[PATCH] ShellPkg/edit: Fix heap access out-of-bounds

[Ruiyu Ni <ruiyu.ni@intel.com>]

The issue was found when heap guard is turned on.
PrintLib somehow receives a non-null terminated string in var-arg.
When the PrintLib implementation reads the string it keeps
reading because no null-terminator is met, which triggers the page
fault set by the heap guard.

The issue is caused by a bug in FileBufferPrintLine().
When "edit" opens a binary file, in FileBufferPrintLine(),
the Line->Buffer may start with \x00 \x00, but the Line->Size is
larger than MainEditor.ScreenSize.Column, it causes the PrintLine is
set to an empty string by below call:
    StrnCpyS (
      PrintLine, BufLen/sizeof(CHAR16), Buffer,
      MIN(Limit, MainEditor.ScreenSize.Column)
      );
But since Limit (equals to Line->Size) is larger than
MainEditor.ScreenSize.Column, below for-loop doesn't successfully
set the whole PrintLine to all-empty-space.
    for (; Limit < MainEditor.ScreenSize.Column; Limit++) {
      PrintLine[Limit] = L' ';
    }
So after the for-loop, PrintLine is still an empty string.
Later in below call, the PrintLine2 is created based on PrintLine.
    ShellCopySearchAndReplace (
      PrintLine, PrintLine2,
      BufLen * 2, L"%", L"^%", FALSE, FALSE
      );
But due to the implementation of ShellCopySearchAndReplace(),
PrintLine2 is untouched and INVALID_PARAMETER is returned.
Finally an uninitialized string is passed to ShellPrintEx()
which causes the #PF exception.

The fix is to reset Limit to StrLen(PrintLine) before for-loop.
So that PrintLine can be converted from an empty string to a
string containing all spaces.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Jaben Carsey <jaben.carsey@intel.com>
Cc: Jian Wang <jian.j.wang@intel.com>
---
 ShellPkg/Library/UefiShellDebug1CommandsLib/Edit/FileBuffer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/ShellPkg/Library/UefiShellDebug1CommandsLib/Edit/FileBuffer.c b/ShellPkg/Library/UefiShellDebug1CommandsLib/Edit/FileBuffer.c
index 56ccd399b0..39a5afb53f 100644
--- a/ShellPkg/Library/UefiShellDebug1CommandsLib/Edit/FileBuffer.c
+++ b/ShellPkg/Library/UefiShellDebug1CommandsLib/Edit/FileBuffer.c
@@ -500,7 +500,7 @@ FileBufferPrintLine (
   PrintLine = AllocatePool (BufLen);
   if (PrintLine != NULL) {
     StrnCpyS (PrintLine, BufLen/sizeof(CHAR16), Buffer, MIN(Limit, MainEditor.ScreenSize.Column));
-    for (; Limit < MainEditor.ScreenSize.Column; Limit++) {
+    for (Limit = StrLen (PrintLine); Limit < MainEditor.ScreenSize.Column; Limit++) {
       PrintLine[Limit] = L' ';
     }
 
-- 
2.16.1.windows.1