UnSafeAllocatePool rule (wdm)

The UnSafeAllocatePool rule is an important security rule that checks that a driver is not using deprecated DDIs to allocate memory.

The UnsafeAllocatePool rule specifies that the driver should not call:

ExAllocatePool

ExAllocatePoolWithTag

ExAllocatePoolWithQuota

ExAllocatePoolWithQuotaTag

ExAllocatePoolWithTagPriority

This rule is available in preview WDK builds 20236 and above.

Driver updates for versions of Windows 10, version 2004 and later

If you are building a driver that targets Windows 10, version 2004 and later, use the replacement APIs ExAllocatePool2 and ExAllocatePool3 instead.

The new APIs will zero pool allocations by default, to help avoid possible memory disclosure bugs.

ExAllocatePool/ExAllocatePoolWithTag

// Old code
PVOID Allocation = ExAllocatePoolWithTag(PagedPool, 100, 'abcd');
RtlZeroMemory(Allocation, 100);

// New code
PVOID Allocation = ExAllocatePool2(POOL_FLAG_PAGED, 100, 'abcd');

The old pool allocation APIs accept a POOL_TYPE argument, but the new allocation APIs accept a POOL_FLAGS argument. Update any associated code to use the new POOL_FLAGS argument.

ExAllocatePoolWithQuota/ExAllocatePoolWithQuotaTag

The new function will now return NULL on allocation failure by default. In order to have the allocator raise an exception on failure instead, the POOL_FLAG_RAISE_ON_FAILURE flag must be passed as discussed in ExAllocatePool2.

// Old code
PVOID Allocation = ExAllocatePoolWithQuotaTag(PagedPool | POOL_QUOTA_FAIL_INSTEAD_OF_RAISE, 100, 'abcd');
RtlZeroMemory(Allocation, 100);

// New code
PVOID Allocation = ExAllocatePool2(POOL_FLAG_PAGED | POOL_FLAG_USE_QUOTA, 100, 'abcd');

ExAllocatePoolWithTagPriority

// Old code
PVOID Allocation = ExAllocatePoolWithTagPriority(PagedPool, 100, 'abcd', HighPoolPriority);
RtlZeroMemory(Allocation, 100);

// New code
POOL_EXTENDED_PARAMETER params = {0};
params.Type = PoolExtendedParameterPriority;
params.Priority = HighPoolPriority;
PVOID Allocation = ExAllocatePool3(POOL_FLAG_PAGED, 100, 'abcd', &params, 1);

Driver updates for versions of Windows earlier than Windows 10, version 2004

If you are building a driver that targets versions of Windows prior to Windows 10, version 2004, you must use the following force inline wrapper functions.

You must also #define POOL_ZERO_DOWN_LEVEL_SUPPORT and call ExInitializeDriverRuntime during driver initialization before calling the pool allocation functions.

Locally defined inline functions

PVOID
NTAPI
ExAllocatePoolZero (
    _In_ __drv_strictTypeMatch(__drv_typeExpr) POOL_TYPE PoolType,
    _In_ SIZE_T NumberOfBytes,
    _In_ ULONG Tag
    )

PVOID
NTAPI
ExAllocatePoolQuotaZero (
    _In_ __drv_strictTypeMatch(__drv_typeExpr) POOL_TYPE PoolType,
    _In_ SIZE_T NumberOfBytes,
    _In_ ULONG Tag
    )

PVOID
NTAPI
ExAllocatePoolPriorityZero (
    _In_ __drv_strictTypeMatch(__drv_typeExpr) POOL_TYPE PoolType,
    _In_ SIZE_T NumberOfBytes,
    _In_ ULONG Tag,
    _In_ EX_POOL_PRIORITY Priority
    )

Refer to the latest wdm.h header for the implementation code for these code wrappers. For example this is the implementation for ExAllocatePoolPriorityZero, showing the use of RtlZeroMemory.

{
    PVOID Allocation;

    Allocation = ExAllocatePoolWithTagPriority((POOL_TYPE) (PoolType | POOL_ZERO_ALLOCATION),
                                               NumberOfBytes,
                                               Tag,
                                               Priority);

#if defined(POOL_ZERO_DOWN_LEVEL_SUPPORT)

    if ((!ExPoolZeroingNativelySupported) && (Allocation != NULL)) {
        RtlZeroMemory(Allocation, NumberOfBytes);
    }

#endif

    return Allocation;
}

Mapping of old APIs to new APIs

Example

// Old code
PVOID Allocation = ExAllocatePoolWithTag(PagedPool, 100, 'abcd');
RtlZeroMemory(Allocation, 100);

// New code

// Before headers are pulled in (or compiler defined)
#define POOL_ZERO_DOWN_LEVEL_SUPPORT

// Once during driver initialization
// Argument can be any value
ExInitializeDriverRuntime(0);

// Replacement for each pool allocation
PVOID Allocation = ExAllocatePoolZero(PagedPool, 100, 'abcd');

Driver model: WDM, Generic

How to test

At compile time:

1. Run Static Driver Verifier and specify the UnSafeAllocatePool rule.

2. Use the following steps (found in Using Static Driver Verifier to Find Defects in Windows Drivers) to run an analysis of your code:

   • Prepare your code (use role type declarations).
   • Run Static Driver Verifier.
   • View and analyze the results.

For more information, see Using Static Driver Verifier to Find Defects in Drivers.