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virtiofsd: sandbox mount namespace

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Use a mount namespace with the shared directory tree mounted at "/" and
no other mounts.

This prevents symlink escape attacks because symlink targets are
resolved only against the shared directory and cannot go outside it.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Peng Tao <tao.peng@linux.alibaba.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2019-03-12 15:51:38 +00:00 committed by Dr. David Alan Gilbert
parent 9f59d175e2
commit 5baa3b8e95
1 changed files with 89 additions and 0 deletions
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89
tools/virtiofsd/passthrough_ll.c
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@ -50,6 +50,7 @@
#include <stdlib.h> #include <stdlib.h>
#include <string.h> #include <string.h>
#include <sys/file.h> #include <sys/file.h>
#include <sys/mount.h>
#include <sys/syscall.h> #include <sys/syscall.h>
#include <sys/xattr.h> #include <sys/xattr.h>
#include <unistd.h> #include <unistd.h>
@ -1943,6 +1944,58 @@ static void print_capabilities(void)
printf("}\n"); printf("}\n");
} }
/* This magic is based on lxc's lxc_pivot_root() */
static void setup_pivot_root(const char *source)
{
int oldroot;
int newroot;
oldroot = open("/", O_DIRECTORY | O_RDONLY | O_CLOEXEC);
if (oldroot < 0) {
fuse_log(FUSE_LOG_ERR, "open(/): %m\n");
exit(1);
}
newroot = open(source, O_DIRECTORY | O_RDONLY | O_CLOEXEC);
if (newroot < 0) {
fuse_log(FUSE_LOG_ERR, "open(%s): %m\n", source);
exit(1);
}
if (fchdir(newroot) < 0) {
fuse_log(FUSE_LOG_ERR, "fchdir(newroot): %m\n");
exit(1);
}
if (syscall(__NR_pivot_root, ".", ".") < 0) {
fuse_log(FUSE_LOG_ERR, "pivot_root(., .): %m\n");
exit(1);
}
if (fchdir(oldroot) < 0) {
fuse_log(FUSE_LOG_ERR, "fchdir(oldroot): %m\n");
exit(1);
}
if (mount("", ".", "", MS_SLAVE | MS_REC, NULL) < 0) {
fuse_log(FUSE_LOG_ERR, "mount(., MS_SLAVE | MS_REC): %m\n");
exit(1);
}
if (umount2(".", MNT_DETACH) < 0) {
fuse_log(FUSE_LOG_ERR, "umount2(., MNT_DETACH): %m\n");
exit(1);
}
if (fchdir(newroot) < 0) {
fuse_log(FUSE_LOG_ERR, "fchdir(newroot): %m\n");
exit(1);
}
close(newroot);
close(oldroot);
}
static void setup_proc_self_fd(struct lo_data *lo) static void setup_proc_self_fd(struct lo_data *lo)
{ {
lo->proc_self_fd = open("/proc/self/fd", O_PATH); lo->proc_self_fd = open("/proc/self/fd", O_PATH);
@ -1952,6 +2005,39 @@ static void setup_proc_self_fd(struct lo_data *lo)
} }
} }
/*
* Make the source directory our root so symlinks cannot escape and no other
* files are accessible.
*/
static void setup_mount_namespace(const char *source)
{
if (unshare(CLONE_NEWNS) != 0) {
fuse_log(FUSE_LOG_ERR, "unshare(CLONE_NEWNS): %m\n");
exit(1);
}
if (mount(NULL, "/", NULL, MS_REC | MS_SLAVE, NULL) < 0) {
fuse_log(FUSE_LOG_ERR, "mount(/, MS_REC|MS_PRIVATE): %m\n");
exit(1);
}
if (mount(source, source, NULL, MS_BIND, NULL) < 0) {
fuse_log(FUSE_LOG_ERR, "mount(%s, %s, MS_BIND): %m\n", source, source);
exit(1);
}
setup_pivot_root(source);
}
/*
* Lock down this process to prevent access to other processes or files outside
* source directory. This reduces the impact of arbitrary code execution bugs.
*/
static void setup_sandbox(struct lo_data *lo)
{
setup_mount_namespace(lo->source);
}
int main(int argc, char *argv[]) int main(int argc, char *argv[])
{ {
struct fuse_args args = FUSE_ARGS_INIT(argc, argv); struct fuse_args args = FUSE_ARGS_INIT(argc, argv);
@ -2052,6 +2138,7 @@ int main(int argc, char *argv[])
} }
lo.root.fd = open(lo.source, O_PATH); lo.root.fd = open(lo.source, O_PATH);
if (lo.root.fd == -1) { if (lo.root.fd == -1) {
fuse_log(FUSE_LOG_ERR, "open(\"%s\", O_PATH): %m\n", lo.source); fuse_log(FUSE_LOG_ERR, "open(\"%s\", O_PATH): %m\n", lo.source);
exit(1); exit(1);
@ -2075,6 +2162,8 @@ int main(int argc, char *argv[])
/* Must be after daemonize to get the right /proc/self/fd */ /* Must be after daemonize to get the right /proc/self/fd */
setup_proc_self_fd(&lo); setup_proc_self_fd(&lo);
setup_sandbox(&lo);
/* Block until ctrl+c or fusermount -u */ /* Block until ctrl+c or fusermount -u */
ret = virtio_loop(se); ret = virtio_loop(se);