MmSecureVirtualMemory function

The MmSecureVirtualMemory routine secures a user-space memory address range so that it cannot be freed and its page protection cannot be made more restrictive.

Syntax

NTKERNELAPI HANDLE MmSecureVirtualMemory(
  __in_data_source(USER_MODE)PVOID  Address,
  __in_data_source(USER_MODE)SIZE_T Size,
  ULONG                             ProbeMode
);

Parameters

Address

The beginning of the user virtual address range to secure.

Size

The size, in bytes, of the virtual address range to secure.

ProbeMode

The most restrictive page protection that is allowed. Use PAGE_READWRITE to specify that the address range must remain both readable and writable, or use PAGE_READONLY to specify that the address range must only remain readable.

ProbeModeMeaning
PAGE_READWRITEProtection cannot be changed to PAGE_NOACCESS or PAGE_READONLY. All other protection changes are allowed.
PAGE_READONLYProtection cannot be changed to PAGE_NOACCESS. All other protection changes are allowed.

Return Value

On success, MmSecureVirtualMemory returns an opaque pointer value that the driver passes to the MmUnsecureVirtualMemory routine to unsecure the memory address range. If the routine is unable to secure the memory address range, it returns NULL.

Remarks

MmSecureVirtualMemory can be used to avoid certain race conditions on user-mode buffers. For example, if a driver checks to see if the buffer is writable, but then the originating user-mode process changes the buffer to be read-only before the driver can write to the buffer, then a race condition can result. The driver can use MmSecureVirtualMemory with PAGE_READWRITE probe mode to guarantee that the buffer will remain writable until the driver calls MmUnsecureVirtualMemory. The routine also protects against the originating user-mode process freeing the buffer. Here are a few guidelines about calling these routines:

  • Calling MmSecureVirtualMemory with PAGE_READONLY does not guarantee that the buffer will remain read-only. The read-only probe mode prevents the user from changing the protection of the buffer to PAGE_NOACCESS. It does not prevent changing the protection to PAGE_READWRITE (or PAGE_WRITECOPY, for mapped views).
  • If a driver calls MmSecureVirtualMemory and does not call MmUnsecureVirtualMemory, the memory is automatically unsecured when the process terminates.

  • If the driver calls MmUnsecureVirtualMemory, it must call it in the context of the process in which the memory was originally secured, and before that process terminates.

  • Typically drivers need to reference the process when they secure the memory, then later call KeStackAttachProcess to switch to the context of that process before calling MmUnsecureVirtualMemory.

  • To detect process termination drivers can use PsSetCreateProcessNotifyRoutine. Alternatively, the process can submit an IRP with a cancel routine that is invoked by the I/O manager when the process is exiting. In the cancel routine the driver can attach to the process and call MmUnsecureVirtualMemory.

While MmSecureVirtualMemory can be used to guarantee that reading or writing user memory will not raise an exception due to insufficient page permissions, it does not protect against other types of exceptions. For example, it does not protect against exceptions raised when the system finds a bad disk block in the page file. Therefore, drivers must still wrap all user memory accesses in a try/except block. Because of this, we recommend that drivers do not use this function. For more information, see Handling Exceptions.

Requirements

   
Windows version Available starting with Windows 2000.
Target Platform Universal
Header ntddk.h (include Ntddk.h)
Library NtosKrnl.lib
DLL NtosKrnl.exe
IRQL "<=APC_LEVEL"
DDI compliance rules IrqlMmApcLte, HwStorPortProhibitedDDIs

See Also

MmUnsecureVirtualMemory