1 files changed, 154 insertions, 1 deletions

diff --git a/auth.c b/auth.c
index a44906174..fd02eff07 100644
--- a/auth.c
+++ b/auth.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: auth.c,v 1.124 2017/09/12 06:32:07 djm Exp $ */
+/* $OpenBSD: auth.c,v 1.125 2018/01/08 15:21:49 markus Exp $ */
 /*
  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
  *
@@ -28,6 +28,7 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/socket.h>
+#include <sys/wait.h>
 
 #include <netinet/in.h>
 
@@ -840,3 +841,155 @@ auth_get_canonical_hostname(struct ssh *ssh, int use_dns)
                 return dnsname;
         }
 }
+
+/*
+ * Runs command in a subprocess wuth a minimal environment.
+ * Returns pid on success, 0 on failure.
+ * The child stdout and stderr maybe captured, left attached or sent to
+ * /dev/null depending on the contents of flags.
+ * "tag" is prepended to log messages.
+ * NB. "command" is only used for logging; the actual command executed is
+ * av[0].
+ */
+pid_t
+subprocess(const char *tag, struct passwd *pw, const char *command,
+    int ac, char **av, FILE **child, u_int flags)
+{
+        FILE *f = NULL;
+        struct stat st;
+        int fd, devnull, p[2], i;
+        pid_t pid;
+        char *cp, errmsg[512];
+        u_int envsize;
+        char **child_env;
+
+        if (child != NULL)
+                *child = NULL;
+
+        debug3("%s: %s command \"%s\" running as %s (flags 0x%x)", __func__,
+            tag, command, pw->pw_name, flags);
+
+        /* Check consistency */
+        if ((flags & SSH_SUBPROCESS_STDOUT_DISCARD) != 0 &&
+            (flags & SSH_SUBPROCESS_STDOUT_CAPTURE) != 0) {
+                error("%s: inconsistent flags", __func__);
+                return 0;
+        }
+        if (((flags & SSH_SUBPROCESS_STDOUT_CAPTURE) == 0) != (child == NULL)) {
+                error("%s: inconsistent flags/output", __func__);
+                return 0;
+        }
+
+        /*
+         * If executing an explicit binary, then verify the it exists
+         * and appears safe-ish to execute
+         */
+        if (*av[0] != '/') {
+                error("%s path is not absolute", tag);
+                return 0;
+        }
+        temporarily_use_uid(pw);
+        if (stat(av[0], &st) < 0) {
+                error("Could not stat %s \"%s\": %s", tag,
+                    av[0], strerror(errno));
+                restore_uid();
+                return 0;
+        }
+        if (safe_path(av[0], &st, NULL, 0, errmsg, sizeof(errmsg)) != 0) {
+                error("Unsafe %s \"%s\": %s", tag, av[0], errmsg);
+                restore_uid();
+                return 0;
+        }
+        /* Prepare to keep the child's stdout if requested */
+        if (pipe(p) != 0) {
+                error("%s: pipe: %s", tag, strerror(errno));
+                restore_uid();
+                return 0;
+        }
+        restore_uid();
+
+        switch ((pid = fork())) {
+        case -1: /* error */
+                error("%s: fork: %s", tag, strerror(errno));
+                close(p[0]);
+                close(p[1]);
+                return 0;
+        case 0: /* child */
+                /* Prepare a minimal environment for the child. */
+                envsize = 5;
+                child_env = xcalloc(sizeof(*child_env), envsize);
+                child_set_env(&child_env, &envsize, "PATH", _PATH_STDPATH);
+                child_set_env(&child_env, &envsize, "USER", pw->pw_name);
+                child_set_env(&child_env, &envsize, "LOGNAME", pw->pw_name);
+                child_set_env(&child_env, &envsize, "HOME", pw->pw_dir);
+                if ((cp = getenv("LANG")) != NULL)
+                        child_set_env(&child_env, &envsize, "LANG", cp);
+
+                for (i = 0; i < NSIG; i++)
+                        signal(i, SIG_DFL);
+
+                if ((devnull = open(_PATH_DEVNULL, O_RDWR)) == -1) {
+                        error("%s: open %s: %s", tag, _PATH_DEVNULL,
+                            strerror(errno));
+                        _exit(1);
+                }
+                if (dup2(devnull, STDIN_FILENO) == -1) {
+                        error("%s: dup2: %s", tag, strerror(errno));
+                        _exit(1);
+                }
+
+                /* Set up stdout as requested; leave stderr in place for now. */
+                fd = -1;
+                if ((flags & SSH_SUBPROCESS_STDOUT_CAPTURE) != 0)
+                        fd = p[1];
+                else if ((flags & SSH_SUBPROCESS_STDOUT_DISCARD) != 0)
+                        fd = devnull;
+                if (fd != -1 && dup2(fd, STDOUT_FILENO) == -1) {
+                        error("%s: dup2: %s", tag, strerror(errno));
+                        _exit(1);
+                }
+                closefrom(STDERR_FILENO + 1);
+
+                /* Don't use permanently_set_uid() here to avoid fatal() */
+                if (setresgid(pw->pw_gid, pw->pw_gid, pw->pw_gid) != 0) {
+                        error("%s: setresgid %u: %s", tag, (u_int)pw->pw_gid,
+                            strerror(errno));
+                        _exit(1);
+                }
+                if (setresuid(pw->pw_uid, pw->pw_uid, pw->pw_uid) != 0) {
+                        error("%s: setresuid %u: %s", tag, (u_int)pw->pw_uid,
+                            strerror(errno));
+                        _exit(1);
+                }
+                /* stdin is pointed to /dev/null at this point */
+                if ((flags & SSH_SUBPROCESS_STDOUT_DISCARD) != 0 &&
+                    dup2(STDIN_FILENO, STDERR_FILENO) == -1) {
+                        error("%s: dup2: %s", tag, strerror(errno));
+                        _exit(1);
+                }
+
+                execve(av[0], av, child_env);
+                error("%s exec \"%s\": %s", tag, command, strerror(errno));
+                _exit(127);
+        default: /* parent */
+                break;
+        }
+
+        close(p[1]);
+        if ((flags & SSH_SUBPROCESS_STDOUT_CAPTURE) == 0)
+                close(p[0]);
+        else if ((f = fdopen(p[0], "r")) == NULL) {
+                error("%s: fdopen: %s", tag, strerror(errno));
+                close(p[0]);
+                /* Don't leave zombie child */
+                kill(pid, SIGTERM);
+                while (waitpid(pid, NULL, 0) == -1 && errno == EINTR)
+                        ;
+                return 0;
+        }
+        /* Success */
+        debug3("%s: %s pid %ld", __func__, tag, (long)pid);
+        if (child != NULL)
+                *child = f;
+        return pid;
+}