On 01/16/20 13:22, Ni, Ray wrote:
> Laszlo,
> Thanks for finding and fixing the bug.
>
> The code change for 5level paging was done many years ago
> before mAddressEncMask was added.
>
> The two lines of *Pd assignment might be caused when resolving
> the local merge conflict.
Ah, that makes total sense. I didn't expect this reason!
> Reviewed-by: Ray Ni <ray...@intel.com>
Thanks!
Laszlo
>
>> -----Original Message-----
>> From: Laszlo Ersek <ler...@redhat.com>
>> Sent: Friday, January 10, 2020 7:25 AM
>> To: edk2-devel-groups-io <devel@edk2.groups.io>
>> Cc: Dong, Eric <eric.d...@intel.com>; Ni, Ray <ray...@intel.com>
>> Subject: [PATCH] UefiCpuPkg/PiSmmCpuDxeSmm: fix 2M->4K page splitting
>> regression for PDEs
>>
>> In commit 4eee0cc7cc0d ("UefiCpuPkg/PiSmmCpu: Enable 5 level paging when
>> CPU supports", 2019-07-12), the Page Directory Entry setting was regressed
>> (corrupted) when splitting a 2MB page to 512 4KB pages, in the
>> InitPaging() function.
>>
>> Consider the following hunk, displayed with
>>
>> $ git show --function-context --ignore-space-change 4eee0cc7cc0db
>>
>>> //
>>> // If it is 2M page, check IsAddressSplit()
>>> //
>>> if (((*Pd & IA32_PG_PS) != 0) && IsAddressSplit (Address)) {
>>> //
>>> // Based on current page table, create 4KB page table for
>>> split area.
>>> //
>>> ASSERT (Address == (*Pd & PHYSICAL_ADDRESS_MASK));
>>>
>>> Pt = AllocatePageTableMemory (1);
>>> ASSERT (Pt != NULL);
>>>
>>> + *Pd = (UINTN) Pt | IA32_PG_RW | IA32_PG_P;
>>> +
>>> // Split it
>>> - for (PtIndex = 0; PtIndex < SIZE_4KB / sizeof(*Pt); PtIndex++) {
>>> - Pt[PtIndex] = Address + ((PtIndex << 12) | mAddressEncMask |
>>> PAGE_ATTRIBUTE_BITS);
>>> + for (PtIndex = 0; PtIndex < SIZE_4KB / sizeof(*Pt); PtIndex++,
>>> Pt++) {
>>> + *Pt = Address + ((PtIndex << 12) | mAddressEncMask |
>>> PAGE_ATTRIBUTE_BITS);
>>> } // end for PT
>>> *Pd = (UINT64)(UINTN)Pt | mAddressEncMask |
>>> PAGE_ATTRIBUTE_BITS;
>>> } // end if IsAddressSplit
>>> } // end for PD
>>
>> First, the new assignment to the Page Directory Entry (*Pd) is
>> superfluous. That's because (a) we set (*Pd) after the Page Table Entry
>> loop anyway, and (b) here we do not attempt to access the memory starting
>> at "Address" (which is mapped by the original value of the Page Directory
>> Entry).
>>
>> Second, appending "Pt++" to the incrementing expression of the PTE loop is
>> a bug. It causes "Pt" to point *right past* the just-allocated Page Table,
>> once we finish the loop. But the PDE assignment that immediately follows
>> the loop assumes that "Pt" still points to the *start* of the new Page
>> Table.
>>
>> The result is that the originally mapped 2MB page disappears from the
>> processor's view. The PDE now points to a "Page Table" that is filled with
>> garbage. The random entries in that "Page Table" will cause some virtual
>> addresses in the original 2MB area to fault. Other virtual addresses in
>> the same range will no longer have a 1:1 physical mapping, but be
>> scattered over random physical page frames.
>>
>> The second phase of the InitPaging() function ("Go through page table and
>> set several page table entries to absent or execute-disable") already
>> manipulates entries in wrong Page Tables, for such PDEs that got split in
>> the first phase.
>>
>> This issue has been caught as follows:
>>
>> - OVMF is started with 2001 MB of guest RAM.
>>
>> - This places the main SMRAM window at 0x7C10_1000.
>>
>> - The SMRAM management in the SMM Core links this SMRAM window into
>> "mSmmMemoryMap", with a FREE_PAGE_LIST record placed at the start of the
>> area.
>>
>> - At "SMM Ready To Lock" time, PiSmmCpuDxeSmm calls InitPaging(). The
>> first phase (quoted above) decides to split the 2MB page at 0x7C00_0000
>> into 512 4KB pages, and corrupts the PDE. The new Page Table is
>> allocated at 0x7CE0_D000, but the PDE is set to 0x7CE0_E000 (plus
>> attributes 0x67).
>>
>> - Due to the corrupted PDE, the second phase of InitPaging() already looks
>> up the PTE for Address=0x7C10_1000 in the wrong place. The second phase
>> goes on to mark bogus PTEs as "NX".
>>
>> - PiSmmCpuDxeSmm calls SetMemMapAttributes(). Address 0x7C10_1000 is at
>> the base of the SMRAM window, therefore it happens to be listed in the
>> SMRAM map as an EfiConventionalMemory region. SetMemMapAttributes()
>> calls SmmSetMemoryAttributes() to mark the region as XP. However,
>> GetPageTableEntry() in ConvertMemoryPageAttributes() fails -- address
>> 0x7C10_1000 is no longer mapped by anything! -- and so the attribute
>> setting fails with RETURN_UNSUPPORTED. This error goes unnoticed, as
>> SetMemMapAttributes() ignores the return value of
>> SmmSetMemoryAttributes().
>>
>> - When SetMemMapAttributes() reaches another entry in the SMRAM map,
>> ConvertMemoryPageAttributes() decides it needs to split a 2MB page, and
>> calls SplitPage().
>>
>> - SplitPage() calls AllocatePageTableMemory() for the new Page Table,
>> which takes us to InternalAllocMaxAddress() in the SMM Core.
>>
>> - The SMM core attempts to read the FREE_PAGE_LIST record at 0x7C10_1000.
>> Because this virtual address is no longer mapped, the firmware crashes
>> in InternalAllocMaxAddress(), when accessing (Pages->NumberOfPages).
>>
>> Remove the useless assignment to (*Pd) from before the loop. Revert the
>> loop incrementing and the PTE assignment to the known good version.
>>
>> Cc: Eric Dong <eric.d...@intel.com>
>> Cc: Ray Ni <ray...@intel.com>
>> Ref: https://bugzilla.redhat.com/show_bug.cgi?id=1789335
>> Fixes: 4eee0cc7cc0db74489b99c19eba056b53eda6358
>> Signed-off-by: Laszlo Ersek <ler...@redhat.com>
>> ---
>>
>> Notes:
>> Repo: https://github.com/lersek/edk2.git
>> Branch: smm_cpu_page_split_corrupt_pde
>>
>> UefiCpuPkg/PiSmmCpuDxeSmm/SmmProfile.c | 6 ++----
>> 1 file changed, 2 insertions(+), 4 deletions(-)
>>
>> diff --git a/UefiCpuPkg/PiSmmCpuDxeSmm/SmmProfile.c
>> b/UefiCpuPkg/PiSmmCpuDxeSmm/SmmProfile.c
>> index c5131526f0c6..c47b5573e366 100644
>> --- a/UefiCpuPkg/PiSmmCpuDxeSmm/SmmProfile.c
>> +++ b/UefiCpuPkg/PiSmmCpuDxeSmm/SmmProfile.c
>> @@ -657,11 +657,9 @@ InitPaging (
>> Pt = AllocatePageTableMemory (1);
>> ASSERT (Pt != NULL);
>>
>> - *Pd = (UINTN) Pt | IA32_PG_RW | IA32_PG_P;
>> -
>> // Split it
>> - for (PtIndex = 0; PtIndex < SIZE_4KB / sizeof(*Pt); PtIndex++,
>> Pt++) {
>> - *Pt = Address + ((PtIndex << 12) | mAddressEncMask |
>> PAGE_ATTRIBUTE_BITS);
>> + for (PtIndex = 0; PtIndex < SIZE_4KB / sizeof(*Pt); PtIndex++) {
>> + Pt[PtIndex] = Address + ((PtIndex << 12) | mAddressEncMask |
>> PAGE_ATTRIBUTE_BITS);
>> } // end for PT
>> *Pd = (UINT64)(UINTN)Pt | mAddressEncMask | PAGE_ATTRIBUTE_BITS;
>> } // end if IsAddressSplit
>> --
>> 2.19.1.3.g30247aa5d201
>
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