[PATCH] x86/sgx: Fix free_cnt counting logic in epc section

[PATCH] x86/sgx: Fix free_cnt counting logic in epc section

[Tianjia Zhang]

Increase `section->free_cnt` in sgx_sanitize_section() is
more reasonable, which is called in ksgxd kernel thread,
instead of assigning it to epc section pages number at
initialization. Although this is unlikely to fail, these
pages cannot be allocated after initialization, and which
need to be reset by ksgxd.

Reported-by: Jia Zhang <zhang.jia@linux.alibaba.com>
Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
---
 arch/x86/kernel/cpu/sgx/main.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
index c519fc5f6948..9e9a3cf7c00b 100644
--- a/arch/x86/kernel/cpu/sgx/main.c
+++ b/arch/x86/kernel/cpu/sgx/main.c
@@ -48,9 +48,10 @@ static void sgx_sanitize_section(struct sgx_epc_section *section)
 					struct sgx_epc_page, list);
 
 		ret = __eremove(sgx_get_epc_virt_addr(page));
-		if (!ret)
+		if (!ret) {
 			list_move(&page->list, &section->page_list);
-		else
+			section->free_cnt += 1;
+		} else
 			list_move_tail(&page->list, &dirty);
 
 		spin_unlock(&section->lock);
@@ -646,7 +647,6 @@ static bool __init sgx_setup_epc_section(u64 phys_addr, u64 size,
 		list_add_tail(&section->pages[i].list, &section->init_laundry_list);
 	}
 
-	section->free_cnt = nr_pages;
 	return true;
 }
 
-- 
2.19.1.3.ge56e4f7

Re: [PATCH] x86/sgx: Fix free_cnt counting logic in epc section

[Sean Christopherson]

On Mon, Jan 18, 2021, Tianjia Zhang wrote:
> Increase `section->free_cnt` in sgx_sanitize_section() is
> more reasonable, which is called in ksgxd kernel thread,
> instead of assigning it to epc section pages number at
> initialization. Although this is unlikely to fail, these
> pages cannot be allocated after initialization, and which
> need to be reset by ksgxd.
> 
> Reported-by: Jia Zhang <zhang.jia@linux.alibaba.com>
> Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>

Reviewed-by: Sean Christopherson <seanjc@google.com> 

> ---
>  arch/x86/kernel/cpu/sgx/main.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> index c519fc5f6948..9e9a3cf7c00b 100644
> --- a/arch/x86/kernel/cpu/sgx/main.c
> +++ b/arch/x86/kernel/cpu/sgx/main.c
> @@ -48,9 +48,10 @@ static void sgx_sanitize_section(struct sgx_epc_section *section)
>  					struct sgx_epc_page, list);
>  
>  		ret = __eremove(sgx_get_epc_virt_addr(page));
> -		if (!ret)
> +		if (!ret) {
>  			list_move(&page->list, &section->page_list);
> -		else
> +			section->free_cnt += 1;
> +		} else

Nit: the the "else" should also have curly braces.

>  			list_move_tail(&page->list, &dirty);
>  
>  		spin_unlock(&section->lock);

FWIW, taking section->lock could be moved inside the !ret flow so that EREMOVE
is done without holding the lock.  I've no idea if that'd actually change
anything in practice, but it's theoretically possible that ksgxd hasn't finished
sanitizing the EPC when userspace starts creating enclaves.

> @@ -646,7 +647,6 @@ static bool __init sgx_setup_epc_section(u64 phys_addr, u64 size,
>  		list_add_tail(&section->pages[i].list, &section->init_laundry_list);
>  	}
>  
> -	section->free_cnt = nr_pages;
>  	return true;
>  }
>  
> -- 
> 2.19.1.3.ge56e4f7
> 

Re: [PATCH] x86/sgx: Fix free_cnt counting logic in epc section

[Jarkko Sakkinen]

On Mon, Jan 18, 2021 at 09:33:47PM +0800, Tianjia Zhang wrote:
> Increase `section->free_cnt` in sgx_sanitize_section() is
> more reasonable, which is called in ksgxd kernel thread,
> instead of assigning it to epc section pages number at
> initialization. Although this is unlikely to fail, these
> pages cannot be allocated after initialization, and which
> need to be reset by ksgxd.
> 
> Reported-by: Jia Zhang <zhang.jia@linux.alibaba.com>
> Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>

There is nothing broken in the logic. Convince me otherwise.

I.e. what is exactly broken, and how?

/Jarkko

> ---
>  arch/x86/kernel/cpu/sgx/main.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> index c519fc5f6948..9e9a3cf7c00b 100644
> --- a/arch/x86/kernel/cpu/sgx/main.c
> +++ b/arch/x86/kernel/cpu/sgx/main.c
> @@ -48,9 +48,10 @@ static void sgx_sanitize_section(struct sgx_epc_section *section)
>  					struct sgx_epc_page, list);
>  
>  		ret = __eremove(sgx_get_epc_virt_addr(page));
> -		if (!ret)
> +		if (!ret) {
>  			list_move(&page->list, &section->page_list);
> -		else
> +			section->free_cnt += 1;
> +		} else
>  			list_move_tail(&page->list, &dirty);
>  
>  		spin_unlock(&section->lock);
> @@ -646,7 +647,6 @@ static bool __init sgx_setup_epc_section(u64 phys_addr, u64 size,
>  		list_add_tail(&section->pages[i].list, &section->init_laundry_list);
>  	}
>  
> -	section->free_cnt = nr_pages;
>  	return true;
>  }
>  
> -- 
> 2.19.1.3.ge56e4f7
> 
>