bhyveload: use a dirfd to support -h

Don't allow lookups from the loader scripts, which in rare cases may be
in guest control depending on the setup, to leave the specified host
root.  Open the root dir and strictly do RESOLVE_BENEATH lookups from
there.

cb_open() has been restructured a bit to work nicely with this, using
fdopendir() in the directory case and just using the fd we already
opened in the regular file case.

hostbase_open() was split out to provide an obvious place to apply
rights(4) if that's something we care to do.

Reviewed by:	allanjude (earlier version), markj
Approved by:	so
Security:	FreeBSD-SA-24:01.bhyveload
Security:	CVE-2024-25940

(cherry picked from commit 6779d44bd878e3cf4723f7386b11da6508ab5431)
(cherry picked from commit 426b28fdf700bcd8c05b25665da34f806b27b1bb)

1 files changed, 57 insertions, 27 deletions

diff --git a/usr.sbin/bhyveload/bhyveload.c b/usr.sbin/bhyveload/bhyveload.c
index 6b2633cac288..4c1dbd583e1f 100644
--- a/usr.sbin/bhyveload/bhyveload.c
+++ b/usr.sbin/bhyveload/bhyveload.c
@@ -88,11 +88,11 @@
 
 #define	NDISKS	32
 
-static char *host_base;
 static struct termios term, oldterm;
 static int disk_fd[NDISKS];
 static int ndisks;
 static int consin_fd, consout_fd;
+static int hostbase_fd = -1;
 
 static int need_reinit;
 
@@ -159,42 +159,61 @@ static int
 cb_open(void *arg __unused, const char *filename, void **hp)
 {
 	struct cb_file *cf;
-	char path[PATH_MAX];
+	struct stat sb;
+	int fd, flags;
 
-	if (!host_base)
+	cf = NULL;
+	fd = -1;
+	flags = O_RDONLY | O_RESOLVE_BENEATH;
+	if (hostbase_fd == -1)
 		return (ENOENT);
 
-	strlcpy(path, host_base, PATH_MAX);
-	if (path[strlen(path) - 1] == '/')
-		path[strlen(path) - 1] = 0;
-	strlcat(path, filename, PATH_MAX);
-	cf = malloc(sizeof(struct cb_file));
-	if (stat(path, &cf->cf_stat) < 0) {
-		free(cf);
+	/* Absolute paths are relative to our hostbase, chop off leading /. */
+	if (filename[0] == '/')
+		filename++;
+
+	/* Lookup of /, use . instead. */
+	if (filename[0] == '\0')
+		filename = ".";
+
+	if (fstatat(hostbase_fd, filename, &sb, AT_RESOLVE_BENEATH) < 0)
 		return (errno);
+
+	if (!S_ISDIR(sb.st_mode) && !S_ISREG(sb.st_mode))
+		return (EINVAL);
+
+	if (S_ISDIR(sb.st_mode))
+		flags |= O_DIRECTORY;
+
+	/* May be opening the root dir */
+	fd = openat(hostbase_fd, filename, flags);
+	if (fd < 0)
+		return (errno);
+
+	cf = malloc(sizeof(struct cb_file));
+	if (cf == NULL) {
+		close(fd);
+		return (ENOMEM);
 	}
 
+	cf->cf_stat = sb;
 	cf->cf_size = cf->cf_stat.st_size;
+
 	if (S_ISDIR(cf->cf_stat.st_mode)) {
 		cf->cf_isdir = 1;
-		cf->cf_u.dir = opendir(path);
-		if (!cf->cf_u.dir)
-			goto out;
-		*hp = cf;
-		return (0);
-	}
-	if (S_ISREG(cf->cf_stat.st_mode)) {
+		cf->cf_u.dir = fdopendir(fd);
+		if (cf->cf_u.dir == NULL) {
+			close(fd);
+			free(cf);
+			return (ENOMEM);
+		}
+	} else {
+		assert(S_ISREG(cf->cf_stat.st_mode));
 		cf->cf_isdir = 0;
-		cf->cf_u.fd = open(path, O_RDONLY);
-		if (cf->cf_u.fd < 0)
-			goto out;
-		*hp = cf;
-		return (0);
+		cf->cf_u.fd = fd;
 	}
-
-out:
-	free(cf);
-	return (EINVAL);
+	*hp = cf;
+	return (0);
 }
 
 static int
@@ -714,6 +733,17 @@ usage(void)
 	exit(1);
 }
 
+static void
+hostbase_open(const char *base)
+{
+
+	if (hostbase_fd != -1)
+		close(hostbase_fd);
+	hostbase_fd = open(base, O_DIRECTORY | O_PATH);
+	if (hostbase_fd == -1)
+		err(EX_OSERR, "open");
+}
+
 int
 main(int argc, char** argv)
 {
@@ -748,7 +778,7 @@ main(int argc, char** argv)
 			break;
 
 		case 'h':
-			host_base = optarg;
+			hostbase_open(optarg);
 			break;
 
 		case 'l':