run XferCommand via exec

system() runs the provided command via a shell, which is subject to
command injection.  Even though pacman already provides a mechanism to
sign and verify the databases containing the urls, certain distributions
have yet to get their act together and start signing databases, leaving
them vulnerable to MITM attacks.  Replacing the system call with an
almost equivalent exec call removes the possibility of a shell-injection
attack for those users.

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>

2 files changed, 115 insertions, 17 deletions

diff --git a/src/pacman/conf.c b/src/pacman/conf.c
index 2d8518c4..f9739928 100644
--- a/src/pacman/conf.c
+++ b/src/pacman/conf.c
@@ -29,6 +29,7 @@
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <sys/utsname.h> /* uname */
+#include <sys/wait.h>
 #include <unistd.h>
 
 /* pacman */
@@ -152,6 +153,7 @@ int config_free(config_t *oldconfig)
 	free(oldconfig->xfercommand);
 	free(oldconfig->print_format);
 	free(oldconfig->arch);
+	wordsplit_free(oldconfig->xfercommand_argv);
 	free(oldconfig);
 
 	return 0;
@@ -201,18 +203,86 @@ static char *get_tempfile(const char *path, const char *filename)
 	return tempfile;
 }
 
+/* system()/exec() hybrid function allowing exec()-style direct execution
+ * of a command with the simplicity of system()
+ * - not thread-safe
+ * - errno may be set by fork(), pipe(), or execvp()
+ */
+static int systemvp(const char *file, char *const argv[])
+{
+	int pid, err = 0, ret = -1, err_fd[2];
+	sigset_t oldblock;
+	struct sigaction sa_ign = { .sa_handler = SIG_IGN }, oldint, oldquit;
+
+	if(pipe(err_fd) != 0) {
+		return -1;
+	}
+
+	sigaction(SIGINT, &sa_ign, &oldint);
+	sigaction(SIGQUIT, &sa_ign, &oldquit);
+	sigaddset(&sa_ign.sa_mask, SIGCHLD);
+	sigprocmask(SIG_BLOCK, &sa_ign.sa_mask, &oldblock);
+
+	pid = fork();
+
+	/* child */
+	if(pid == 0) {
+		close(err_fd[0]);
+		fcntl(err_fd[1], F_SETFD, FD_CLOEXEC);
+
+		/* restore signal handling for the child to inherit */
+		sigaction(SIGINT, &oldint, NULL);
+		sigaction(SIGQUIT, &oldquit, NULL);
+		sigprocmask(SIG_SETMASK, &oldblock, NULL);
+
+		execvp(file, argv);
+
+		/* execvp failed, pass the error back to the parent */
+		while(write(err_fd[1], &errno, sizeof(errno)) == -1 && errno == EINTR);
+		_Exit(127);
+	}
+
+	/* parent */
+	close(err_fd[1]);
+
+	if(pid != -1)  {
+		int wret;
+		while((wret = waitpid(pid, &ret, 0)) == -1 && errno == EINTR);
+		if(wret > 0) {
+			while(read(err_fd[0], &err, sizeof(err)) == -1 && errno == EINTR);
+		}
+	} else {
+		/* fork failed, make sure errno is preserved after cleanup */
+		err = errno;
+	}
+
+	close(err_fd[0]);
+
+	sigaction(SIGINT, &oldint, NULL);
+	sigaction(SIGQUIT, &oldquit, NULL);
+	sigprocmask(SIG_SETMASK, &oldblock, NULL);
+
+	if(err) {
+		errno = err;
+		ret = -1;
+	}
+
+	return ret;
+}
+
 /** External fetch callback */
 static int download_with_xfercommand(const char *url, const char *localpath,
 		int force)
 {
 	int ret = 0, retval;
 	int usepart = 0;
-	int cwdfd;
+	int cwdfd = -1;
 	struct stat st;
-	char *parsedcmd, *tempcmd;
 	char *destfile, *tempfile, *filename;
+	const char **argv;
+	size_t i;
 
-	if(!config->xfercommand) {
+	if(!config->xfercommand_argv) {
 		return -1;
 	}
 
@@ -230,17 +300,26 @@ static int download_with_xfercommand(const char *url, const char *localpath,
 		unlink(destfile);
 	}
 
-	tempcmd = strdup(config->xfercommand);
-	/* replace all occurrences of %o with fn.part */
-	if(strstr(tempcmd, "%o")) {
-		usepart = 1;
-		parsedcmd = strreplace(tempcmd, "%o", tempfile);
-		free(tempcmd);
-		tempcmd = parsedcmd;
+	if((argv = calloc(config->xfercommand_argc + 1, sizeof(char*))) == NULL) {
+		size_t bytes = (config->xfercommand_argc + 1) * sizeof(char*);
+		pm_printf(ALPM_LOG_ERROR,
+				_n("malloc failure: could not allocate %zu byte\n",
+				   "malloc failure: could not allocate %zu bytes\n",
+					 bytes),
+				bytes);
+		goto cleanup;
+	}
+
+	for(i = 0; i <= config->xfercommand_argc; i++) {
+		const char *val = config->xfercommand_argv[i];
+		if(val && strcmp(val, "%o") == 0) {
+			usepart = 1;
+			val = tempfile;
+		} else if(val && strcmp(val, "%u") == 0) {
+			val = url;
+		}
+		argv[i] = val;
 	}
-	/* replace all occurrences of %u with the download URL */
-	parsedcmd = strreplace(tempcmd, "%u", url);
-	free(tempcmd);
 
 	/* save the cwd so we can restore it later */
 	do {
@@ -256,9 +335,15 @@ static int download_with_xfercommand(const char *url, const char *localpath,
 		ret = -1;
 		goto cleanup;
 	}
-	/* execute the parsed command via /bin/sh -c */
-	pm_printf(ALPM_LOG_DEBUG, "running command: %s\n", parsedcmd);
-	retval = system(parsedcmd);
+
+	if(config->logmask & ALPM_LOG_DEBUG) {
+		char *cmd = arg_to_string(config->xfercommand_argc, (char**)argv);
+		if(cmd) {
+			pm_printf(ALPM_LOG_DEBUG, "running command: %s\n", cmd);
+			free(cmd);
+		}
+	}
+	retval = systemvp(argv[0], (char**)argv);
 
 	if(retval == -1) {
 		pm_printf(ALPM_LOG_WARNING, _("running XferCommand: fork failed!\n"));
@@ -296,7 +381,7 @@ cleanup:
 	}
 	free(destfile);
 	free(tempfile);
-	free(parsedcmd);
+	free(argv);
 
 	return ret;
 }
@@ -544,6 +629,17 @@ static int _parse_options(const char *key, char *value,
 				pm_printf(ALPM_LOG_DEBUG, "config: logfile: %s\n", value);
 			}
 		} else if(strcmp(key, "XferCommand") == 0) {
+			char **c;
+			if((config->xfercommand_argv = wordsplit(value)) == NULL) {
+				pm_printf(ALPM_LOG_WARNING,
+						_("config file %s, line %d: invalid value for '%s' : '%s'\n"),
+						file, linenum, "XferCommand", value);
+				return 1;
+			}
+			config->xfercommand_argc = 0;
+			for(c = config->xfercommand_argv; *c; c++) {
+				config->xfercommand_argc++;
+			}
 			config->xfercommand = strdup(value);
 			pm_printf(ALPM_LOG_DEBUG, "config: xfercommand: %s\n", value);
 		} else if(strcmp(key, "CleanMethod") == 0) {
diff --git a/src/pacman/conf.h b/src/pacman/conf.h
index f45ed436..47df979d 100644
--- a/src/pacman/conf.h
+++ b/src/pacman/conf.h
@@ -125,6 +125,8 @@ typedef struct __config_t {
 	alpm_list_t *noextract;
 	alpm_list_t *overwrite_files;
 	char *xfercommand;
+	char **xfercommand_argv;
+	size_t xfercommand_argc;
 
 	/* our connection to libalpm */
 	alpm_handle_t *handle;