daemon-helper: add read-file-as-user

Add a new command to read the content of a file after switching to the
given user. This command can be used to enforce Unix filesystem
permissions when accessing a file on behalf of a user.

src/libnm-std-aux/nm-std-utils.c

@@ -4,10 +4,14 @@
 
 #include "nm-std-utils.h"
 
-#include <stdint.h>
 #include <assert.h>
+#include <fcntl.h>
+#include <grp.h>
 #include <limits.h>
 #include <net/if.h>
+#include <pwd.h>
+#include <stdint.h>
+#include <sys/types.h>
 
 /*****************************************************************************/
 
@@ -95,6 +99,114 @@ out_huge:
 
 /*****************************************************************************/
 
+bool
+nm_utils_set_effective_user(const char *user, char *errbuf, size_t errbuf_len)
+{
+    struct passwd *pwentry;
+    int            errsv;
+    char           error[1024];
+
+    errno   = 0;
+    pwentry = getpwnam(user);
+    if (!pwentry) {
+        errsv = errno;
+        if (errsv == 0) {
+            snprintf(errbuf, errbuf_len, "user not found");
+        } else {
+            snprintf(errbuf,
+                     errbuf_len,
+                     "error getting user entry: %d (%s)\n",
+                     errsv,
+                     strerror_r(errsv, error, sizeof(error)));
+        }
+        return false;
+    }
+
+    if (setgid(pwentry->pw_gid) != 0) {
+        errsv = errno;
+        snprintf(errbuf,
+                 errbuf_len,
+                 "failed to change group to %u: %d (%s)\n",
+                 pwentry->pw_gid,
+                 errsv,
+                 strerror_r(errsv, error, sizeof(error)));
+        return false;
+    }
+
+    if (initgroups(user, pwentry->pw_gid) != 0) {
+        errsv = errno;
+        snprintf(errbuf,
+                 errbuf_len,
+                 "failed to reset supplementary group list to %u: %d (%s)\n",
+                 pwentry->pw_gid,
+                 errsv,
+                 strerror_r(errsv, error, sizeof(error)));
+        return false;
+    }
+
+    if (setuid(pwentry->pw_uid) != 0) {
+        errsv = errno;
+        snprintf(errbuf,
+                 errbuf_len,
+                 "failed to change user to %u: %d (%s)\n",
+                 pwentry->pw_uid,
+                 errsv,
+                 strerror_r(errsv, error, sizeof(error)));
+        return false;
+    }
+
+    return true;
+}
+
+/*****************************************************************************/
+
+bool
+nm_utils_read_file_to_stdout(const char *filename, char *errbuf, size_t errbuf_len)
+{
+    nm_auto_close int fd = -1;
+    char              buffer[4096];
+    char              error[1024];
+    ssize_t           bytes_read;
+    int               errsv;
+
+    fd = open(filename, O_RDONLY);
+    if (fd == -1) {
+        errsv = errno;
+        snprintf(errbuf,
+                 errbuf_len,
+                 "error opening the file: %d (%s)",
+                 errsv,
+                 strerror_r(errsv, error, sizeof(error)));
+        return false;
+    }
+
+    while ((bytes_read = read(fd, buffer, sizeof(buffer))) > 0) {
+        if (fwrite(buffer, 1, bytes_read, stdout) != (size_t) bytes_read) {
+            errsv = errno;
+            snprintf(errbuf,
+                     errbuf_len,
+                     "error writing to stdout: %d (%s)",
+                     errsv,
+                     strerror_r(errsv, error, sizeof(error)));
+            return false;
+        }
+    }
+
+    if (bytes_read < 0) {
+        errsv = errno;
+        snprintf(errbuf,
+                 errbuf_len,
+                 "error reading the file: %d (%s)",
+                 errsv,
+                 strerror_r(errsv, error, sizeof(error)));
+        return false;
+    }
+
+    return true;
+}
+
+/*****************************************************************************/
+
 /**
  * _nm_strerror_r:
  * @errsv: the errno passed to strerror_r()

src/libnm-std-aux/nm-std-utils.h

@@ -37,4 +37,8 @@ size_t nm_utils_get_next_realloc_size(bool true_realloc, size_t requested);
 
 const char *_nm_strerror_r(int errsv, char *buf, size_t buf_size);
 
+bool nm_utils_set_effective_user(const char *user, char *errbuf, size_t errbuf_size);
+
+bool nm_utils_read_file_to_stdout(const char *filename, char *errbuf, size_t errbuf_len);
+
 #endif /* __NM_STD_UTILS_H__ */

src/nm-helpers/README.md

@@ -7,12 +7,13 @@ components.
 nm-daemon-helper
 ----------------
 
-A internal helper application that is spawned by NetworkManager
-to perform certain actions.
+A internal helper application that is spawned by NetworkManager to
+perform certain actions which can't be done in the daemon. 
 
-Currently all it does is doing a reverse DNS lookup, which
-cannot be done by NetworkManager because the operation requires
-to reconfigure the libc resolver (which is a process-wide operation).
+Currently it's used to do a reverse DNS lookup after reconfiguring the
+libc resolver (which is a process-wide operation), and to read files
+on behalf of unprivileged users (which requires a seteuid that affects
+all the threads of the process).
 
 This is not directly useful to the user.
 

src/nm-helpers/nm-daemon-helper.c

@@ -137,6 +137,37 @@ cmd_resolve_address(void)
     return RETURN_ERROR;
 }
 
+static int
+cmd_read_file_as_user(void)
+{
+    nm_auto_free char *user     = NULL;
+    nm_auto_free char *filename = NULL;
+    char               error[1024];
+
+    user = read_arg();
+    if (!user)
+        return RETURN_INVALID_ARGS;
+
+    filename = read_arg();
+    if (!filename)
+        return RETURN_INVALID_ARGS;
+
+    if (more_args())
+        return RETURN_INVALID_ARGS;
+
+    if (!nm_utils_set_effective_user(user, error, sizeof(error))) {
+        fprintf(stderr, "Failed to set effective user '%s': %s", user, error);
+        return RETURN_ERROR;
+    }
+
+    if (!nm_utils_read_file_to_stdout(filename, error, sizeof(error))) {
+        fprintf(stderr, "Failed to read file '%s' as user '%s': %s", filename, user, error);
+        return RETURN_ERROR;
+    }
+
+    return RETURN_SUCCESS;
+}
+
 int
 main(int argc, char **argv)
 {
@@ -150,6 +181,8 @@ main(int argc, char **argv)
         return cmd_version();
     if (nm_streq(cmd, "resolve-address"))
         return cmd_resolve_address();
+    if (nm_streq(cmd, "read-file-as-user"))
+        return cmd_read_file_as_user();
 
     return RETURN_INVALID_CMD;
 }