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

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

-- 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