[edk2-devel][PATCH v1 1/1] MdeModulePkg: HeapGuard: Don't Assume Pool Head Allocated In First Page

[Oliver Smith-Denny]

Currently, HeapGuard, when in the GuardAlignedToTail mode, assumes that
the pool head has been allocated in the first page of memory that was
allocated. This is not the case for ARM64 platforms when allocating
runtime pools, as RUNTIME_PAGE_ALLOCATION_GRANULARITY is 64k, unlike
X64, which has RUNTIME_PAGE_ALLOCATION_GRANULARITY as 4k.

When a runtime pool is allocated on ARM64, the minimum number of pages
allocated is 16, to match the runtime granularity. When a small pool is
allocated and GuardAlignedToTail is true, HeapGuard instructs the pool
head to be placed as (MemoryAllocated + EFI_PAGES_TO_SIZE(Number of Pages)
- SizeRequiredForPool).

This gives this scenario:

|Head Guard|Large Free Number of Pages|PoolHead|TailGuard|

When this pool goes to be freed, HeapGuard instructs the pool code to
free from (PoolHead & ~EFI_PAGE_MASK). However, this assumes that the
PoolHead is in the first page allocated, which as shown above is not true
in this case. For the 4k granularity case (i.e. where the correct number of
pages are allocated for this pool), this logic does work.

In this failing case, HeapGuard then instructs the pool code to free 16
(or more depending) pages from the page the pool head was allocated on,
which as seen above means we overrun the pool and attempt to free memory
far past the pool. We end up running into the tail guard and getting an
access flag fault.

This causes ArmVirtQemu to fail to boot with an access flag fault when
GuardAlignedToTail is set to true (and pool guard enabled for runtime
memory). It should also cause all ARM64 platforms to fail in this
configuration, for exactly the same reason, as this is core code making
the assumption.

This patch removes HeapGuard's assumption that the pool head is allocated
on the first page and instead undoes the same logic that HeapGuard did
when allocating the pool head in the first place.

With this patch in place, ArmVirtQemu boots with GuardAlignedToTail
set to true (and when it is false, also).

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=4521
Github PR: https://github.com/tianocore/edk2/pull/4731

Cc: Leif Lindholm <quic_llindhol at quicinc.com>
Cc: Ard Biesheuvel <ardb+tianocore at kernel.org>
Cc: Jian J Wang <jian.j.wang at intel.com>
Cc: Liming Gao <gaoliming at byosoft.com.cn>
Cc: Dandan Bi <dandan.bi at intel.com>

Signed-off-by: Oliver Smith-Denny <osde at linux.microsoft.com>
---
 MdeModulePkg/Core/Dxe/Mem/HeapGuard.h |  7 ++++++-
 MdeModulePkg/Core/Dxe/Mem/HeapGuard.c | 14 +++++++++++---
 MdeModulePkg/Core/Dxe/Mem/Pool.c      |  2 +-
 3 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/MdeModulePkg/Core/Dxe/Mem/HeapGuard.h b/MdeModulePkg/Core/Dxe/Mem/HeapGuard.h
index 9a32b4dd51d5..24b4206c0e02 100644
--- a/MdeModulePkg/Core/Dxe/Mem/HeapGuard.h
+++ b/MdeModulePkg/Core/Dxe/Mem/HeapGuard.h
@@ -378,12 +378,17 @@ AdjustPoolHeadA (
   Get the page base address according to pool head address.
 
   @param[in]  Memory    Head address of pool to free.
+  @param[in]  NoPages   Number of pages actually allocated.
+  @param[in]  Size      Size of memory requested.
+                        (plus pool head/tail overhead)
 
   @return Address of pool head.
 **/
 VOID *
 AdjustPoolHeadF (
-  IN EFI_PHYSICAL_ADDRESS  Memory
+  IN EFI_PHYSICAL_ADDRESS  Memory,
+  IN UINTN                 NoPages,
+  IN UINTN                 Size
   );
 
 /**
diff --git a/MdeModulePkg/Core/Dxe/Mem/HeapGuard.c b/MdeModulePkg/Core/Dxe/Mem/HeapGuard.c
index 9377f620c5a5..0c0ca61872b4 100644
--- a/MdeModulePkg/Core/Dxe/Mem/HeapGuard.c
+++ b/MdeModulePkg/Core/Dxe/Mem/HeapGuard.c
@@ -1037,12 +1037,17 @@ AdjustPoolHeadA (
   Get the page base address according to pool head address.
 
   @param[in]  Memory    Head address of pool to free.
+  @param[in]  NoPages   Number of pages actually allocated.
+  @param[in]  Size      Size of memory requested.
+                        (plus pool head/tail overhead)
 
   @return Address of pool head.
 **/
 VOID *
 AdjustPoolHeadF (
-  IN EFI_PHYSICAL_ADDRESS  Memory
+  IN EFI_PHYSICAL_ADDRESS  Memory,
+  IN UINTN                 NoPages,
+  IN UINTN                 Size
   )
 {
   if ((Memory == 0) || ((PcdGet8 (PcdHeapGuardPropertyMask) & BIT7) != 0)) {
@@ -1053,9 +1058,12 @@ AdjustPoolHeadF (
   }
 
   //
-  // Pool head is put near the tail Guard
+  // Pool head is put near the tail Guard. We need to exactly undo the addition done in AdjustPoolHeadA
+  // because we may not have allocated the pool head on the first allocated page, since we are aligned to
+  // the tail and on some architectures, the runtime page allocation granularity is > one page. So we allocate
+  // more pages than we need and put the pool head somewhere past the first page.
   //
-  return (VOID *)(UINTN)(Memory & ~EFI_PAGE_MASK);
+  return (VOID *)(UINTN)(Memory + Size - EFI_PAGES_TO_SIZE (NoPages));
 }
 
 /**
diff --git a/MdeModulePkg/Core/Dxe/Mem/Pool.c b/MdeModulePkg/Core/Dxe/Mem/Pool.c
index b20cbfdedbab..716dd045f9fd 100644
--- a/MdeModulePkg/Core/Dxe/Mem/Pool.c
+++ b/MdeModulePkg/Core/Dxe/Mem/Pool.c
@@ -783,7 +783,7 @@ CoreFreePoolI (
     NoPages  = EFI_SIZE_TO_PAGES (Size) + EFI_SIZE_TO_PAGES (Granularity) - 1;
     NoPages &= ~(UINTN)(EFI_SIZE_TO_PAGES (Granularity) - 1);
     if (IsGuarded) {
-      Head = AdjustPoolHeadF ((EFI_PHYSICAL_ADDRESS)(UINTN)Head);
+      Head = AdjustPoolHeadF ((EFI_PHYSICAL_ADDRESS)(UINTN)Head, NoPages, Size);
       CoreFreePoolPagesWithGuard (
         Pool->MemoryType,
         (EFI_PHYSICAL_ADDRESS)(UINTN)Head,
-- 
2.40.1



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