VMD: revise check for regular files on disks

[Carlos Cardenas]

Howdy.

Attached is a patch to address a TOCTOU issue with checking to
ensure disks are regular files, reported by jca@ .

Comments? Ok?

+--+
Carlos

Index: config.c
===================================================================
RCS file: /home/los/cvs/src/usr.sbin/vmd/config.c,v
retrieving revision 1.38
diff -u -p -a -u -r1.38 config.c
--- config.c    3 Jan 2018 05:39:56 -0000       1.38
+++ config.c    4 Jan 2018 03:55:47 -0000
@@ -262,23 +262,23 @@ config_setvm(struct privsep *ps, struct 
        /* Open disk images for child */
        for (i = 0 ; i < vcp->vcp_ndisks; i++) {
                 /* Stat disk[i] to ensure it is a regular file */
-               if (stat(vcp->vcp_disks[i], &stat_buf) == -1) {
+               if ((diskfds[i] =
+                   open(vcp->vcp_disks[i], O_RDWR)) == -1) {
                        log_warn("%s: can't open disk %s", __func__,
                            vcp->vcp_disks[i]);
                        errno = VMD_DISK_MISSING;
                        goto fail;
                }
-               if (S_ISREG(stat_buf.st_mode) == 0) {
-                       log_warn("%s: disk %s is not a regular file", __func__,
+               if (fstat(diskfds[i], &stat_buf) == -1) {
+                       log_warn("%s: can't open disk %s", __func__,
                            vcp->vcp_disks[i]);
-                       errno = VMD_DISK_INVALID;
+                       errno = VMD_DISK_MISSING;
                        goto fail;
                }
-               if ((diskfds[i] =
-                   open(vcp->vcp_disks[i], O_RDWR)) == -1) {
-                       log_warn("%s: can't open disk %s", __func__,
+               if (S_ISREG(stat_buf.st_mode) == 0) {
+                       log_warn("%s: disk %s is not a regular file", __func__,
                            vcp->vcp_disks[i]);
-                       errno = VMD_DISK_MISSING;
+                       errno = VMD_DISK_INVALID;
                        goto fail;
                }
        }