From 56658c22a674062e46f5bfd2d4da71a88bf3c0e5 Mon Sep 17 00:00:00 2001
From: Ruiyu Ni <ruiyu.ni@intel.com>
Date: Fri, 2 Feb 2018 18:47:18 +0800
Subject: [PATCH] MdePkg/SafeString: Fix potential out-of-bound memory access

Today's implementation of [Ascii]StrnCpyS/[Ascii]StrnCatS calls
StrnLenS () to get the length of source string but supplies the
destination buffer size as max size.
It's a bug that may cause out-of-bound memory access.
For example:
  StrnCpyS (Dest[10], 10, "hello", 6)
  -> StrnLenS ("hello", 10) //< cause out-of bound memory access

In a pool guard enabled environment, when using shell to edit an
existing file which contains empty line, the page fault is met.

The patch fixes the four library functions to avoid such
out-of-bound memory access.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
---
 MdePkg/Library/BaseLib/SafeString.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/MdePkg/Library/BaseLib/SafeString.c b/MdePkg/Library/BaseLib/SafeString.c
index 68c33e9b7b..29310889ca 100644
--- a/MdePkg/Library/BaseLib/SafeString.c
+++ b/MdePkg/Library/BaseLib/SafeString.c
@@ -1,7 +1,7 @@
 /** @file
   Safe String functions.
 
-  Copyright (c) 2014 - 2017, Intel Corporation. All rights reserved.<BR>
+  Copyright (c) 2014 - 2018, Intel Corporation. All rights reserved.<BR>
   This program and the accompanying materials
   are licensed and made available under the terms and conditions of the BSD License
   which accompanies this distribution.  The full text of the license may be found at
@@ -342,7 +342,7 @@ StrnCpyS (
   //
   // 4. If Length is not less than DestMax, then DestMax shall be greater than StrnLenS(Source, DestMax).
   //
-  SourceLen = StrnLenS (Source, DestMax);
+  SourceLen = StrnLenS (Source, MIN (DestMax, Length));
   if (Length >= DestMax) {
     SAFE_STRING_CONSTRAINT_CHECK ((DestMax > SourceLen), RETURN_BUFFER_TOO_SMALL);
   }
@@ -361,7 +361,7 @@ StrnCpyS (
   // pointed to by Destination. If no null character was copied from Source, then Destination[Length] is set to a null
   // character.
   //
-  while ((*Source != 0) && (SourceLen > 0)) {
+  while ((SourceLen > 0) && (*Source != 0)) {
     *(Destination++) = *(Source++);
     SourceLen--;
   }
@@ -551,7 +551,7 @@ StrnCatS (
   //
   // 5. If Length is not less than CopyLen, then CopyLen shall be greater than StrnLenS(Source, CopyLen).
   //
-  SourceLen = StrnLenS (Source, CopyLen);
+  SourceLen = StrnLenS (Source, MIN (CopyLen, Length));
   if (Length >= CopyLen) {
     SAFE_STRING_CONSTRAINT_CHECK ((CopyLen > SourceLen), RETURN_BUFFER_TOO_SMALL);
   }
@@ -572,7 +572,7 @@ StrnCatS (
   // a null character.
   //
   Destination = Destination + DestLen;
-  while ((*Source != 0) && (SourceLen > 0)) {
+  while ((SourceLen > 0) && (*Source != 0)) {
     *(Destination++) = *(Source++);
     SourceLen--;
   }
@@ -1916,7 +1916,7 @@ AsciiStrnCpyS (
   //
   // 4. If Length is not less than DestMax, then DestMax shall be greater than AsciiStrnLenS(Source, DestMax).
   //
-  SourceLen = AsciiStrnLenS (Source, DestMax);
+  SourceLen = AsciiStrnLenS (Source, MIN (DestMax, Length));
   if (Length >= DestMax) {
     SAFE_STRING_CONSTRAINT_CHECK ((DestMax > SourceLen), RETURN_BUFFER_TOO_SMALL);
   }
@@ -1935,7 +1935,7 @@ AsciiStrnCpyS (
   // pointed to by Destination. If no null character was copied from Source, then Destination[Length] is set to a null
   // character.
   //
-  while ((*Source != 0) && (SourceLen > 0)) {
+  while ((SourceLen > 0) && (*Source != 0)) {
     *(Destination++) = *(Source++);
     SourceLen--;
   }
@@ -2115,7 +2115,7 @@ AsciiStrnCatS (
   //
   // 5. If Length is not less than CopyLen, then CopyLen shall be greater than AsciiStrnLenS(Source, CopyLen).
   //
-  SourceLen = AsciiStrnLenS (Source, CopyLen);
+  SourceLen = AsciiStrnLenS (Source, MIN (CopyLen, Length));
   if (Length >= CopyLen) {
     SAFE_STRING_CONSTRAINT_CHECK ((CopyLen > SourceLen), RETURN_BUFFER_TOO_SMALL);
   }
@@ -2136,7 +2136,7 @@ AsciiStrnCatS (
   // a null character.
   //
   Destination = Destination + DestLen;
-  while ((*Source != 0) && (SourceLen > 0)) {
+  while ((SourceLen > 0) && (*Source != 0)) {
     *(Destination++) = *(Source++);
     SourceLen--;
   }
-- 
2.39.2