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README.md
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README.md
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qssb (quite simple sandbox)
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===========================
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qssb.h is a simple header only library for easy sandboxing of
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applications.
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It aims to provide an interface to avoid the annoying details that
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using Seccomp and Linux Namespaces requires.
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Features
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========
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Systemcall filtering, restricting file system access, dropping
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privileges, isolating the application from the network, etc.
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Requirements
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============
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Kernel x.y.z.
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Status
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======
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No release yet, API is unstable.
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Documentation
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=============
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To be written
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Examples
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========
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Real world project: cgit sandboxed: https://git.quitesimple.org/cgitsb
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Contributing
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============
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Contributations are very welcome. Options:
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1) Pull-Request: github.com/quitesimpleorg/qssb
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2) Mail to qssb at quitesimple.org with instructions
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on where to pull the changes.
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3) Mailing a classic patch.
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License
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=======
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ISC
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401
qssb.h
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401
qssb.h
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#ifndef QSSB_H
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#define QSSB_H
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#define _GNU_SOURCE
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#include <sched.h>
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#include <stdio.h>
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#include <errno.h>
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#include <sys/stat.h>
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#include <sys/types.h>
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#include <sys/mount.h>
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#include <sys/prctl.h>
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#include <stdlib.h>
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#include <string.h>
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#include <unistd.h>
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#include <fcntl.h>
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#include <linux/limits.h>
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#include <linux/filter.h>
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#include <linux/seccomp.h>
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#include <sys/capability.h>
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#include <stddef.h>
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//TODO: stolen from kernel samples/seccomp, GPLv2...?
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#define ALLOW \
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BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW)
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#define DENY \
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BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL)
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#define SYSCALL(nr, jt) \
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BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (nr), 0, 1), jt
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#define LOAD_SYSCALL_NR \
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BPF_STMT(BPF_LD+BPF_W+BPF_ABS, \
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offsetof(struct seccomp_data, nr))
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#define QSSB_ISOLATE_NETWORK 1<<1
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#define QSSB_LOG_ERROR(...) fprintf(stderr, __VA_ARGS__)
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/* Policy tells qssb what to do */
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struct qssb_policy
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{
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int drop_caps;
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int preserve_cwd;
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int not_dumpable;
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int no_new_privs;
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int namespace_options;
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int syscall_default_policy;
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int *blacklisted_syscalls;
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int *allowed_syscalls;
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char *chroot_target_path;
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char *chdir_path;
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char **readonly_paths;
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char **writable_paths;
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};
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/* Creates the default policy
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* Must be freed using qssb_free_policy
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* @returns: default policy */
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struct qssb_policy *qssb_init_policy()
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{
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struct qssb_policy *result = calloc(1, sizeof(struct qssb_policy));
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result->drop_caps = 1;
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result->not_dumpable = 1;
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result->no_new_privs = 1;
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result->namespace_options = 0;
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result->chdir_path = "/";
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result->chroot_target_path = NULL;
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result->readonly_paths = NULL;
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result->writable_paths = NULL;
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return result;
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}
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/* Creates a directory and all necessary parent directories
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*
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* @returns: 0 on success, -ERRNO on failure
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* */
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static int mkdir_structure(const char *p, mode_t mode)
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{
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char path[PATH_MAX + 2] = { 0 };
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snprintf(path, sizeof(path), "%s/", p);
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char *begin = path;
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char *end = begin+1;
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while(*end)
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{
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if(*end == '/')
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{
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*end = 0;
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if(mkdir(begin, mode) < 0)
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{
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if(errno == EEXIST)
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{
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//TODO: stat, test if it is a directory, if not, err
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}
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else
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{
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QSSB_LOG_ERROR("Failed to create directory for chroot: %s\n", begin);
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return -1;
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}
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}
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*end = '/';
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++end;
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while(*end == '/')
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{
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++end;
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}
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}
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else
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{
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++end;
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}
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}
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return 0;
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}
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/* Helper to mount directories into the chroot path "chroot_target_path"
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* Paths will be created if necessary
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* @returns: 0 on sucess, -ERRNO on failure */
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static int mount_to_chroot(const char *chroot_target_path, char **paths, unsigned long flags)
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{
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if(paths == NULL)
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{
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return 0;
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}
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char *path = *paths;
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while(path != NULL)
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{
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char path_inside_chroot[PATH_MAX + 1];
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snprintf(path_inside_chroot, sizeof(path_inside_chroot), "%s/%s", chroot_target_path, path);
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int ret = mkdir_structure(path_inside_chroot, 0700);
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if(ret < 0)
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{
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QSSB_LOG_ERROR("Error creating directory structure while mounting paths to chroot. %s\n", strerror(errno));
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return ret;
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}
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ret = mount(path, path_inside_chroot, NULL, flags, NULL);
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if(ret < 0 )
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{
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QSSB_LOG_ERROR("Error: Failed to mount %s to %s: %s\n", path, path_inside_chroot, strerror(errno));
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return ret;
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}
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path = *(++paths);
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}
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return 0;
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}
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/* Ends the policy as best as possible. */
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/* TODO: can this function do actually anything useful?*/
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static int qssb_end_policy(struct qssb_policy *ctxt)
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{
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}
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/*
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* Frees the memory taken by a qssb_policy object
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*/
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static void qssb_free_policy(struct qssb_policy *ctxt)
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{
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free(ctxt);
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}
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/* Enters the user and mount namespaces */
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static int enter_namespaces()
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{
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int ret = unshare(CLONE_NEWUSER);
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if(ret == -1)
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{
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QSSB_LOG_ERROR("Error: Failed to unshare user namespaces: %s\n", strerror(errno));
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return ret;
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}
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uid_t current_uid = getuid();
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gid_t current_gid = getgid();
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//TODO: check errors
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FILE *fp = fopen("/proc/self/setgroups", "w");
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fprintf(fp, "deny");
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fclose(fp);
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fp = fopen("/proc/self/uid_map", "w");
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fprintf(fp, "0 %i", current_uid);
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fclose(fp);
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fp = fopen("/proc/self/gid_map", "w");
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fprintf(fp, "0 %i", current_gid);
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fclose(fp);
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ret = unshare(CLONE_NEWNS);
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if(ret == -1)
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{
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QSSB_LOG_ERROR("Error: Failed to unshare mount namespaces: %s\n", strerror(errno));
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return ret;
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}
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return 0;
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}
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/* Drops all capabiltiies held by the process
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*
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* @returns: 0 on sucess, -1 on error
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*/
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static int drop_caps()
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{
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int cap = 0;
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int res = 0;
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while((res = prctl(PR_CAPBSET_DROP, cap, 0, 0, 0)) == 0)
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{
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++cap;
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}
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if(res == -1 && errno != EINVAL)
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{
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QSSB_LOG_ERROR("Failed to drop the capability bounding set!");
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return -errno;
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}
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//TODO: systems that are not 64 bit
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struct __user_cap_header_struct h = { 0 };
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h.pid = 0;
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h.version = _LINUX_CAPABILITY_VERSION_3;
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struct __user_cap_data_struct drop[2];
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drop[0].effective = 0;
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drop[0].permitted = 0;
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drop[0].inheritable = 0;
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drop[1].effective = 0;
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drop[1].permitted = 0;
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drop[1].inheritable = 0;
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if(capset(&h, drop) == -1)
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{
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QSSB_LOG_ERROR("Failed to drop capabilities: %s\n", strerror(errno));;
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return -errno;
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}
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return 0;
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}
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/*
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* Enables the per_syscall seccomp action for system calls
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*
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* syscalls: array of system calls numbers. -1 must be the last entry.
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* per_syscall: action to apply for each system call
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* default_action: the default action at the end
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*
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* @returns: 0 on success, -1 on error
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*/
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static int seccomp_enable(int *syscalls, int per_syscall, int default_action)
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{
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struct sock_filter filter[1024] =
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{
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LOAD_SYSCALL_NR,
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};
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int current_filter_index = 1;
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while(*syscalls != -1)
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{
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struct sock_filter syscall = BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, *syscalls, 0, 1);
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struct sock_filter action = BPF_STMT(BPF_RET+BPF_K, per_syscall);
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filter[current_filter_index++] = syscall;
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filter[current_filter_index++] = action;
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++syscalls;
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}
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struct sock_filter da = BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL);
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filter[current_filter_index] = da;
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struct sock_fprog prog = {
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.len = current_filter_index + 1,
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.filter = filter,
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};
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if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog) == -1)
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{
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QSSB_LOG_ERROR("prctl SET_SECCOMP %s\n", strerror(errno));
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return -1;
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}
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return 0;
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}
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/*
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* Blacklists the specified systemcalls.
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*
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* syscalls: array of system calls numbers. -1 must be the last entry.
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*/
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static int seccomp_enable_blacklist(int *syscalls)
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{
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return seccomp_enable(syscalls, SECCOMP_RET_KILL, SECCOMP_RET_ALLOW);
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}
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/*
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* Blacklists the specified systemcalls.
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*
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* syscalls: array of system calls numbers. -1 must be the last entry.
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*/
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static int seccomp_enable_whitelist(int *syscalls)
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{
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return seccomp_enable(syscalls, SECCOMP_RET_ALLOW, SECCOMP_RET_KILL);
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}
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/* Enables the specified qssb_policy.
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*
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* The calling process is supposed *TO BE WRITTEN* if
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* this function fails.
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* @returns: 0 on sucess, <0 on error
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*/
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int qssb_enable_policy(struct qssb_policy *policy)
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{
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if(policy->blacklisted_syscalls != NULL && policy->allowed_syscalls != NULL)
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{
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QSSB_LOG_ERROR("Error: Cannot mix blacklisted and whitelisted systemcalls\n");
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return -EINVAL;
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}
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if(policy->chroot_target_path == NULL)
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{
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policy->chroot_target_path = "/tmp/.TODOIMPLEMENT"; //TODO: implement
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}
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if(enter_namespaces() < 0)
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{
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QSSB_LOG_ERROR("Error while trying to enter namespaces\n");
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return -1;
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}
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if(policy->readonly_paths != NULL || policy->writable_paths != NULL)
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{
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if(mount_to_chroot(policy->chroot_target_path, policy->readonly_paths, MS_BIND | MS_RDONLY) < 0)
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{
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return -1;
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}
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if(mount_to_chroot(policy->chroot_target_path, policy->writable_paths, MS_BIND) < 0)
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{
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return -1;
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}
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if(chroot(policy->chroot_target_path) < 0)
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{
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QSSB_LOG_ERROR("chroot: failed to enter %s\n", policy->chroot_target_path);
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return -1;
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}
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}
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if(chdir(policy->chdir_path) < 0)
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{
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return -1;
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}
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if(policy->drop_caps)
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{
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drop_caps();
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}
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if(policy->not_dumpable)
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{
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if(prctl(PR_SET_DUMPABLE, 0) == -1)
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{
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QSSB_LOG_ERROR("prctl: PR_SET_DUMPABLE failed\n");
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return -1;
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}
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}
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if(policy->no_new_privs)
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{
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if(prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) == -1)
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{
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QSSB_LOG_ERROR("prctl: PR_SET_NO_NEW_PRIVS failed: %s\n", strerror(errno));
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return -1;
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}
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}
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if(policy->allowed_syscalls != NULL)
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{
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if(seccomp_enable_whitelist(policy->allowed_syscalls) <0)
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{
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QSSB_LOG_ERROR("seccomp_enable_whitelist failed\n");
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return -1;
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}
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}
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if(policy->blacklisted_syscalls != NULL)
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{
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if(seccomp_enable_blacklist(policy->blacklisted_syscalls) <0)
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{
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QSSB_LOG_ERROR("seccomp_enable_blacklist failed\n");
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return -1;
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}
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}
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return 0;
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}
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#endif
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