1 files changed, 220 insertions, 0 deletions

diff --git a/miscutils/drop.c b/miscutils/drop.c
new file mode 100644
index 000000000..bef1fa52b
--- /dev/null
+++ b/miscutils/drop.c
@@ -0,0 +1,220 @@
+/* vi: set sw=4 ts=4: */
+/*
+ * drop - run a command without elevated privileges.
+ *
+ * Copyright (c) 2023 Ronald M Yorston
+ *
+ * Licensed under GPLv2 or later, see file LICENSE in this source tree.
+ */
+//config:config DROP
+//config:       bool "drop"
+//config:       default y
+//config:       depends on PLATFORM_MINGW32 && SH_IS_ASH
+//config:       help
+//config:       Run a command without elevated privileges
+
+//config:config CDROP
+//config:       bool "cdrop"
+//config:       default y
+//config:       depends on PLATFORM_MINGW32
+//config:       help
+//config:       Run a command without elevated privileges using cmd.exe
+
+//config:config PDROP
+//config:       bool "pdrop"
+//config:       default y
+//config:       depends on PLATFORM_MINGW32
+//config:       help
+//config:       Run a command without elevated privileges using PowerShell
+
+//applet:IF_DROP(APPLET(drop, BB_DIR_USR_BIN, BB_SUID_DROP))
+//applet:IF_CDROP(APPLET_ODDNAME(cdrop, drop, BB_DIR_USR_BIN, BB_SUID_DROP, cdrop))
+//applet:IF_PDROP(APPLET_ODDNAME(pdrop, drop, BB_DIR_USR_BIN, BB_SUID_DROP, pdrop))
+
+//kbuild:lib-$(CONFIG_DROP) += drop.o
+//kbuild:lib-$(CONFIG_CDROP) += drop.o
+//kbuild:lib-$(CONFIG_PDROP) += drop.o
+
+//usage:#define drop_trivial_usage
+//usage:        "[COMMAND [ARG...] | [-s SHELL] -c CMD_STRING [ARG...]]"
+//usage:#define drop_full_usage "\n\n"
+//usage:        "Drop elevated privileges and run a command. If no command\n"
+//usage:        "is provided run a shell (by default, the BusyBox shell).\n"
+
+//usage:#define cdrop_trivial_usage
+//usage:        "[COMMAND [ARG...] | -c CMD_STRING [ARG...]]"
+//usage:#define cdrop_full_usage "\n\n"
+//usage:        "Drop elevated privileges and run a command. If no command\n"
+//usage:        "is provided run cmd.exe.\n"
+
+//usage:#define pdrop_trivial_usage
+//usage:        "[COMMAND [ARG...] | -c CMD_STRING [ARG...]]"
+//usage:#define pdrop_full_usage "\n\n"
+//usage:        "Drop elevated privileges and run a command. If no command\n"
+//usage:        "is provided run PowerShell.\n"
+
+#include "libbb.h"
+#include <winsafer.h>
+#include <lazyload.h>
+#include "NUM_APPLETS.h"
+
+// Set an environment variable to the name of the unprivileged user,
+// but only if it was previously unset or contained "root".
+static void setenv_name(const char *key)
+{
+        const char *name = get_user_name();
+        const char *oldname = getenv(key);
+
+        if (name && (!oldname || strcmp(oldname, "root") == 0)) {
+                setenv(key, name, 1);
+        }
+}
+
+int drop_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
+int drop_main(int argc UNUSED_PARAM, char **argv)
+{
+        SAFER_LEVEL_HANDLE safer;
+        HANDLE token;
+        STARTUPINFO si;
+        PROCESS_INFORMATION pi;
+        TOKEN_MANDATORY_LABEL TIL;
+        // Medium integrity level S-1-16-8192
+        unsigned char medium[12] = {
+                0x01, 0x01, 0x00, 0x00,
+                0x00, 0x00, 0x00, 0x10,
+                0x00, 0x20, 0x00, 0x00
+        };
+        DWORD code;
+        // This shouldn't be necessary but without it the binary complains
+        // it can't find CreateProcessAsUserA on older versions of Windows.
+        DECLARE_PROC_ADDR(BOOL, CreateProcessAsUserA, HANDLE, LPCSTR, LPSTR,
+                        LPSECURITY_ATTRIBUTES, LPSECURITY_ATTRIBUTES, BOOL, DWORD,
+                        LPVOID, LPCSTR, LPSTARTUPINFOA, LPPROCESS_INFORMATION);
+
+        if (!INIT_PROC_ADDR(advapi32.dll, CreateProcessAsUserA))
+                bb_simple_error_msg_and_die("not supported");
+
+        /*
+         * Run a shell using a token with reduced privilege.  Hints from:
+         *
+         *    https://stackoverflow.com/questions/17765568/
+         */
+        if (SaferCreateLevel(SAFER_SCOPEID_USER, SAFER_LEVELID_NORMALUSER,
+                                        SAFER_LEVEL_OPEN, &safer, NULL) &&
+                        SaferComputeTokenFromLevel(safer, NULL, &token, 0, NULL)) {
+
+                // Set medium integrity
+                TIL.Label.Sid = (PSID)medium;
+                TIL.Label.Attributes = SE_GROUP_INTEGRITY;
+                if (SetTokenInformation(token, TokenIntegrityLevel, &TIL,
+                                                sizeof(TOKEN_MANDATORY_LABEL))) {
+                        char *opt_command = NULL;
+                        char *opt_shell = NULL;
+                        char **a;
+                        const char *opt, *arg;
+                        char *exe, *cmd, *q;
+
+                        if (*applet_name == 'd')
+                                getopt32(argv, "c:s:", &opt_command, &opt_shell);
+                        else
+                                getopt32(argv, "c:", &opt_command);
+                        a = argv + optind;
+                        opt = "-c";
+
+                        if (*a == NULL || opt_command) {
+                                switch (*applet_name) {
+#if ENABLE_PDROP
+                                case 'p':
+                                        arg = "powershell.exe";
+                                        exe = find_first_executable(arg);
+                                        break;
+#endif
+#if ENABLE_CDROP
+                                case 'c':
+                                        opt = "/c";
+                                        arg = "cmd.exe";
+                                        exe = find_first_executable(arg);
+                                        break;
+#endif
+#if ENABLE_DROP
+                                case 'd':
+                                        if (opt_shell) {
+                                                arg = bb_basename(opt_shell);
+                                                if (strcasecmp(arg, "cmd.exe") == 0)
+                                                        opt = "/c";
+                                                exe = file_is_win32_exe(opt_shell);
+                                        } else {
+                                                arg = "sh";
+                                                exe = xstrdup(bb_busybox_exec_path);
+                                        }
+                                        break;
+#endif
+                                default:
+                                        // Never executed, just to silence warnings.
+                                        arg = argv[0];
+                                        exe = NULL;
+                                        break;
+                                }
+                        } else {
+                                arg = *a++;
+
+#if ENABLE_FEATURE_PREFER_APPLETS && NUM_APPLETS > 1
+                                if (!has_path(arg) && find_applet_by_name(arg) >= 0) {
+                                        exe = xstrdup(bb_busybox_exec_path);
+                                } else
+#endif
+                                if (has_path(arg)) {
+                                        exe = file_is_win32_exe(arg);
+                                } else {
+                                        exe = find_first_executable(arg);
+                                }
+                        }
+
+                        if (exe == NULL) {
+                                xfunc_error_retval = 127;
+                                bb_error_msg_and_die("can't find '%s'", arg);
+                        }
+
+                        slash_to_bs(exe);
+                        cmd = quote_arg(arg);
+                        if (opt_command) {
+                                cmd = xappendword(cmd, opt);
+                                q = quote_arg(opt_command);
+                                cmd = xappendword(cmd, q);
+                                free(q);
+                        }
+
+                        // Build the command line
+                        while (*a) {
+                                q = quote_arg(*a++);
+                                cmd = xappendword(cmd, q);
+                                free(q);
+                        }
+
+                        ZeroMemory(&si, sizeof(STARTUPINFO));
+                        si.cb = sizeof(STARTUPINFO);
+                        si.hStdInput = GetStdHandle(STD_INPUT_HANDLE);
+                        si.hStdOutput = GetStdHandle(STD_OUTPUT_HANDLE);
+                        si.hStdError = GetStdHandle(STD_ERROR_HANDLE);
+                        si.dwFlags = STARTF_USESTDHANDLES;
+
+                        setenv_name("USER");
+                        setenv_name("LOGNAME");
+
+                        if (!CreateProcessAsUserA(token, exe, cmd, NULL, NULL, TRUE,
+                                                0, NULL, NULL, &si, &pi)) {
+                                xfunc_error_retval = 126;
+                                bb_error_msg_and_die("can't execute '%s'", exe);
+                        }
+
+                        kill_child_ctrl_handler(pi.dwProcessId);
+                        SetConsoleCtrlHandler(kill_child_ctrl_handler, TRUE);
+                        WaitForSingleObject(pi.hProcess, INFINITE);
+                        if (GetExitCodeProcess(pi.hProcess, &code)) {
+                                return exit_code_to_posix(code);
+                        }
+                }
+        }
+
+        return EXIT_FAILURE;
+}