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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>

authored by

Kees Cook and committed by
Jens Axboe 9e4bf6a0 f788893d

+10 -9
+10 -9
drivers/block/rnbd/rnbd-srv.c
··· 586 586 { 587 587 char *full_path; 588 588 char *a, *b; 589 + int len; 589 590 590 591 full_path = kmalloc(PATH_MAX, GFP_KERNEL); 591 592 if (!full_path) ··· 598 597 */ 599 598 a = strnstr(dev_search_path, "%SESSNAME%", sizeof(dev_search_path)); 600 599 if (a) { 601 - int len = a - dev_search_path; 600 + len = a - dev_search_path; 602 601 603 602 len = snprintf(full_path, PATH_MAX, "%.*s/%s/%s", len, 604 603 dev_search_path, srv_sess->sessname, dev_name); 605 - if (len >= PATH_MAX) { 606 - pr_err("Too long path: %s, %s, %s\n", 607 - dev_search_path, srv_sess->sessname, dev_name); 608 - kfree(full_path); 609 - return ERR_PTR(-EINVAL); 610 - } 611 604 } else { 612 - snprintf(full_path, PATH_MAX, "%s/%s", 613 - dev_search_path, dev_name); 605 + len = snprintf(full_path, PATH_MAX, "%s/%s", 606 + dev_search_path, dev_name); 607 + } 608 + if (len >= PATH_MAX) { 609 + pr_err("Too long path: %s, %s, %s\n", 610 + dev_search_path, srv_sess->sessname, dev_name); 611 + kfree(full_path); 612 + return ERR_PTR(-EINVAL); 614 613 } 615 614 616 615 /* eliminitate duplicated slashes */