MCCP Buffer Protection

Starting with Windows Vista, the RPC Marshalling Engine takes further steps to try to prevent client-side buffer overruns due to returned data. This facility is called Mini Compute Conformance Protection (MCCP).

When the client passes a pointer to an existing buffer to an [out] or [in,out] parameter, returned data for that parameter is copied into the existing buffer. If the returned data is larger than the passed buffer, a buffer overrun can occur when RPC copies the returned data into the too-small buffer. See Top-Level and Embedded Pointers.

With MCCP, RPC attempts to detect this condition and reject the call if it is detected. For buffers with a correlation value, such as [size_is], if the returned data does not fit in the specified buffer size, the call is rejected and RPC_X_BAD_STUB_DATA exception is raised. For unsized strings, the call is rejected if the existing string size (length until the null terminator) is insufficient to hold the returned string, the call is rejected. RPC cannot detect buffer overruns in all conditions, so the developer is advised to continue to take normal precautions against buffer overruns.

If the client does not pass an existing buffer for an [out] parameter, but instead passes a dereferenced pointer to NULL, RPC will follow normal rules to allocate a new buffer on the client’s behalf. This buffer will be allocated with sufficient space to hold the returned data.

A second protection is that for correlated parameters, RPC will enforce that a non-null buffer is passed when the correlation count variable is non-null.

HRESULT PassString( [in] DWORD Length, [in, unique, string, size_is( Length )]LPWSTR MyString );

If MyString is NULL, RPC will reject the call unless Length is set to 0. Note that RPC will allow Length to be 0 while MyString is non-NULL, and RPC will treat MyString as a 0-length buffer allocation.

 

 

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