[PATCH] MdeModulePkg/NonDiscoverable: fix memory override bug

[Heyi Guo <heyi.guo@linaro.org>]

For PciIoPciRead interface, memory prior to Buffer would be written
with zeros if Offset was larger than sizeof (Dev->ConfigSpace), which
would cause serious system exception.

So we add a pre-check branch to avoid memory override.

Cc: Star Zeng <star.zeng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Heyi Guo <heyi.guo@linaro.org>
---
 .../Bus/Pci/NonDiscoverablePciDeviceDxe/NonDiscoverablePciDeviceIo.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/MdeModulePkg/Bus/Pci/NonDiscoverablePciDeviceDxe/NonDiscoverablePciDeviceIo.c b/MdeModulePkg/Bus/Pci/NonDiscoverablePciDeviceDxe/NonDiscoverablePciDeviceIo.c
index c836ad6..0e42ae4 100644
--- a/MdeModulePkg/Bus/Pci/NonDiscoverablePciDeviceDxe/NonDiscoverablePciDeviceIo.c
+++ b/MdeModulePkg/Bus/Pci/NonDiscoverablePciDeviceDxe/NonDiscoverablePciDeviceIo.c
@@ -465,6 +465,11 @@ PciIoPciRead (
   Address = (UINT8 *)&Dev->ConfigSpace + Offset;
   Length = Count << ((UINTN)Width & 0x3);
 
+  if (Offset >= sizeof (Dev->ConfigSpace)) {
+    ZeroMem (Buffer, Length);
+    return EFI_SUCCESS;
+  }
+
   if (Offset + Length > sizeof (Dev->ConfigSpace)) {
     //
     // Read all zeroes for config space accesses beyond the first
-- 
1.9.1