Re: [Libguestfs] [nbdkit PATCH 2/2] server: Sanitize stdin/out before running plugin code

As shown in the previous patch, plugins may choose to use stdin or stdout during .config. But from .get_ready onwards, well-written plugins shouldn't be needing any further use of stdin/out. We already swapped stdin/out to /dev/null while daemonizing, but did not do do during -f or --run, which leads to some surprising inconsistency when trying to debug a plugin that works in the foreground but fails when daemonized. What's more, while the task executed by --run should use the original stdin/out (and we even have tests that would fail without this), we don't want the plugin to interfere with whatever --run is doing. So it's time to move the swap over to /dev/null into a central location, right before .get_ready. Signed-off-by: Eric Blake <eblake(a)redhat.com&gt; --- server/internal.h | 2 ++ server/background.c | 12 ++++-------- server/captive.c | 10 ++++++++-- server/connections.c | 12 ------------ server/main.c | 33 ++++++++++++++++++++++++++++++++- tests/test-layers-plugin.c | 12 +++++++++++- tests/test-layers.c | 4 +++- 7 files changed, 60 insertions(+), 25 deletions(-) diff --git a/server/internal.h b/server/internal.h index 67eb6a32..79e1906c 100644 --- a/server/internal.h +++ b/server/internal.h @@ -132,6 +132,8 @@ extern bool tls_verify_peer; extern char *unixsocket; extern const char *user, *group; extern bool verbose; +extern int orig_in; +extern int orig_out; /* Linked list of backends. Each backend struct is followed by either * a filter or plugin struct. "top" points to the first one. They diff --git a/server/background.c b/server/background.c index 531ba470..8388d9c8 100644 --- a/server/background.c +++ b/server/background.c @@ -1,5 +1,5 @@ /* nbdkit - * Copyright (C) 2019 Red Hat Inc. + * Copyright (C) 2019-2020 Red Hat Inc. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions are @@ -70,13 +70,9 @@ fork_into_background (void) chdir ("/"); #pragma GCC diagnostic pop - /* Close stdin/stdout and redirect to /dev/null. */ - close (0); - close (1); - open ("/dev/null", O_RDONLY); - open ("/dev/null", O_WRONLY); - - /* If not verbose, set stderr to the same as stdout as well. */ + /* By this point, stdin/out have been redirected to /dev/null. + * If not verbose, set stderr to the same as stdout as well. + */ if (!verbose) dup2 (1, 2); diff --git a/server/captive.c b/server/captive.c index 0a243d2b..1ff10192 100644 --- a/server/captive.c +++ b/server/captive.c @@ -1,5 +1,5 @@ /* nbdkit - * Copyright (C) 2019 Red Hat Inc. + * Copyright (C) 2019-2020 Red Hat Inc. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions are @@ -151,7 +151,13 @@ run_command (void) } if (pid > 0) { /* Parent process is the run command. */ - r = system (cmd); + /* Restore original stdin/out */ + if (dup2 (orig_in, STDIN_FILENO) == -1 || + dup2 (orig_out, STDOUT_FILENO) == -1) { + r = -1; + } + else + r = system (cmd); if (r == -1) { nbdkit_error ("failure to execute external command: %m"); r = EXIT_FAILURE; diff --git a/server/connections.c b/server/connections.c index c54c71c1..c7b55ca1 100644 --- a/server/connections.c +++ b/server/connections.c @@ -335,18 +335,6 @@ free_connection (struct connection *conn) return; conn->close (); - if (listen_stdin) { - int fd; - - /* Restore something to stdin/out so the rest of our code can - * continue to assume that all new fds will be above stderr. - * Swap directions to get EBADF on improper use of stdin/out. - */ - fd = open ("/dev/null", O_WRONLY | O_CLOEXEC); - assert (fd == 0); - fd = open ("/dev/null", O_RDONLY | O_CLOEXEC); - assert (fd == 1); - } /* Don't call the plugin again if quit has been set because the main * thread will be in the process of unloading it. The plugin.unload diff --git a/server/main.c b/server/main.c index 748122fc..596dd306 100644 --- a/server/main.c +++ b/server/main.c @@ -101,6 +101,8 @@ const char *user, *group; /* -u & -g */ bool verbose; /* -v */ bool vsock; /* --vsock */ unsigned int socket_activation /* $LISTEN_FDS and $LISTEN_PID set */; +int orig_in = -1; /* dup'd stdin during -s/--run */ +int orig_out = -1; /* dup'd stdout during -s/--run */ /* The linked list of zero or more filters, and one plugin. */ struct backend *top; @@ -694,6 +696,35 @@ main (int argc, char *argv[]) top->config_complete (top); + /* Sanitize stdin/stdout to /dev/null, after saving the originals + * when needed. We are still single-threaded at this point, and + * already checked that stdin/out were open, so we don't have to + * worry about other threads accidentally grabbing our intended fds, + * or races on FD_CLOEXEC. + */ + if (listen_stdin || run) { +#ifndef F_DUPFD_CLOEXEC +#define F_DUPFD_CLOEXEC F_DUPFD +#endif + orig_in = fcntl (STDIN_FILENO, F_DUPFD_CLOEXEC, STDERR_FILENO + 1); + orig_out = fcntl (STDOUT_FILENO, F_DUPFD_CLOEXEC, STDERR_FILENO + 1); +#if F_DUPFD == F_DUPFD_CLOEXEC + orig_in = set_cloexec (orig_in); + orig_out = set_cloexec (orig_out); +#endif + if (orig_in == -1 || orig_out == -1) { + perror ("fcntl"); + exit (EXIT_FAILURE); + } + } + close (STDIN_FILENO); + close (STDOUT_FILENO); + if (open ("/dev/null", O_RDONLY) != STDIN_FILENO || + open ("/dev/null", O_WRONLY) != STDOUT_FILENO) { + perror ("open"); + exit (EXIT_FAILURE); + } + /* Select the correct thread model based on config. */ lock_init_thread_model (); @@ -918,7 +949,7 @@ start_serving (void) change_user (); write_pidfile (); threadlocal_new_server_thread (); - handle_single_connection (0, 1); + handle_single_connection (orig_in, orig_out); return; }

diff --git a/tests/test-layers-plugin.c b/tests/test-layers-plugin.c index 8858bede..5a6b3020 100644 --- a/tests/test-layers-plugin.c +++ b/tests/test-layers-plugin.c @@ -1,5 +1,5 @@ /* nbdkit - * Copyright (C) 2018-2019 Red Hat Inc. + * Copyright (C) 2018-2020 Red Hat Inc. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions are @@ -36,6 +36,7 @@ #include <stdlib.h> #include <stdint.h> #include <string.h> +#include <unistd.h> #define NBDKIT_API_VERSION 2 #include <nbdkit-plugin.h> @@ -75,7 +76,16 @@ test_layers_plugin_config_complete (void) static int test_layers_plugin_get_ready (void) { + char c = 'X'; DEBUG_FUNCTION; + + /* Test that stdin/stdout have been sanitized */ + if (write (STDOUT_FILENO, &c, 1) != 1 || + read (STDIN_FILENO, &c, 1) != 0) { + nbdkit_error ("unusual stdio behavior"); + return -1; + } + return 0; } diff --git a/tests/test-layers.c b/tests/test-layers.c index 33ae5a75..5bdce54e 100644 --- a/tests/test-layers.c +++ b/tests/test-layers.c @@ -1,5 +1,5 @@ /* nbdkit - * Copyright (C) 2018-2019 Red Hat Inc. + * Copyright (C) 2018-2020 Red Hat Inc. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions are @@ -125,6 +125,8 @@ main (int argc, char *argv[]) if (pid == 0) { /* Child. */ dup2 (sfd[1], 0); dup2 (sfd[1], 1); + close (sfd[0]); + close (sfd[1]); close (pfd[0]); dup2 (pfd[1], 2); close (pfd[1]);