util/mkhtemp: O_TMPFILE fast path on linux

linux itself provides much of the hardening we need,
and avoids the need for some of our tests. use this
on linux (fall back to openat still, on e.g. bsd)

Signed-off-by: Leah Rowe <leah@libreboot.org>

1 files changed, 107 insertions, 1 deletions

diff --git a/util/libreboot-utils/lib/mkhtemp.c b/util/libreboot-utils/lib/mkhtemp.c
index dff05793..419bb0b3 100644
--- a/util/libreboot-utils/lib/mkhtemp.c
+++ b/util/libreboot-utils/lib/mkhtemp.c
@@ -19,10 +19,16 @@
 #include <string.h>
 #include <unistd.h>
 
-/* for openat2: */
+/* for openat2 / fast path: */
 #ifdef __linux__
 #include <linux/openat2.h>
 #include <sys/syscall.h>
+#ifndef O_TMPFILE
+#define O_TMPFILE 020000000
+#endif
+#ifndef AT_EMPTY_PATH
+#define AT_EMPTY_PATH 0x1000
+#endif
 #endif
 
 #include "../include/common.h"
@@ -655,6 +661,18 @@ mkhtemp_try_create(int dirfd,
 		goto err;
 
 	if (type == MKHTEMP_FILE) {
+#ifdef __linux__
+		/* try O_TMPFILE fast path */
+		if (mkhtemp_tmpfile_linux(dirfd,
+		    st_dir_initial, fname_copy,
+		    p, xc, fd, st) == 0) {
+
+			errno = saved_errno;
+			rval = 1;
+			goto out;
+		}
+#endif
+
 		*fd = openat2p(dirfd, fname_copy,
 		    O_RDWR | O_CREAT | O_EXCL |
 		    O_NOFOLLOW | O_CLOEXEC | O_NOCTTY, 0600);
@@ -746,6 +764,94 @@ out:
 	return rval;
 }
 
+/* linux has its own special hardening
+   available specifically for tmpfiles,
+   which eliminates many race conditions.
+
+   we still use openat() on bsd, which is
+   still ok with our other mitigations
+ */
+#ifdef __linux__
+int
+mkhtemp_tmpfile_linux(int dirfd,
+    struct stat *st_dir_initial,
+    char *fname_copy,
+    char *p,
+    size_t xc,
+    int *fd,
+    struct stat *st)
+{
+	int saved_errno = errno;
+	int tmpfd = -1;
+	size_t retries;
+	int linked = 0;
+
+	if (fd == NULL || st == NULL ||
+	    fname_copy == NULL || p == NULL ||
+	    st_dir_initial == NULL) {
+		errno = EFAULT;
+		return -1;
+	}
+
+	/* create unnamed tmpfile */
+	tmpfd = openat(dirfd, ".",
+	    O_TMPFILE | O_RDWR | O_CLOEXEC, 0600);
+
+	if (tmpfd < 0)
+		return -1;
+
+	if (fd_verify_dir_identity(dirfd, st_dir_initial) < 0)
+		goto err;
+
+	for (retries = 0; retries < MKHTEMP_RETRY_MAX; retries++) {
+
+		if (mkhtemp_fill_random(p, xc) < 0)
+			goto err;
+
+		if (fd_verify_dir_identity(dirfd,
+		    st_dir_initial) < 0)
+			goto err;
+
+		if (linkat(tmpfd, "",
+		    dirfd, fname_copy,
+		    AT_EMPTY_PATH) == 0) {
+
+			linked = 1; /* file created */
+
+			if (fd_verify_dir_identity(dirfd, st_dir_initial) < 0)
+				goto err;
+
+			/* success */
+			*fd = tmpfd;
+
+			if (fstat(*fd, st) < 0)
+				goto err;
+
+			if (secure_file(fd, st, st,
+			    O_APPEND, 1, 1, 0600) < 0)
+				goto err;
+
+			errno = saved_errno;
+			return 0;
+		}
+
+		if (errno != EEXIST)
+			goto err;
+
+		/* retry on collision */
+	}
+
+	errno = EEXIST;
+
+err:
+	if (linked)
+		(void) unlinkat(dirfd, fname_copy, 0);
+
+	close_no_err(&tmpfd);
+	return -1;
+}
+#endif
+
 int
 mkhtemp_fill_random(char *p, size_t xc)
 {