Re: [PATCH V2 5/5] x86/sgx: Ensure no data in PCMD page after truncate

[Jarkko Sakkinen <jarkko@kernel.org>]

On Mon, May 09, 2022 at 02:48:03PM -0700, Reinette Chatre wrote:
> A PCMD (Paging Crypto MetaData) page contains the PCMD
> structures of enclave pages that have been encrypted and
> moved to the shmem backing store. When all enclave pages
> sharing a PCMD page are loaded in the enclave, there is no
> need for the PCMD page and it can be truncated from the
> backing store.
> 
> A few issues appeared around the truncation of PCMD pages. The
> known issues have been addressed but the PCMD handling code could
> be made more robust by loudly complaining if any new issue appears
> in this area.
> 
> Add a check that will complain once with a WARN if the PCMD page is not
> actually empty after it has been truncated. There should never be data
> in the PCMD page at this point since it is always updated with the
> enclave mutex held.
> 
> Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
> ---
>  arch/x86/kernel/cpu/sgx/encl.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
> index d1d4e8572702..af972dbad965 100644
> --- a/arch/x86/kernel/cpu/sgx/encl.c
> +++ b/arch/x86/kernel/cpu/sgx/encl.c
> @@ -183,12 +183,19 @@ static int __sgx_encl_eldu(struct sgx_encl_page *encl_page,
>  	kunmap_atomic(pcmd_page);
>  	kunmap_atomic((void *)(unsigned long)pginfo.contents);
>  
> +	get_page(b.pcmd);
>  	sgx_encl_put_backing(&b);
>  
>  	sgx_encl_truncate_backing_page(encl, page_index);
>  
> -	if (pcmd_page_empty && !pcmd_page_in_use(encl, pcmd_first_page))
> +	if (pcmd_page_empty && !pcmd_page_in_use(encl, pcmd_first_page)) {
>  		sgx_encl_truncate_backing_page(encl, PFN_DOWN(page_pcmd_off));
> +		pcmd_page = kmap_atomic(b.pcmd);
> +		WARN_ON_ONCE(memchr_inv(pcmd_page, 0, PAGE_SIZE));

Is WARN necessary, or would it make more sense to use pr_warn()?

It would give a better chance to collect information if "panic_on_warn" is
set for the running kernel.

> +		kunmap_atomic(pcmd_page);
> +	}
> +
> +	put_page(b.pcmd);
>  
>  	return ret;
>  }
> -- 
> 2.25.1
> 

BR, Jarkko