RE: [RFC RFT PATCH 1/4] hv: Leak pages if set_memory_encrypted() fails

[Michael Kelley]

From: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx> Sent: Wednesday, February 21, 2024 6:10 PM
> 

Historically, the preferred Subject prefix for changes to connection.c has
been "Drivers: hv: vmbus:", not just "hv:".  Sometimes that preference
isn't followed, but most of the time it is.

> On TDX it is possible for the untrusted host to cause

I'd argue that this is for CoCo VMs in general, not just TDX.  I don't know
all the failure modes for SEV-SNP, but the code paths you are changing
are run in both TDX and SEV-SNP CoCo VMs.

> set_memory_encrypted() or set_memory_decrypted() to fail such that an
> error is returned and the resulting memory is shared. Callers need to take
> care to handle these errors to avoid returning decrypted (shared) memory to
> the page allocator, which could lead to functional or security issues.
> 
> Hyperv could free decrypted/shared pages if set_memory_encrypted() fails.

It's not Hyper-V doing the freeing.  Maybe say "VMBus code could
free ...."

> Leak the pages if this happens.
> 
> Only compile tested.
> 
> Cc: "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx>
> Cc: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
> Cc: Wei Liu <wei.liu@xxxxxxxxxx>
> Cc: Dexuan Cui <decui@xxxxxxxxxxxxx>
> Cc: linux-hyperv@xxxxxxxxxxxxxxx
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> ---
>  drivers/hv/connection.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/hv/connection.c b/drivers/hv/connection.c
> index 3cabeeabb1ca..e39493421bbb 100644
> --- a/drivers/hv/connection.c
> +++ b/drivers/hv/connection.c
> @@ -315,6 +315,7 @@ int vmbus_connect(void)
> 
>  void vmbus_disconnect(void)
>  {
> +	int ret;
>  	/*
>  	 * First send the unload request to the host.
>  	 */
> @@ -337,11 +338,13 @@ void vmbus_disconnect(void)
>  		vmbus_connection.int_page = NULL;
>  	}
> 
> -	set_memory_encrypted((unsigned long)vmbus_connection.monitor_pages[0], 1);
> -	set_memory_encrypted((unsigned long)vmbus_connection.monitor_pages[1], 1);
> +	ret = set_memory_encrypted((unsigned long)vmbus_connection.monitor_pages[0], 1);
> +	ret |= set_memory_encrypted((unsigned long)vmbus_connection.monitor_pages[1], 1);
> 
> -	hv_free_hyperv_page(vmbus_connection.monitor_pages[0]);
> -	hv_free_hyperv_page(vmbus_connection.monitor_pages[1]);
> +	if (!ret) {
> +		hv_free_hyperv_page(vmbus_connection.monitor_pages[0]);
> +		hv_free_hyperv_page(vmbus_connection.monitor_pages[1]);
> +	}

Of course, this will leak the memory for both pages if only one of the
set_memory_encrypted() calls fails, but I'm OK with that.  It doesn't
seem worth the additional complexity to treat each page separately.

>  	vmbus_connection.monitor_pages[0] = NULL;
>  	vmbus_connection.monitor_pages[1] = NULL;
>  }
> --
> 2.34.1