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

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

Can you just allocate 1 more page for split?
If new one need split, you can just use the additional page.
If not, you can free it later.

Thank you
Yao Jiewen


> -----Original Message-----
> From: Wang, Jian J
> Sent: Thursday, November 30, 2017 9:17 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
> 
> 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