Re: [PATCH 0/2] Enable page table write protection

["Wang, Jian J" <jian.j.wang@intel.com>]

I did think of protecting new page table in cpu driver. But I think there's risks
to do it, which is that there might be a chance that, whenever you're trying to
mark one page used as page table to be read-only, you need to split its page
table in advance, and again and again, until all page tables are for 4KB pages.

Although this is a rare case, or the page table "split" will just happen for a few
times, it will slow down the boot process a little bit anyway. So I though  it's
safer not to protect new page tables created after DxeIpl.

Maybe it's just over worrying about it. Maybe we just need to add a PCD to 
turn on/off it just in case. Do you have any ideas in mind?

> -----Original Message-----
> From: Yao, Jiewen
> Sent: Wednesday, November 29, 2017 9:35 PM
> To: Wang, Jian J <jian.j.wang@intel.com>
> Cc: edk2-devel@lists.01.org
> Subject: Re: [edk2] [PATCH 0/2] Enable page table write protection
> 
> I do think we need update CPU driver to protect new allocated split page.
> 
> If you verified in shell env, I think it will exposed, please add a test to trigger
> page split in CPU driver.
> 
> I recommend to write some unit test to parse page table in shell.
> 
> thank you!
> Yao, Jiewen
> 
> 
> > 在 2017年11月29日，下午8:15，Wang, Jian J <jian.j.wang@intel.com> 写
> 道：
> >
> > It's in the DxeIplPeim.
> >
> > By the way, there's an issue in this patch. I forgot to protect page table for 32-
> bit mode.
> > So this patch works only for 64-bit mode. I'll add it in v2 patch.
> >
> >> -----Original Message-----
> >> From: Yao, Jiewen
> >> Sent: Wednesday, November 29, 2017 6:56 PM
> >> To: Wang, Jian J <jian.j.wang@intel.com>
> >> Cc: edk2-devel@lists.01.org
> >> Subject: Re: [edk2] [PATCH 0/2] Enable page table write protection
> >>
> >> Is this code in CPU driver?
> >>
> >> thank you!
> >> Yao, Jiewen
> >>
> >>
> >>> 在 2017年11月29日，下午6:24，Wang, Jian J <jian.j.wang@intel.com>
> 写
> >> 道：
> >>>
> >>> Yes, I validated them manually with JTAG debug tool.
> >>>
> >>> if ((L3PageTable[Index3] & IA32_PG_PS) != 0) {
> >>>  // 1G page. Split to 2M.
> >>>  L2PageTable = AllocatePages (1);
> >>>  ASSERT (L2PageTable != NULL);
> >>>   PhysicalAddress = L3PageTable[Index3] & PAGING_1G_ADDRESS_MASK_64;
> >>>   for (Index = 0; Index < EFI_PAGE_SIZE/sizeof (UINT64); ++Index) {
> >>>    L2PageTable[Index] = PhysicalAddress  | AddressEncMask |
> >>>                         IA32_PG_PS | IA32_PG_P | IA32_PG_RW;
> >>>    PhysicalAddress += SIZE_2MB;
> >>>  }
> >>>  L3PageTable[Index3] = (UINT64) (UINTN) L2PageTable | AddressEncMask |
> >>>                                         IA32_PG_P | IA32_PG_RW;
> >>>  SetPageReadOnly (PageTableBase,
> >> (EFI_PHYSICAL_ADDRESS)(UINTN)L2PageTable);
> >>> }
> >>>
> >>> The newly allocated page table is set in the SetPageReadOnly() itself
> >> recursively, like
> >>> above code in which L2PageTable is allocated and then set it to be read-only
> >> after
> >>> initializing the table content.
> >>>
> >>>> -----Original Message-----
> >>>> From: Yao, Jiewen
> >>>> Sent: Wednesday, November 29, 2017 5:16 PM
> >>>> To: Wang, Jian J <jian.j.wang@intel.com>
> >>>> Cc: edk2-devel@lists.01.org
> >>>> Subject: Re: [edk2] [PATCH 0/2] Enable page table write protection
> >>>>
> >>>> Thanks.
> >>>>
> >>>> May I know if this is validated in uefi shell, that all page table is readonly?
> >>>>
> >>>> I did not find the code to set new allocated split page to be readonly. Can
> you
> >>>> give me a hand on that?
> >>>>
> >>>> thank you!
> >>>> Yao, Jiewen
> >>>>
> >>>>
> >>>>> 在 2017年11月29日，下午4:47，Jian J Wang <jian.j.wang@intel.com>
> >> 写
> >>>> 道：
> >>>>>
> >>>>> Write Protect feature (CR0.WP) is always enabled in driver
> >>>> UefiCpuPkg/CpuDxe.
> >>>>> But the memory pages used for page table are not set as read-only in the
> >>>> driver
> >>>>> DxeIplPeim, after the paging is setup. This might jeopardize the page
> table
> >>>>> integrity if there's buffer overflow occured in other part of system.
> >>>>>
> >>>>> This patch series will change this situation by clearing R/W bit in page
> >> attribute
> >>>>> of the pages used as page table.
> >>>>>
> >>>>> Validation works include booting Windows (10/server 2016) and Linux
> >>>> (Fedora/Ubuntu)
> >>>>> on OVMF and Intel real platform.
> >>>>>
> >>>>> Jian J Wang (2):
> >>>>> UefiCpuPkg/CpuDxe: Check CR0.WP before changing page table
> >>>>> MdeModulePkg/DxeIpl: Mark page table as read-only
> >>>>>
> >>>>> MdeModulePkg/Core/DxeIplPeim/X64/VirtualMemory.c | 166
> >>>> +++++++++++++++++++++++
> >>>>> MdeModulePkg/Core/DxeIplPeim/X64/VirtualMemory.h |  14 ++
> >>>>> UefiCpuPkg/CpuDxe/CpuPageTable.c                 |  65 ++++++++-
> >>>>> 3 files changed, 241 insertions(+), 4 deletions(-)
> >>>>>
> >>>>> --
> >>>>> 2.14.1.windows.1
> >>>>>
> >>>>> _______________________________________________
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