Re: [PATCH v9 3/7] x86/sgx: Initial poison handling for dirty and free pages

[Sean Christopherson <seanjc@google.com>]

On Mon, Oct 11, 2021, Tony Luck wrote:
> A memory controller patrol scrubber can report poison in a page
> that isn't currently being used.
> 
> Add "poison" field in the sgx_epc_page that can be set for an
> sgx_epc_page. Check for it:
> 1) When sanitizing dirty pages
> 2) When freeing epc pages
> 
> Poison is a new field separated from flags to avoid having to make
> all updates to flags atomic, or integrate poison state changes into
> some other locking scheme to protect flags.

Explain why atomic would be needed.  I lived in this code for a few years and
still had to look at the source to remember that the reclaimer can set flags
without taking node->lock.

> In both cases place the poisoned page on a list of poisoned epc pages
> to make sure it will not be reallocated.
> 
> Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
> Tested-by: Reinette Chatre <reinette.chatre@intel.com>
> Signed-off-by: Tony Luck <tony.luck@intel.com>
> ---
>  arch/x86/kernel/cpu/sgx/main.c | 14 +++++++++++++-
>  arch/x86/kernel/cpu/sgx/sgx.h  |  3 ++-
>  2 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> index 09fa42690ff2..653bace26100 100644
> --- a/arch/x86/kernel/cpu/sgx/main.c
> +++ b/arch/x86/kernel/cpu/sgx/main.c
> @@ -43,6 +43,7 @@ static nodemask_t sgx_numa_mask;
>  static struct sgx_numa_node *sgx_numa_nodes;
>  
>  static LIST_HEAD(sgx_dirty_page_list);
> +static LIST_HEAD(sgx_poison_page_list);
>  
>  /*
>   * Reset post-kexec EPC pages to the uninitialized state. The pages are removed
> @@ -62,6 +63,12 @@ static void __sgx_sanitize_pages(struct list_head *dirty_page_list)
>  
>  		page = list_first_entry(dirty_page_list, struct sgx_epc_page, list);
>  
> +		if (page->poison) {

Does this need READ_ONCE (and WRITE_ONCE in the writer) to prevent reloading
page->poison since the sanitizer doesn't hold node->lock, i.e. page->poison can
be set any time?  Honest question, I'm terrible with memory ordering rules...

> +			list_del(&page->list);
> +			list_add(&page->list, &sgx_poison_page_list);

list_move()

> +			continue;
> +		}
> +
>  		ret = __eremove(sgx_get_epc_virt_addr(page));
>  		if (!ret) {
>  			/*
> @@ -626,7 +633,11 @@ void sgx_free_epc_page(struct sgx_epc_page *page)
>  
>  	spin_lock(&node->lock);
>  
> -	list_add_tail(&page->list, &node->free_page_list);
> +	page->owner = NULL;
> +	if (page->poison)
> +		list_add(&page->list, &sgx_poison_page_list);

sgx_poison_page_list is a global list, whereas node->lock is, well, per node.
On a system with multiple EPCs, this could corrupt sgx_poison_page_list if
multiple poisoned pages from different nodes are freed simultaneously.

> +	else
> +		list_add_tail(&page->list, &node->free_page_list);
>  	sgx_nr_free_pages++;
>  	page->flags = 0;
>  
> @@ -658,6 +669,7 @@ static bool __init sgx_setup_epc_section(u64 phys_addr, u64 size,
>  		section->pages[i].section = index;
>  		section->pages[i].flags = SGX_EPC_PAGE_IN_USE;
>  		section->pages[i].owner = NULL;
> +		section->pages[i].poison = 0;
>  		list_add_tail(&section->pages[i].list, &sgx_dirty_page_list);
>  	}
>  
> diff --git a/arch/x86/kernel/cpu/sgx/sgx.h b/arch/x86/kernel/cpu/sgx/sgx.h
> index f9202d3d6278..a990a4c9a00f 100644
> --- a/arch/x86/kernel/cpu/sgx/sgx.h
> +++ b/arch/x86/kernel/cpu/sgx/sgx.h
> @@ -31,7 +31,8 @@
>  
>  struct sgx_epc_page {
>  	unsigned int section;
> -	unsigned int flags;
> +	u16 flags;
> +	u16 poison;
>  	struct sgx_encl_page *owner;
>  	struct list_head list;
>  };
> 
> -- 
> 2.31.1
>