Re: [PATCH v2] KVM: guest_memfd: Elaborate on how release() vs. get_pfn() is safe against UAF

[Yan Zhao]

On Wed, Aug 26, 2026 at 09:56:47AM -0700, 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().
> 
> See the extended on-list discussion[*] for more details about exactly what
> KVM guards against, and how.
> 
> No functional change intended.
> 
> Link: https://lore.kernel.org/all/CAEvNRgGmyd1yqQXsnz5hWRZpBZUs%3DpiEWbEaqP9%2Bcz9ZqEMQ6g@xxxxxxxxxxxxxx [*]
> Cc: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> Cc: Vishal Annapurve <vannapurve@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
> 
> v2: Explain how this all works in even gorier detail. [Yan]
> 
> v1: https://lore.kernel.org/all/20251113232229.1698886-1-seanjc@xxxxxxxxxx
> 
>  virt/kvm/guest_memfd.c | 50 +++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 44 insertions(+), 6 deletions(-)
> 
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index b596486d184c..7f1c6a0f8039 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -300,17 +300,55 @@ 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.

Nit:

Do we need to update the code comment in __kvm_gmem_unbind() from
	/*
	 * synchronize_srcu(&kvm->srcu) ensured that kvm_gmem_get_pfn()
	 * cannot see this memslot.
	 */
to
	/*
	 * synchronize_srcu_expedited() in kvm_swap_active_memslots() ensured
	 * that kvm_gmem_get_pfn() cannot see this memslot.
	 */

to align with the above comment.

> +	 *
> +	 * 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).
> +	 *
> +	 * Unlike most other users of get_file_rcu(), where callers don't care
> +	 * if they race with a write, only that they have a reference to _a_
> +	 * live file, kvm_gmem_get_pfn() needs to get the exact file that is
> +	 * associated with the memslot.  Without the aforementioned SRCU
> +	 * synchronization, the following could happen:
> +	 *
> +	 *  CPU0				CPU1
> +	 *  kvm_gmem_get_pfn()
> +	 *    f = X (from slot->gmem.file)
> +	 *					kvm_gmem_release())
> +	 *					  slot->gmem.file = NULL
> +	 *
> +	 *					kvm_set_memory_region()
> +	 *					  slot deleted
> +	 *
> +	 *					kvm_set_memory_region()
> +	 *					  slot created
> +	 *					  slot->gmem.file = f (alloc the same object)
> +	 *
> +	 *  get_file_active()
> +	 *    file = f
> +	 *    file_reloaded = f
> +	 *
> +	 * <KVM does weird things with an old memslot+file>
> +	 *
> +	 * Obviously KVM would be broken in many places if the synchronization
> +	 * were omitted, but it's important to note that get_file_active() does
> +	 * NOT guarantee a reference to the correct file was obtained, only
> +	 * that the file doesn't point at a reallocated object.
reallocated -> reloaded?

> +	 */
>  	xa_for_each(&f->bindings, index, slot)
>  		WRITE_ONCE(slot->gmem.file, NULL);
With the nits addressed,

Reviewed-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>