block/rnbd-srv: Check for unlikely string overflow

Since "dev_search_path" can technically be as large as PATH_MAX,
there was a risk of truncation when copying it and a second string
into "full_path" since it was also PATH_MAX sized. The W=1 builds were
reporting this warning:

drivers/block/rnbd/rnbd-srv.c: In function 'process_msg_open.isra':
drivers/block/rnbd/rnbd-srv.c:616:51: warning: '%s' directive output may be truncated writing up to 254 bytes into a region of size between 0 and 4095 [-Wformat-truncation=]
  616 |                 snprintf(full_path, PATH_MAX, "%s/%s",
      |                                                   ^~
In function 'rnbd_srv_get_full_path',
    inlined from 'process_msg_open.isra' at drivers/block/rnbd/rnbd-srv.c:721:14: drivers/block/rnbd/rnbd-srv.c:616:17: note: 'snprintf' output between 2 and 4351 bytes into a destination of size 4096
  616 |                 snprintf(full_path, PATH_MAX, "%s/%s",
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  617 |                          dev_search_path, dev_name);
      |                          ~~~~~~~~~~~~~~~~~~~~~~~~~~

To fix this, unconditionally check for truncation (as was already done
for the case where "%SESSNAME%" was present).

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202312100355.lHoJPgKy-lkp@intel.com/
Cc: Md. Haris Iqbal <haris.iqbal@ionos.com>
Cc: Jack Wang <jinpu.wang@ionos.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc:  <linux-block@vger.kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Acked-by: Jack Wang <jinpu.wang@ionos.com>
Link: https://lore.kernel.org/r/20231212214738.work.169-kees@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/drivers/block/rnbd/rnbd-srv.c b/drivers/block/rnbd/rnbd-srv.c
index 29733b6157541997720f72f4c00787de83e6ee64..3a0d5dcec6f2559f85b13c432248533651c27c44 100644 (file)
--- a/drivers/block/rnbd/rnbd-srv.c
+++ b/drivers/block/rnbd/rnbd-srv.c
@@ -586,6 +586,7 @@ static char *rnbd_srv_get_full_path(struct rnbd_srv_session *srv_sess,
 {
        char *full_path;
        char *a, *b;
+       int len;
 
        full_path = kmalloc(PATH_MAX, GFP_KERNEL);
        if (!full_path)
@@ -597,19 +598,19 @@ static char *rnbd_srv_get_full_path(struct rnbd_srv_session *srv_sess,
         */
        a = strnstr(dev_search_path, "%SESSNAME%", sizeof(dev_search_path));
        if (a) {
-               int len = a - dev_search_path;
+               len = a - dev_search_path;
 
                len = snprintf(full_path, PATH_MAX, "%.*s/%s/%s", len,
                               dev_search_path, srv_sess->sessname, dev_name);
-               if (len >= PATH_MAX) {
-                       pr_err("Too long path: %s, %s, %s\n",
-                              dev_search_path, srv_sess->sessname, dev_name);
-                       kfree(full_path);
-                       return ERR_PTR(-EINVAL);
-               }
        } else {
-               snprintf(full_path, PATH_MAX, "%s/%s",
-                        dev_search_path, dev_name);
+               len = snprintf(full_path, PATH_MAX, "%s/%s",
+                              dev_search_path, dev_name);
+       }
+       if (len >= PATH_MAX) {
+               pr_err("Too long path: %s, %s, %s\n",
+                      dev_search_path, srv_sess->sessname, dev_name);
+               kfree(full_path);
+               return ERR_PTR(-EINVAL);
        }
 
        /* eliminitate duplicated slashes */