kdb: Fix buffer overflow during tab-complete

Currently, when the user attempts symbol completion with the Tab key, kdb
will use strncpy() to insert the completed symbol into the command buffer.
Unfortunately it passes the size of the source buffer rather than the
destination to strncpy() with predictably horrible results. Most obviously
if the command buffer is already full but cp, the cursor position, is in
the middle of the buffer, then we will write past the end of the supplied
buffer.

Fix this by replacing the dubious strncpy() calls with memmove()/memcpy()
calls plus explicit boundary checks to make sure we have enough space
before we start moving characters around.

Reported-by: Justin Stitt <justinstitt@google.com>
Closes: https://lore.kernel.org/all/CAFhGd8qESuuifuHsNjFPR-Va3P80bxrw+LqvC8deA8GziUJLpw@mail.gmail.com/
Cc: stable@vger.kernel.org
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Tested-by: Justin Stitt <justinstitt@google.com>
Link: https://lore.kernel.org/r/20240424-kgdb_read_refactor-v3-1-f236dbe9828d@linaro.org
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>

diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
index 9443bc63c5a245548a1390ee85dd096091649607..06dfbccb103368a240a1725591778da07bc4c88b 100644 (file)
--- a/kernel/debug/kdb/kdb_io.c
+++ b/kernel/debug/kdb/kdb_io.c
@@ -367,14 +367,19 @@ poll_again:
                        kdb_printf(kdb_prompt_str);
                        kdb_printf("%s", buffer);
                } else if (tab != 2 && count > 0) {
-                       len_tmp = strlen(p_tmp);
-                       strncpy(p_tmp+len_tmp, cp, lastchar-cp+1);
-                       len_tmp = strlen(p_tmp);
-                       strncpy(cp, p_tmp+len, len_tmp-len + 1);
-                       len = len_tmp - len;
-                       kdb_printf("%s", cp);
-                       cp += len;
-                       lastchar += len;
+                       /* How many new characters do we want from tmpbuffer? */
+                       len_tmp = strlen(p_tmp) - len;
+                       if (lastchar + len_tmp >= bufend)
+                               len_tmp = bufend - lastchar;
+
+                       if (len_tmp) {
+                               /* + 1 ensures the '\0' is memmove'd */
+                               memmove(cp+len_tmp, cp, (lastchar-cp) + 1);
+                               memcpy(cp, p_tmp+len, len_tmp);
+                               kdb_printf("%s", cp);
+                               cp += len_tmp;
+                               lastchar += len_tmp;
+                       }
                }
                kdb_nextline = 1; /* reset output line number */
                break;