linux-kernel - Re: [PATCH][v2] virt: tdx-guest: Don't free decrypted memory

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Date: Fri, 14 Jun 2024 16:13:46 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
	"lirongqing@...du.com" <lirongqing@...du.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "dave.hansen@...ux.intel.com"
	<dave.hansen@...ux.intel.com>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH][v2] virt: tdx-guest: Don't free decrypted memory

On Fri, 2024-06-14 at 13:14 +0800, Li RongQing wrote:
> In CoCo VMs it is possible for the untrusted host to cause
> 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.
> 
> So when set_memory_decrypted fails, leak decrypted memory, and
> print an error message
> 
> Signed-off-by: Li RongQing <lirongqing@...du.com>
> ---
> diff with v1: leak the page, and print error
> 
>  drivers/virt/coco/tdx-guest/tdx-guest.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/virt/coco/tdx-guest/tdx-guest.c b/drivers/virt/coco/tdx-
> guest/tdx-guest.c
> index 1253bf7..3a6e76c8 100644
> --- a/drivers/virt/coco/tdx-guest/tdx-guest.c
> +++ b/drivers/virt/coco/tdx-guest/tdx-guest.c
> @@ -125,7 +125,7 @@ static void *alloc_quote_buf(void)
>                 return NULL;
>  
>         if (set_memory_decrypted((unsigned long)addr, count)) {
> -               free_pages_exact(addr, len);
> +               pr_err("Failed to set Quote buffer decrypted, leak the
> buffer\n");
>                 return NULL;
>         }

I'm not sure we need the error message, because the set_memory() failure we are
most worried about already has a WARN. But, I could be convinced either way. It
seems to fit with the other code in the file.

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@...el.com>