tigervnc/0008-Add-sanity-checks-for-PixelFormat-shift-values.patch

From cd1d650c532a46e95a1229dffaf281c76a50cdfe Mon Sep 17 00:00:00 2001
From: Pierre Ossman <ossman@cendio.se>
Date: Tue, 10 Sep 2019 16:07:50 +0200
Subject: [PATCH] Add sanity checks for PixelFormat shift values

Otherwise we might be tricked in to reading and writing things at
incorrect offsets for pixels which ultimately could result in an
attacker writing things to the stack or heap and executing things
they shouldn't.

This only affects the server as the client never uses the pixel
format suggested by th server.

Issue found by Pavel Cheremushkin from Kaspersky Lab.
---
 common/rfb/PixelFormat.cxx | 7 +++++++
 tests/unit/pixelformat.cxx | 6 ++++++
 2 files changed, 13 insertions(+)

diff --git a/common/rfb/PixelFormat.cxx b/common/rfb/PixelFormat.cxx
index 2d8142d1..789c43ed 100644
--- a/common/rfb/PixelFormat.cxx
+++ b/common/rfb/PixelFormat.cxx
@@ -682,6 +682,13 @@ bool PixelFormat::isSane(void)
   if (totalBits > depth)
     return false;
 
+  if ((bits(redMax) + redShift) > bpp)
+    return false;
+  if ((bits(greenMax) + greenShift) > bpp)
+    return false;
+  if ((bits(blueMax) + blueShift) > bpp)
+    return false;
+
   if (((redMax << redShift) & (greenMax << greenShift)) != 0)
     return false;
   if (((redMax << redShift) & (blueMax << blueShift)) != 0)
diff --git a/tests/unit/pixelformat.cxx b/tests/unit/pixelformat.cxx
index 7b6087f7..46fecfb4 100644
--- a/tests/unit/pixelformat.cxx
+++ b/tests/unit/pixelformat.cxx
@@ -108,6 +108,12 @@ int main(int argc, char** argv)
 
     doTest(true, 32, 16, false, true, 255, 255, 255, 0, 8, 16);
 
+    /* Invalid shift values */
+
+    doTest(true, 32, 24, false, true, 255, 255, 255, 25, 8, 16);
+    doTest(true, 32, 24, false, true, 255, 255, 255, 0, 25, 16);
+    doTest(true, 32, 24, false, true, 255, 255, 255, 0, 8, 25);
+
     /* Overlapping channels */
 
     doTest(true, 32, 24, false, true, 255, 255, 255, 0, 7, 16);
-- 
2.16.4