ubifs: wbuf: Don't leak kernel memory to flash

Write buffers use a kmalloc()'ed buffer, they can leak
up to seven bytes of kernel memory to flash if writes are not
aligned.
So use ubifs_pad() to fill these gaps with padding bytes.
This was never a problem while scanning because the scanner logic
manually aligns node lengths and skips over these gaps.

Cc: <stable@vger.kernel.org>
Fixes: 1e51764a ("UBIFS: add new flash file system")
Signed-off-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

fs/ubifs/io.c

@@ -319,7 +319,7 @@ void ubifs_pad(const struct ubifs_info *c, void *buf, int pad)
 {
 	uint32_t crc;
 
-	ubifs_assert(c, pad >= 0 && !(pad & 7));
+	ubifs_assert(c, pad >= 0);
 
 	if (pad >= UBIFS_PAD_NODE_SZ) {
 		struct ubifs_ch *ch = buf;
@@ -764,6 +764,10 @@ int ubifs_wbuf_write_nolock(struct ubifs_wbuf *wbuf, void *buf, int len)
 		 * write-buffer.
 		 */
 		memcpy(wbuf->buf + wbuf->used, buf, len);
+		if (aligned_len > len) {
+			ubifs_assert(c, aligned_len - len < 8);
+			ubifs_pad(c, wbuf->buf + wbuf->used + len, aligned_len - len);
+		}
 
 		if (aligned_len == wbuf->avail) {
 			dbg_io("flush jhead %s wbuf to LEB %d:%d",
@@ -856,13 +860,18 @@ int ubifs_wbuf_write_nolock(struct ubifs_wbuf *wbuf, void *buf, int len)
 	}
 
 	spin_lock(&wbuf->lock);
-	if (aligned_len)
+	if (aligned_len) {
 		/*
 		 * And now we have what's left and what does not take whole
 		 * max. write unit, so write it to the write-buffer and we are
 		 * done.
 		 */
 		memcpy(wbuf->buf, buf + written, len);
+		if (aligned_len > len) {
+			ubifs_assert(c, aligned_len - len < 8);
+			ubifs_pad(c, wbuf->buf + len, aligned_len - len);
+		}
+	}
 
 	if (c->leb_size - wbuf->offs >= c->max_write_size)
 		wbuf->size = c->max_write_size;