Fix buffer overflow in input handling

kpress function in x.c previously relied on the wrong understanding
of XmbLookupString behavior. When the composed string is longer
than the available buffer, the buffer is not initialized and the
actual length of the data available to read is returned, not
the amount of data written. When that amount is later used to
process the contents of this buffer, not only will the random
contents of the uninitialized buffer be sent directly to the
terminal and whichever application is now running in it, but
possibly also whatever is in the memory after that buffer,
leading to undefined behavior and possible random command execution.

1 files changed, 33 insertions, 10 deletions

diff --git a/x.c b/x.c
index 017238f..b5405ed 100644
--- a/x.c
+++ b/x.c
@@ -2018,8 +2018,10 @@ kpress(XEvent *ev)
 {
 	XKeyEvent *e = &ev->xkey;
 	KeySym ksym;
-	char buf[64], *customkey;
-	int len;
+	char *buf = NULL, *customkey;
+	int len = 0;
+	int buf_size = 64;
+	int critical = - 1;
 	Rune c;
 	Status status;
 	Shortcut *bp;
@@ -2027,27 +2029,44 @@ kpress(XEvent *ev)
 	if (IS_SET(MODE_KBDLOCK))
 		return;
 
-	if (xw.ime.xic)
-		len = XmbLookupString(xw.ime.xic, e, buf, sizeof buf, &ksym, &status);
-	else
-		len = XLookupString(e, buf, sizeof buf, &ksym, NULL);
+reallocbuf:
+	if (critical > 0)
+		goto cleanup;
+	if (buf)
+		free(buf);
+
+	buf = xmalloc((buf_size) * sizeof(char));
+	critical += 1;
+
+	if (xw.ime.xic) {
+		len = XmbLookupString(xw.ime.xic, e, buf, buf_size, &ksym, &status);
+		if (status == XBufferOverflow) {
+			buf_size = len;
+			goto reallocbuf;
+		}
+	} else {
+		// Not sure how to fix this and if it is fixable
+		// but at least it does write something into the buffer
+		// so it is not as critical
+		len = XLookupString(e, buf, buf_size, &ksym, NULL);
+	}
 	/* 1. shortcuts */
 	for (bp = shortcuts; bp < shortcuts + LEN(shortcuts); bp++) {
 		if (ksym == bp->keysym && match(bp->mod, e->state)) {
 			bp->func(&(bp->arg));
-			return;
+			goto cleanup;
 		}
 	}
 
 	/* 2. custom keys from config.h */
 	if ((customkey = kmap(ksym, e->state))) {
 		ttywrite(customkey, strlen(customkey), 1);
-		return;
+		goto cleanup;
 	}
 
 	/* 3. composed string from input method */
 	if (len == 0)
-		return;
+		goto cleanup;
 	if (len == 1 && e->state & Mod1Mask) {
 		if (IS_SET(MODE_8BIT)) {
 			if (*buf < 0177) {
@@ -2060,7 +2079,11 @@ kpress(XEvent *ev)
 			len = 2;
 		}
 	}
-	ttywrite(buf, len, 1);
+	if (len <= buf_size)
+		ttywrite(buf, len, 1);
+cleanup:
+	if (buf)
+		free(buf);
 }
 
 void