On 2/8/2023 10:49 AM, Ard Biesheuvel wrote:
On Wed, 8 Feb 2023 at 19:32, Marvin Häuser <mhaeu...@posteo.de> wrote:
Thanks!! :) Comments inline.
On 8. Feb 2023, at 18:58, Ard Biesheuvel <a...@kernel.org> wrote:
The DXE core implementation of PcdDxeNxMemoryProtectionPolicy already
contains an assertion that EfiConventionalMemory and EfiBootServicesData
are subjected to the same policy when it comes to the use of NX
permissions. The reason for this is that we may otherwise end up with
unbounded recursion in the page table code, given that allocating a page
table would then involve a permission attribute change, and this could
result in the need for a block entry to be split, which would trigger
the allocation of a page table recursively.
For the same reason, a shortcut exists in ApplyMemoryProtectionPolicy()
where, instead of setting the memory attributes unconditionally, we
compare the NX policies and avoid touching the page tables if they are
the same for the old and the new memory types. Without this shortcut, we
may end up in a situation where, as the CPU arch protocol DXE driver is
ramping up, the same unbounded recursion is triggered, due to the fact
that the NX policy for EfiConventionalMemory has not been applied yet.
To break this cycle, let's remap all EfiConventionalMemory regions
according to the NX policy for EfiBootServicesData before exposing the
CPU arch protocol to the DXE core and other drivers. This ensures that
creating EfiBootServicesData allocations does not result in memory
attribute changes, and therefore no recursion.
Signed-off-by: Ard Biesheuvel <a...@kernel.org>
---
ArmPkg/Drivers/CpuDxe/CpuDxe.c | 77 ++++++++++++++++++++
ArmPkg/Drivers/CpuDxe/CpuDxe.inf | 2 +
2 files changed, 79 insertions(+)
diff --git a/ArmPkg/Drivers/CpuDxe/CpuDxe.c b/ArmPkg/Drivers/CpuDxe/CpuDxe.c
index d04958e79e52..83fd6fd4e476 100644
--- a/ArmPkg/Drivers/CpuDxe/CpuDxe.c
+++ b/ArmPkg/Drivers/CpuDxe/CpuDxe.c
@@ -11,6 +11,8 @@
#include <Guid/IdleLoopEvent.h>
+#include <Library/MemoryAllocationLib.h>
+
BOOLEAN mIsFlushingGCD;
/**
@@ -227,6 +229,69 @@ InitializeDma (
CpuArchProtocol->DmaBufferAlignment = ArmCacheWritebackGranule ();
}
+STATIC
+VOID
+RemapUnusedMemoryNx (
+ VOID
+ )
+{
This feels somewhat hacky but it's strictly better than what we have
right now and a value add so I'm all for this change.
+ UINT64 TestBit;
+ UINTN MemoryMapSize;
+ UINTN MapKey;
+ UINTN DescriptorSize;
+ UINT32 DescriptorVersion;
+ EFI_MEMORY_DESCRIPTOR *MemoryMap;
+ EFI_MEMORY_DESCRIPTOR *MemoryMapEntry;
+ EFI_MEMORY_DESCRIPTOR *MemoryMapEnd;
+ EFI_STATUS Status;
+
+ TestBit = LShiftU64 (1, EfiBootServicesData);
+ if ((PcdGet64 (PcdDxeNxMemoryProtectionPolicy) & TestBit) == 0) {
+ return;
+ }
+
+ MemoryMapSize = 0;
+ MemoryMap = NULL;
+
+ Status = gBS->GetMemoryMap (
+ &MemoryMapSize,
+ MemoryMap,
+ &MapKey,
+ &DescriptorSize,
+ &DescriptorVersion
+ );
+ ASSERT (Status == EFI_BUFFER_TOO_SMALL);
+ do {
+ MemoryMap = (EFI_MEMORY_DESCRIPTOR *)AllocatePool (MemoryMapSize);
nitpick: *If* there is a V2, maybe drop the cast?
+ ASSERT (MemoryMap != NULL);
I know ASSERTing for NULL is a common pattern for some reason, but I'd rather
not have more code with this added.
+ Status = gBS->GetMemoryMap (
+ &MemoryMapSize,
+ MemoryMap,
+ &MapKey,
+ &DescriptorSize,
+ &DescriptorVersion
+ );
Another nitpick, isn't it common practice to call the Core* functions directly
within *Core? I know there is code that doesn't, but the former should be more
efficient.
+ if (EFI_ERROR (Status)) {
+ FreePool (MemoryMap);
+ }
+ } while (Status == EFI_BUFFER_TOO_SMALL);
Is this guaranteed to terminate? I mean, I get the idea - try again with the
larger size and when allocating the bigger buffer, its potential new entry will
already be accounted for. However, I saw downstream code that tried something
like this (they actually added a constant overhead ahead of time) bounded by
something like 5 iterations. Might just have been defensive programming -
probably was -, but it's not trivial to verify, I think, is it?
Regarding the added constant, the spec actually says the following, which
obviously is just to shortcut a second round of GetMemoryMap(), but still:
"The actual size of the buffer allocated for the consequent call to GetMemoryMap()
should be bigger then the value returned in MemoryMapSize"
It appears the spec did not define a canonical way to call GetMemoryMap(), did
it? :(
This is all copy-pasted from MdeModulePkg/Core/Dxe/Misc/MemoryProtection.c
+
+ ASSERT_EFI_ERROR (Status);
+
+ MemoryMapEntry = MemoryMap;
+ MemoryMapEnd = (EFI_MEMORY_DESCRIPTOR *)((UINT8 *)MemoryMap +
MemoryMapSize);
+ while ((UINTN)MemoryMapEntry < (UINTN)MemoryMapEnd) {
+ if (MemoryMapEntry->Type == EfiConventionalMemory) {
+ ArmSetMemoryRegionNoExec (
+ MemoryMapEntry->PhysicalStart,
+ EFI_PAGES_TO_SIZE (MemoryMapEntry->NumberOfPages)
+ );
+ }
+
+ MemoryMapEntry = NEXT_MEMORY_DESCRIPTOR (MemoryMapEntry, DescriptorSize);
+ }
+}
+
EFI_STATUS
CpuDxeInitialize (
IN EFI_HANDLE ImageHandle,
@@ -240,6 +305,18 @@ CpuDxeInitialize (
InitializeDma (&mCpu);
+ //
+ // Once we install the CPU arch protocol, the DXE core's memory
+ // protection routines will invoke them to manage the permissions of page
+ // allocations as they are created. Given that this includes pages
+ // allocated for page tables by this driver, we must ensure that unused
+ // memory is mapped with the same permissions as boot services data
+ // regions. Otherwise, we may end up with unbounded recursion, due to the
+ // fact that updating permissions on a newly allocated page table may trigger
+ // a block entry split, which triggers a page table allocation, etc etc
+ //
+ RemapUnusedMemoryNx ();
Hmm. I might be too tired, but why is this not already done by
InitializeDxeNxMemoryProtectionPolicy(), assuming BS_Data and ConvMem have the
same XP configuration?
This reassures me that the CPU Arch protocol producers should be linked into
DxeCore rather than loaded at some arbitrary point in time... Unrelated to the
patch, of course.
The ordering here is a bit tricky. As soon as the CPU arch protocol is
exposed, every allocation will be remapped individually, resulting in
page table splits and therefore recursion.
As Ard says, this is done in InitializeDxeNxMemoryProtectionPolicy(),
but at that point the calls to ApplyMemoryProtectionPolicy() will cause
the infinite recursion issue when you no longer skip applying attributes
if the to-from types have the same NX policy.
Yes, it is arbitrary that protection is linked to the CPU Arch Protocol.
But, it would also be bad practice to apply the memory protection policy
before the interface for manipulating attributes has been published for
use by modules outside of the core. Page table and translation table
manipulation is architecturally defined, so I think a good long term
goal should be to allow the manipulation of attributes via a library
rather than a protocol in DXE as ARM platforms currently do.
+
Status = gBS->InstallMultipleProtocolInterfaces (
&mCpuHandle,
&gEfiCpuArchProtocolGuid,
diff --git a/ArmPkg/Drivers/CpuDxe/CpuDxe.inf b/ArmPkg/Drivers/CpuDxe/CpuDxe.inf
index e732e21cb94a..8fd0f4133088 100644
--- a/ArmPkg/Drivers/CpuDxe/CpuDxe.inf
+++ b/ArmPkg/Drivers/CpuDxe/CpuDxe.inf
@@ -48,6 +48,7 @@ [LibraryClasses]
DefaultExceptionHandlerLib
DxeServicesTableLib
HobLib
+ MemoryAllocationLib
PeCoffGetEntryPointLib
UefiDriverEntryPoint
UefiLib
@@ -64,6 +65,7 @@ [Guids]
[Pcd.common]
gArmTokenSpaceGuid.PcdVFPEnabled
+ gEfiMdeModulePkgTokenSpaceGuid.PcdDxeNxMemoryProtectionPolicy
[FeaturePcd.common]
gArmTokenSpaceGuid.PcdDebuggerExceptionSupport
--
2.39.1
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