Graphics library - prevent buffer overrun in (T)BCP encoding

Bug #706494 "Buffer Overflow in s_xBCPE_process"

As described in detail in the bug report, if the write buffer is filled
to one byte less than full, and we then try to write an escaped
character, we overrun the buffer because we don't check before
writing two bytes to it.

This just checks if we have two bytes before starting to write an
escaped character and exits if we don't (replacing the consumed byte
of the input).

Up for further discussion; why do we even permit a BCP encoding filter
anyway ? I think we should remove this, at least when SAFER is true.

1 files changed, 9 insertions, 1 deletions

diff --git a/base/sbcp.c b/base/sbcp.c
index 979ae0992..47fc233ec 100644
--- a/base/sbcp.c
+++ b/base/sbcp.c
@@ -1,4 +1,4 @@
-/* Copyright (C) 2001-2021 Artifex Software, Inc.
+/* Copyright (C) 2001-2023 Artifex Software, Inc.
    All Rights Reserved.
 
    This software is provided AS-IS with no warranty, either express or
@@ -50,6 +50,14 @@ s_xBCPE_process(stream_state * st, stream_cursor_read * pr,
         byte ch = *++p;
 
         if (ch <= 31 && escaped[ch]) {
+            /* Make sure we have space to store two characters in the write buffer,
+             * if we don't then exit without consuming the input character, we'll process
+             * that on the next time round.
+             */
+            if (pw->limit - q < 2) {
+                p--;
+                break;
+            }
             if (p == rlimit) {
                 p--;
                 break;