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

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

When you split page tables, you need to allocate new pages for new page table.
Since you have new page tables added, you need to mark them to be read-only
as well, right? When you do this, the page table for the memory newly allocated
might still needs to be split. This is the worst case but there's still chance of it,
right? We cannot guarantee, theoretically, the page table for the newly allocated
page tables is in the same as just split ones. So, in worst case, once we want to
mark new page table to be read-only, we need to split the page table managing
those memory, and if we need to do split, we need to allocate more new pages.
This might fall into a recursive situation until all the smallest page table are created.
In practice it's almost impossible to let it happen but the chances are we will fall into 
such recursive situation more than one level.

> -----Original Message-----
> From: Yao, Jiewen
> Sent: Thursday, November 30, 2017 8:52 AM
> 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
> 
> -- 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
> [Jiewen] Sorry, I do not quite understand the problem statement.
> To set a page to be ReadOnly, you just flip the R/W bit in the page entry.
> The step I expect is:
> 1) You split page table.
> 2) Fill the data structure in the new entry.
> 3) Flip R/W bit (<== This is the new step).
> 4) FlushPageTable.
> 
> Thank you
> Yao Jiewen
> 
> > -----Original Message-----
> > From: Wang, Jian J
> > Sent: Thursday, November 30, 2017 8:44 AM
> > To: Yao, Jiewen <jiewen.yao@intel.com>
> > Cc: edk2-devel@lists.01.org
> > Subject: RE: [edk2] [PATCH 0/2] Enable page table write protection
> >
> > 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
> > > >>>>>
> > > >>>>> _______________________________________________
> > > >>>>> edk2-devel mailing list
> > > >>>>> edk2-devel@lists.01.org
> > > >>>>> https://lists.01.org/mailman/listinfo/edk2-devel