kdb: Fix buffer overflow during tab-complete

commit e9730744bf3af04cda23799029342aa3cddbc454 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
index 6735ac36b7187edb6d18c9786c0b376b90e35a57..dcc7e13e98b389ac03b20be5dce5ab6cb1009109 100644 (file)
--- a/kernel/debug/kdb/kdb_io.c
+++ b/kernel/debug/kdb/kdb_io.c
@@ -354,14 +354,19 @@ 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;