[PATCH 5.10 303/317] kdb: Fix buffer overflow during tab-complete

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5.10-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Daniel Thompson <daniel.thompson@xxxxxxxxxx>

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@xxxxxxxxxx>
Closes: https://lore.kernel.org/all/CAFhGd8qESuuifuHsNjFPR-Va3P80bxrw+LqvC8deA8GziUJLpw@xxxxxxxxxxxxxx/
Cc: stable@xxxxxxxxxxxxxxx
Reviewed-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
Reviewed-by: Justin Stitt <justinstitt@xxxxxxxxxx>
Tested-by: Justin Stitt <justinstitt@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20240424-kgdb_read_refactor-v3-1-f236dbe9828d@xxxxxxxxxx
Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/debug/kdb/kdb_io.c |   21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

--- 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;






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