util/nvmutil: Use unveil, and harden pledges

After /dev/urandom (for MAC address randomisation) and
the GbE file have been handled, unveil them. Unveil is
a system call provided by OpenBSD that, when called,
restricts access only to the files and/or directories
specified, each given specific permissions.

You can learn more about unveil here:

https://man.openbsd.org/unveil.2

An ifdef rule makes nvmutil only use unveil on OpenBSD,
because it's not available anywhere else. This is the same
as with the pledge() system call.

Where invalid arguments are given, and no action performed,
pledge promises are also reduced to just stdio, preventing
any writes to files, or reads from files.

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

1 files changed, 21 insertions, 6 deletions

diff --git a/util/nvmutil/nvmutil.c b/util/nvmutil/nvmutil.c
index 524e678..26c31d2 100644
--- a/util/nvmutil/nvmutil.c
+++ b/util/nvmutil/nvmutil.c
@@ -57,6 +57,7 @@ void setWord(int pos16, int partnum, uint16_t val16);
 void byteswap(int n, int partnum);
 void writeGbeFile(int *fd, const char *filename, size_t nw);
 void xpledge(const char *promises, const char *execpromises);
+void xunveil(const char *path, const char *permissions);
 
 #define FILENAME argv[1]
 #define COMMAND argv[2]
@@ -79,7 +80,7 @@ uint8_t big_endian;
 int
 main(int argc, char *argv[])
 {
-	xpledge("stdio rpath wpath", NULL);
+	xpledge("stdio rpath wpath unveil", NULL);
 	size_t nr = 128;
 	int fd, flags = O_RDWR;
 	void (*cmd)(void) = NULL;
@@ -92,7 +93,7 @@ main(int argc, char *argv[])
 
 	if (argc == 3) {
 		if (strcmp(COMMAND, "dump") == 0) {
-			xpledge("stdio rpath", NULL);
+			xpledge("stdio rpath unveil", NULL);
 			flags = O_RDONLY;
 			cmd = &cmd_dump;
 		} else if (strcmp(COMMAND, "setmac") == 0) {
@@ -125,16 +126,20 @@ main(int argc, char *argv[])
 			(cmd == &cmd_setchecksum) | (cmd == &cmd_brick);
 		readGbeFile(&fd, FILENAME, flags, nr);
 		(void)rhex();
-		if (flags == O_RDONLY)
+		xunveil("/dev/urandom", "r");
+		if (flags == O_RDONLY) {
 			xpledge("stdio", NULL);
-		else
-			xpledge("stdio wpath", NULL);
+		} else {
+			xpledge("stdio wpath unveil", NULL);
+			xunveil(FILENAME, "w");
+		}
 		if (strMac != NULL)
 			cmd_setmac(strMac); /* nvm gbe.bin setmac */
 		else if (cmd != NULL)
 			(*cmd)(); /* all other commands except setmac */
 		writeGbeFile(&fd, FILENAME, nr);
-	}
+	} else
+		xpledge("stdio", NULL);
 
 	if ((errno != 0) && (cmd != &cmd_dump))
 		err(errno, NULL);
@@ -381,3 +386,13 @@ xpledge(const char *promises, const char *execpromises)
 		err(errno, NULL);
 #endif
 }
+
+void
+xunveil(const char *path, const char *permissions)
+{
+	(void)path; (void)permissions;
+#ifdef __OpenBSD__
+	if (unveil(path, permissions) == -1)
+		err(errno, NULL);
+#endif
+}