In function BasePrintLibSPrintMarker(), when processing ASCII format
strings, if the format string walker pointer 'Format' is pointing at the
end of the format string (i.e. '\0'), the following expression:
*(Format + 1)
will read an undefined value.

Though this value won't affect the functionality, since it will be masked
by variable 'FormatMask':
(*(Format + 1) << 8)) & FormatMask
(FormatMask is 0xff for ASCII format string)

This commit adds additional logic to avoid reading undefined content.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
---
 MdePkg/Library/BasePrintLib/PrintLibInternal.c | 48 ++++++++++++++++----
 1 file changed, 40 insertions(+), 8 deletions(-)

diff --git a/MdePkg/Library/BasePrintLib/PrintLibInternal.c b/MdePkg/Library/BasePrintLib/PrintLibInternal.c
index 9b15a07ac0..d665b7b1d2 100644
--- a/MdePkg/Library/BasePrintLib/PrintLibInternal.c
+++ b/MdePkg/Library/BasePrintLib/PrintLibInternal.c
@@ -653,7 +653,11 @@ BasePrintLibSPrintMarker (
   //
   // Get the first character from the format string
   //
-  FormatCharacter = ((*Format & 0xff) | (*(Format + 1) << 8)) & FormatMask;
+  if (BytesPerFormatCharacter == 1) {
+    FormatCharacter = (*Format & 0xff) & FormatMask;
+  } else {
+    FormatCharacter = ((*Format & 0xff) | (*(Format + 1) << 8)) & FormatMask;
+  }
 
   //
   // Loop until the end of the format string is reached or the output buffer is full
@@ -685,7 +689,11 @@ BasePrintLibSPrintMarker (
       //
       for (Done = FALSE; !Done; ) {
         Format += BytesPerFormatCharacter;
-        FormatCharacter = ((*Format & 0xff) | (*(Format + 1) << 8)) & FormatMask;
+        if (BytesPerFormatCharacter == 1) {
+          FormatCharacter = (*Format & 0xff) & FormatMask;
+        } else {
+          FormatCharacter = ((*Format & 0xff) | (*(Format + 1) << 8)) & FormatMask;
+        }
         switch (FormatCharacter) {
         case '.': 
           Flags |= PRECISION; 
@@ -738,7 +746,11 @@ BasePrintLibSPrintMarker (
           for (Count = 0; ((FormatCharacter >= '0') &&  (FormatCharacter <= '9')); ){
             Count = (Count * 10) + FormatCharacter - '0';
             Format += BytesPerFormatCharacter;
-            FormatCharacter = ((*Format & 0xff) | (*(Format + 1) << 8)) & FormatMask;
+            if (BytesPerFormatCharacter == 1) {
+              FormatCharacter = (*Format & 0xff) & FormatMask;
+            } else {
+              FormatCharacter = ((*Format & 0xff) | (*(Format + 1) << 8)) & FormatMask;
+            }
           }
           Format -= BytesPerFormatCharacter;
           if ((Flags & PRECISION) == 0) {
@@ -1017,7 +1029,11 @@ BasePrintLibSPrintMarker (
 
       case '\r':
         Format += BytesPerFormatCharacter;
-        FormatCharacter = ((*Format & 0xff) | (*(Format + 1) << 8)) & FormatMask;
+        if (BytesPerFormatCharacter == 1) {
+          FormatCharacter = (*Format & 0xff) & FormatMask;
+        } else {
+          FormatCharacter = ((*Format & 0xff) | (*(Format + 1) << 8)) & FormatMask;
+        }
         if (FormatCharacter == '\n') {
           //
           // Translate '\r\n' to '\r\n'
@@ -1038,7 +1054,11 @@ BasePrintLibSPrintMarker (
         //
         ArgumentString = "\r\n";
         Format += BytesPerFormatCharacter;
-        FormatCharacter = ((*Format & 0xff) | (*(Format + 1) << 8)) & FormatMask;
+        if (BytesPerFormatCharacter == 1) {
+          FormatCharacter = (*Format & 0xff) & FormatMask;
+        } else {
+          FormatCharacter = ((*Format & 0xff) | (*(Format + 1) << 8)) & FormatMask;
+        }
         if (FormatCharacter != '\r') {
           Format   -= BytesPerFormatCharacter;
         }
@@ -1057,7 +1077,11 @@ BasePrintLibSPrintMarker (
  
     case '\r':
       Format += BytesPerFormatCharacter;
-      FormatCharacter = ((*Format & 0xff) | (*(Format + 1) << 8)) & FormatMask;
+      if (BytesPerFormatCharacter == 1) {
+        FormatCharacter = (*Format & 0xff) & FormatMask;
+      } else {
+        FormatCharacter = ((*Format & 0xff) | (*(Format + 1) << 8)) & FormatMask;
+      }
       if (FormatCharacter == '\n') {
         //
         // Translate '\r\n' to '\r\n'
@@ -1078,7 +1102,11 @@ BasePrintLibSPrintMarker (
       //
       ArgumentString = "\r\n";
       Format += BytesPerFormatCharacter;
-      FormatCharacter = ((*Format & 0xff) | (*(Format + 1) << 8)) & FormatMask;
+      if (BytesPerFormatCharacter == 1) {
+        FormatCharacter = (*Format & 0xff) & FormatMask;
+      } else {
+        FormatCharacter = ((*Format & 0xff) | (*(Format + 1) << 8)) & FormatMask;
+      }
       if (FormatCharacter != '\r') {
         Format   -= BytesPerFormatCharacter;
       }
@@ -1206,7 +1234,11 @@ BasePrintLibSPrintMarker (
     //
     // Get the next character from the format string
     //
-    FormatCharacter = ((*Format & 0xff) | (*(Format + 1) << 8)) & FormatMask;
+    if (BytesPerFormatCharacter == 1) {
+      FormatCharacter = (*Format & 0xff) & FormatMask;
+    } else {
+      FormatCharacter = ((*Format & 0xff) | (*(Format + 1) << 8)) & FormatMask;
+    }
   }
 
   if ((Flags & COUNT_ONLY_NO_PRINT) != 0) {
-- 
2.12.0.windows.1

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