Re: [PATCH] UefiCpuPkg:PiSmmCpu: Set correct attribute on split.

["Yao, Jiewen" <jiewen.yao@intel.com>]

Comments below:

From: Laszlo Ersek [mailto:lersek@redhat.com]
Sent: Wednesday, November 30, 2016 5:54 AM
To: Yao, Jiewen <jiewen.yao@intel.com>; edk2-devel@ml01.01.org
Cc: Kinney, Michael D <michael.d.kinney@intel.com>; Fan, Jeff <jeff.fan@intel.com>
Subject: Re: [edk2] [PATCH] UefiCpuPkg:PiSmmCpu: Set correct attribute on split.

On 11/29/16 08:39, Jiewen Yao wrote:
> PiSmmCpu driver may split page for page attribute request.
> Current logic will propagate the super page attribute attribute.
> However, it might be wrong because we cannot clear protection
> without touch super page attribute.
>
> We should always clear protection on super page and set
> protection on end page for easy clear later.
>
> Cc: Jeff Fan <jeff.fan@intel.com<mailto:jeff.fan@intel.com>>
> Cc: Michael D Kinney <michael.d.kinney@intel.com<mailto:michael.d.kinney@intel.com>>
> Cc: Laszlo Ersek <lersek@redhat.com<mailto:lersek@redhat.com>>
> Contributed-under: TianoCore Contribution Agreement 1.0
> Signed-off-by: Jiewen Yao <jiewen.yao@intel.com<mailto:jiewen.yao@intel.com>>
> ---
>  UefiCpuPkg/PiSmmCpuDxeSmm/SmmCpuMemoryManagement.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/UefiCpuPkg/PiSmmCpuDxeSmm/SmmCpuMemoryManagement.c b/UefiCpuPkg/PiSmmCpuDxeSmm/SmmCpuMemoryManagement.c
> index accc11e..d0f41a8 100644
> --- a/UefiCpuPkg/PiSmmCpuDxeSmm/SmmCpuMemoryManagement.c
> +++ b/UefiCpuPkg/PiSmmCpuDxeSmm/SmmCpuMemoryManagement.c
> @@ -303,7 +303,7 @@ SplitPage (
>        for (Index = 0; Index < SIZE_4KB / sizeof(UINT64); Index++) {
>          NewPageEntry[Index] = BaseAddress + SIZE_4KB * Index + ((*PageEntry) & PAGE_PROGATE_BITS);
>        }
> -      (*PageEntry) = (UINT64)(UINTN)NewPageEntry + ((*PageEntry) & PAGE_PROGATE_BITS);
> +      (*PageEntry) = (UINT64)(UINTN)NewPageEntry + PAGE_ATTRIBUTE_BITS;
>        return RETURN_SUCCESS;
>      } else {
>        return RETURN_UNSUPPORTED;
> @@ -324,7 +324,7 @@ SplitPage (
>        for (Index = 0; Index < SIZE_4KB / sizeof(UINT64); Index++) {
>          NewPageEntry[Index] = BaseAddress + SIZE_2MB * Index + IA32_PG_PS + ((*PageEntry) & PAGE_PROGATE_BITS);
>        }
> -      (*PageEntry) = (UINT64)(UINTN)NewPageEntry + ((*PageEntry) & PAGE_PROGATE_BITS);
> +      (*PageEntry) = (UINT64)(UINTN)NewPageEntry + PAGE_ATTRIBUTE_BITS;
>        return RETURN_SUCCESS;
>      } else {
>        return RETURN_UNSUPPORTED;
>

I had to stare a while at this, to get a superficial understanding :)
But, it does seem to make sense (I checked PAGE_ATTRIBUTE_BITS and
PAGE_PROGATE_BITS too, just to be sure). So, this change preserves the
protection inheritance for the leaf pages, but clears NX and sets Dirty
/ Accessed / Writeable / Present on the relevant parent entry. (I see
hat User mode access is enabled as well; I don't know why that is useful
here.)
[Jiewen] Yes. You are right.

Some notes about the commit message:

- we have "attribute attribute". I think we should either drop one of
those words, or say "super page attribute to leaf page attribute".
[Jiewen] Agree. I will update.

- "end page" might be more clearly stated as "leaf page" (just a guess)
[Jiewen] Agree. I will update.

- I think it would be useful to mention, for the uninitiated like me :),
that the effective protection is (apparently) the strictest combination
across the levels.
[Jiewen] Agree. I will update.

- What do you think of the following subject line?
UefiCpuPkg/PiSmmCpuDxeSmm: relax superpage protection on page split
[Jiewen] Agree. I will update.

Anyway, to the extent that I understand this, I agree:

Acked-by: Laszlo Ersek <lersek@redhat.com<mailto:lersek@redhat.com>>

I gave the patch a bit of testing in my usual environment; it seems to
cause no problems.
[Jiewen] Thank you.

Tested-by: Laszlo Ersek <lersek@redhat.com<mailto:lersek@redhat.com>>

Thanks
Laszlo