM: guest_memfd: Remove bindings on
memslot deletion when gmem is dying"), unbind() and release() are mutually
exclusive, i.e., both protected by slots_lock, mentioning "race" here is
confusing. So, that commit addressed the mishandling in unbind() after
kvm_gmem_get_file() returning NULL?

> No functional change intended.
> 
> Cc: Yan Zhao <yan.y.zhao@...el.com>
> Cc: Vishal Annapurve <vannapurve@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  virt/kvm/guest_memfd.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index fdaea3422c30..2e09d7ec0cfc 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -338,17 +338,25 @@ static int kvm_gmem_release(struct inode *inode, struct file *file)
>  	 * dereferencing the slot for existing bindings needs to be protected
>  	 * against memslot updates, specifically so that unbind doesn't race
>  	 * and free the memslot (kvm_gmem_get_file() will return NULL).
> -	 *
> -	 * Since .release is called only when the reference count is zero,
> -	 * after which file_ref_get() and get_file_active() fail,
> -	 * kvm_gmem_get_pfn() cannot be using the file concurrently.
> -	 * file_ref_put() provides a full barrier, and get_file_active() the
> -	 * matching acquire barrier.
>  	 */

>  	mutex_lock(&kvm->slots_lock);
>  
>  	filemap_invalidate_lock(inode->i_mapping);
> 
> +	/*
> +	 * Note!  synchronize_srcu() is _not_ needed after nullifying memslot
> +	 * bindings as slot->gmem.file cannot be set back to a non-null value
> +	 * without the memslot first being deleted.  I.e. this relies on the
> +	 * synchronize_srcu_expedited() in kvm_swap_active_memslots() to ensure
> +	 * kvm_gmem_get_pfn() (which runs with kvm->srcu held for read) can't
> +	 * grab a reference to slot->gmem.file even if the struct file object
> +	 * is reallocated.
Do you mean that
as kvm_gmem_get_pfn() can't find a stale slot, it can't grab reference to stale
slot->gmem.file, even if slot->gmem.file has been set to a different value,
i.e., after invoking unbind(), bind() ?

But I'm not sure why to put the kvm_gmem_get_pfn() relying on
synchronize_srcu_expedited() in kvm_swap_active_memslots() in the comment of
release(). 
Without the guard of kvm_gmem_get_file(), kvm_gmem_get_pfn() may need to
provide some RCU read-critial section too for release() to wait by
synchronize_srcu()?

So
	 * Since .release is called only when the reference count is zero,
	 * after which file_ref_get() and get_file_active() fail,
is still helpful?

> +	 *
> +	 * file_ref_put() provides a full barrier, and __get_file_rcu() the
> +	 * matching acquire barrier, to ensure that kvm_gmem_get_file() (via
> +	 * __get_file_rcu()) sees refcount==0 or fails the "file reloaded"
> +	 * check (file != NULL due to nullifying the file pointer here).
> +	 */

>  	xa_for_each(&f->bindings, index, slot)
>  		WRITE_ONCE(slot->gmem.file, NULL);
>  
> 
> base-commit: 16ec4fb4ac95d878b879192d280db2baeec43272
> -- 
> 2.52.0.rc1.455.g30608eb744-goog
>