media: s5p-jpeg: prevent buffer overflows

The current logic allows word to be less than 2. If this happens,
there will be buffer overflows, as reported by smatch. Add extra
checks to prevent it.

While here, remove an unused word = 0 assignment.

Fixes: 6c96dbbc2aa9 ("[media] s5p-jpeg: add support for 5433")
Cc: [e-mail]
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Reviewed-by: Jacek Anaszewski <[e-mail]>

drivers/media/platform/samsung/s5p-jpeg/jpeg-core.c

···
 		(unsigned long)vb2_plane_vaddr(&vb->vb2_buf, 0) + ctx->out_q.sos + 2;
 	jpeg_buffer.curr = 0;
 
-	word = 0;
-
 	if (get_word_be(&jpeg_buffer, &word))
 		return;
-	jpeg_buffer.size = (long)word - 2;
+
+	if (word < 2)
+		jpeg_buffer.size = 0;
+	else
+		jpeg_buffer.size = (long)word - 2;
+
 	jpeg_buffer.data += 2;
 	jpeg_buffer.curr = 0;
 
···
 	if (byte == -1)
 		return -1;
 	*word = (unsigned int)byte | temp;
+
 	return 0;
 }
 
···
 			if (get_word_be(&jpeg_buffer, &word))
 				break;
 			length = (long)word - 2;
-			if (!length)
+			if (length <= 0)
 				return false;
 			sof = jpeg_buffer.curr; /* after 0xffc0 */
 			sof_len = length;
···
 			if (get_word_be(&jpeg_buffer, &word))
 				break;
 			length = (long)word - 2;
-			if (!length)
+			if (length <= 0)
 				return false;
 			if (n_dqt >= S5P_JPEG_MAX_MARKER)
 				return false;
···
 			if (get_word_be(&jpeg_buffer, &word))
 				break;
 			length = (long)word - 2;
-			if (!length)
+			if (length <= 0)
 				return false;
 			if (n_dht >= S5P_JPEG_MAX_MARKER)
 				return false;
···
 			if (get_word_be(&jpeg_buffer, &word))
 				break;
 			length = (long)word - 2;
+			/* No need to check underflows as skip() does it  */
 			skip(&jpeg_buffer, length);
 			break;
 		}