[Libguestfs] [nbdkit PATCH v2 1/3] server: Add nbdkit_stdio_safe

Trying to read a password or inline script from stdin when the -s option was used to connect to our client over stdin is not going to work. It would be nice to fail such usage as soon as possible, by giving plugins a means to decide if it is safe to use stdio during .config. Update nbdkit_read_password and the sh plugin to take advantage of the new entry point. Signed-off-by: Eric Blake <eblake(a)redhat.com&gt; --- docs/nbdkit-plugin.pod | 23 ++++++++++++++++++++++- plugins/sh/nbdkit-sh-plugin.pod | 4 +++- include/nbdkit-common.h | 1 + server/nbdkit.syms | 1 + server/public.c | 18 +++++++++++++++++- server/test-public.c | 23 +++++++++++++++++++++-- plugins/sh/sh.c | 7 ++++++- 7 files changed, 71 insertions(+), 6 deletions(-) diff --git a/docs/nbdkit-plugin.pod b/docs/nbdkit-plugin.pod index 6c1311eb..a12ecfd5 100644 --- a/docs/nbdkit-plugin.pod +++ b/docs/nbdkit-plugin.pod @@ -388,6 +388,10 @@ should probably look at other plugins and follow the same conventions. If the value is a relative path, then note that the server changes directory when it starts up. See L</FILENAMES AND PATHS> above. +If C<nbdkit_stdio_safe> returns true, the value of the configuration +parameter may be used to trigger reading additional data through stdin +(such as a password or inline script). + If the C<.config> callback is not provided by the plugin, and the user tries to specify any C<key=value> arguments, then nbdkit will exit with an error. @@ -1148,6 +1152,23 @@ C<str> can be a string containing a case-insensitive form of various common toggle values. The function returns 0 or 1 if the parse was successful. If there was an error, it returns C<-1>. +=head2 Interacting with stdin and stdout + +The C<nbdkit_stdio_safe> utility function returns non-zero if it is +safe to interact with stdin and stdout during C<.config>. When nbdkit +is started under the single connection mode (C<-s>), the plugin must +not directly interact with stdin, because that would interfere with +the client. The result of this function only matters prior to +C<.config_complete>; once nbdkit reaches C<.get_ready>, the plugin +should assume that nbdkit may have closed the original stdin and +stdout in order to become a daemon. + + int nbdkit_stdio_safe (void); + +As an example, the L<nbdkit-sh-plugin(3)> uses this function to +determine whether it is safe to support C<script=-> to read a script +from stdin. + =head2 Reading passwords The C<nbdkit_read_password> utility function can be used to read @@ -1180,7 +1201,7 @@ used directly on the command line, eg: nbdkit myplugin password=mostsecret But more securely this function can also read a password -interactively: +interactively (if C<nbdkit_stdio_safe> returns true): nbdkit myplugin password=- diff --git a/plugins/sh/nbdkit-sh-plugin.pod b/plugins/sh/nbdkit-sh-plugin.pod index ffd0310f..20a2b785 100644 --- a/plugins/sh/nbdkit-sh-plugin.pod +++ b/plugins/sh/nbdkit-sh-plugin.pod @@ -48,7 +48,9 @@ as the name of the script, like this: EOF By default the inline script runs under F</bin/sh>. You can add a -shebang (C<#!>) to use other scripting languages. +shebang (C<#!>) to use other scripting languages. Of course, reading +an inline script from stdin is incompatible with the C<-s> +(C<--single>) mode of nbdkit that connects a client on stdin. =head1 WRITING AN NBDKIT SH PLUGIN diff --git a/include/nbdkit-common.h b/include/nbdkit-common.h index 9d1d89d0..47288050 100644 --- a/include/nbdkit-common.h +++ b/include/nbdkit-common.h @@ -106,6 +106,7 @@ extern int nbdkit_parse_int64_t (const char *what, const char *str, int64_t *r); extern int nbdkit_parse_uint64_t (const char *what, const char *str, uint64_t *r); +extern int nbdkit_stdio_safe (void); extern int nbdkit_read_password (const char *value, char **password); extern char *nbdkit_realpath (const char *path); extern int nbdkit_nanosleep (unsigned sec, unsigned nsec); diff --git a/server/nbdkit.syms b/server/nbdkit.syms index 111223f2..20c390a9 100644 --- a/server/nbdkit.syms +++ b/server/nbdkit.syms @@ -65,6 +65,7 @@ nbdkit_realpath; nbdkit_set_error; nbdkit_shutdown; + nbdkit_stdio_safe; nbdkit_vdebug; nbdkit_verror; diff --git a/server/public.c b/server/public.c index 3fd11253..4bb1f2e0 100644 --- a/server/public.c +++ b/server/public.c @@ -1,5 +1,5 @@ /* nbdkit - * Copyright (C) 2013-2019 Red Hat Inc. + * Copyright (C) 2013-2020 Red Hat Inc. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions are @@ -404,6 +404,13 @@ nbdkit_parse_bool (const char *str) return -1; } +/* Return true if it is safe to read from stdin during configuration. */ +int +nbdkit_stdio_safe (void) +{ + return !listen_stdin; +} + /* Read a password from configuration value. */ static int read_password_from_fd (const char *what, int fd, char **password); @@ -419,6 +426,11 @@ nbdkit_read_password (const char *value, char **password) /* Read from stdin. */ if (strcmp (value, "-") == 0) { + if (!nbdkit_stdio_safe ()) { + nbdkit_error ("stdin is not available for reading password"); + return -1; + } + printf ("password: "); /* Set no echo. */ @@ -455,6 +467,10 @@ nbdkit_read_password (const char *value, char **password) if (nbdkit_parse_int ("password file descriptor", &value[1], &fd) == -1) return -1; + if (!nbdkit_stdio_safe () && fd < STDERR_FILENO) { + nbdkit_error ("stdin/out is not available for reading password"); + return -1; + } if (read_password_from_fd (&value[1], fd, password) == -1) return -1; } diff --git a/server/test-public.c b/server/test-public.c index fe347d44..d4903420 100644 --- a/server/test-public.c +++ b/server/test-public.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 @@ -53,6 +53,8 @@ nbdkit_error (const char *fs, ...) error_flagged = true; } +bool listen_stdin; + volatile int quit; int quit_fd = -1; @@ -427,7 +429,24 @@ test_nbdkit_read_password (void) pass = false; } - /* XXX Testing reading from stdin would require setting up a pty */ + /* XXX Testing reading from stdin would require setting up a pty. But + * we can test that it is forbidden with -s. + */ + listen_stdin = true; + if (nbdkit_read_password ("-", &pw) != -1) { + fprintf (stderr, "Failed to diagnose failed password from stdin with -s\n"); + pass = false; + } + else if (pw != NULL) { + fprintf (stderr, "Failed to set password to NULL on failure\n"); + pass = false; + } + else if (!error_flagged) { + fprintf (stderr, "Wrong error message handling\n"); + pass = false; + } + error_flagged = false; + return pass; } diff --git a/plugins/sh/sh.c b/plugins/sh/sh.c index 736b8ef0..c8a321f1 100644 --- a/plugins/sh/sh.c +++ b/plugins/sh/sh.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 @@ -116,6 +116,11 @@ inline_script (void) char *filename = NULL; CLEANUP_FREE char *cmd = NULL; + if (!nbdkit_stdio_safe ()) { + nbdkit_error ("inline script is incompatible with -s"); + return NULL; + } + if (asprintf (&filename, "%s/%s", tmpdir, scriptname) == -1) { nbdkit_error ("asprintf: %m"); goto err; -- 2.26.0