Re: [PATCH V3 5/5] x86/sgx: Ensure no data in PCMD page after truncate

Jarkko Sakkinen
2022-05-13T14:44:24Z

On Thu, May 12, 2022 at 02:51:01PM -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 with a warning 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 was just checked to be empty
> and truncated with enclave mutex held and is updated with the
> enclave mutex held.
>
> Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> Tested-by: Haitao Huang <haitao.huang@intel.com>
> Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
> ---
> Changes since V2:
> - Change WARN_ON_ONCE() to pr_warn(). (Jarkko).
> - Add Haitao's Tested-by tag.
>
> Changes since RFC v1:
> - New patch.
>
> arch/x86/kernel/cpu/sgx/encl.c | 10 +++++++++-
> 1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
> index 243f3bd78145..3c24e6124d95 100644
> --- a/arch/x86/kernel/cpu/sgx/encl.c
> +++ b/arch/x86/kernel/cpu/sgx/encl.c
> @@ -187,12 +187,20 @@ 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 && !reclaimer_writing_to_pcmd(encl, pcmd_first_page))
> + if (pcmd_page_empty && !reclaimer_writing_to_pcmd(encl, pcmd_first_page)) {
> sgx_encl_truncate_backing_page(encl, PFN_DOWN(page_pcmd_off));
> + pcmd_page = kmap_atomic(b.pcmd);
> + if (memchr_inv(pcmd_page, 0, PAGE_SIZE))
> + pr_warn("PCMD page not empty after truncate.\n");
> + kunmap_atomic(pcmd_page);
> + }
> +
> + put_page(b.pcmd);
>
> return ret;
> }
> --
> 2.25.1
>

Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>

BR, Jarkko