RE: [RFC PATCH V4 06/17] x86/hyperv: decrypt VMBus pages for sev-snp enlightened guest

[Michael Kelley (LINUX)]

From: Tianyu Lan <ltykernel@xxxxxxxxx> Sent: Monday, April 3, 2023 10:44 AM
> 

The Subject prefix for this patch is still wrong.  I previously commented on
this. :-(   It should be Drivers: hv: vmbus: 


> VMBus post msg, synic event and message pages are shared
> with hypervisor and so decrypt these pages in the sev-snp guest.
> 
> Signed-off-by: Tianyu Lan <tiala@xxxxxxxxxxxxx>
> ---
> Change since RFC V3:
>        * Set encrypt page back in the hv_synic_free()
> 
> Change since RFC V2:
>        * Fix error in the error code path and encrypt
>        	 pages correctly when decryption failure happens.
> ---
>  drivers/hv/hv.c | 42 +++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 41 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/hv/hv.c b/drivers/hv/hv.c
> index 008234894d28..e09cea8f2f04 100644
> --- a/drivers/hv/hv.c
> +++ b/drivers/hv/hv.c
> @@ -20,6 +20,7 @@
>  #include <linux/interrupt.h>
>  #include <clocksource/hyperv_timer.h>
>  #include <asm/mshyperv.h>
> +#include <linux/set_memory.h>
>  #include "hyperv_vmbus.h"
> 
>  /* The one and only */
> @@ -117,7 +118,7 @@ int hv_post_message(union hv_connection_id connection_id,
> 
>  int hv_synic_alloc(void)
>  {
> -	int cpu;
> +	int cpu, ret;
>  	struct hv_per_cpu_context *hv_cpu;
> 
>  	/*
> @@ -168,9 +169,39 @@ int hv_synic_alloc(void)
>  			pr_err("Unable to allocate post msg page\n");
>  			goto err;
>  		}
> +
> +		if (hv_isolation_type_en_snp()) {
> +			ret = set_memory_decrypted((unsigned long)
> +				hv_cpu->synic_message_page, 1);
> +			if (ret)
> +				goto err;
> +
> +			ret = set_memory_decrypted((unsigned long)
> +				hv_cpu->synic_event_page, 1);
> +			if (ret)
> +				goto err_decrypt_event_page;
> +
> +			ret = set_memory_decrypted((unsigned long)
> +				hv_cpu->post_msg_page, 1);
> +			if (ret)
> +				goto err_decrypt_msg_page;
> +
> +			memset(hv_cpu->synic_message_page, 0, PAGE_SIZE);
> +			memset(hv_cpu->synic_event_page, 0, PAGE_SIZE);
> +			memset(hv_cpu->post_msg_page, 0, PAGE_SIZE);
> +		}
>  	}
> 
>  	return 0;
> +
> +err_decrypt_msg_page:
> +	set_memory_encrypted((unsigned long)
> +		hv_cpu->synic_event_page, 1);
> +
> +err_decrypt_event_page:
> +	set_memory_encrypted((unsigned long)
> +		hv_cpu->synic_message_page, 1);
> +
>  err:
>  	/*
>  	 * Any memory allocations that succeeded will be freed when
> @@ -191,6 +222,15 @@ void hv_synic_free(void)
>  		free_page((unsigned long)hv_cpu->synic_event_page);
>  		free_page((unsigned long)hv_cpu->synic_message_page);
>  		free_page((unsigned long)hv_cpu->post_msg_page);
> +
> +		if (hv_isolation_type_en_snp()) {
> +			set_memory_encrypted((unsigned long)
> +				hv_cpu->synic_message_page, 1);
> +			set_memory_encrypted((unsigned long)
> +				hv_cpu->synic_event_page, 1);
> +			set_memory_encrypted((unsigned long)
> +				hv_cpu->post_msg_page, 1);
> +		}

The re-encryption must be done *before* pages are freed!

Furthermore, if the re-encryption fails, we should not free
the page as it would pollute the free memory pool.  The best
we can do is leak the memory.  See Patch 5 in Dexuan's
TDX series, which does the same thing (but still doesn't
get it quite right, per my comments).

Michael

>  	}
> 
>  	kfree(hv_context.hv_numa_map);
> --
> 2.25.1