Re: [Xen-devel] [PATCH v5.1 7/8] os-posix: Provide new -runas <uid>:<gid> facility

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

CC'ing the maintainers (scripts/get_maintainer.pl is your friend)

On Fri, 20 Oct 2017, Ian Jackson wrote:
> This allows the caller to specify a uid and gid to use, even if there
> is no corresponding password entry.  This will be useful in certain
> Xen configurations.
> 
> We don't support just -runas <uid> because: (i) deprivileging without
> calling setgroups would be ineffective (ii) given only a uid we don't
> know what gid we ought to use (since uids may eppear in multiple
> passwd file entries with different gids).
> 
> Signed-off-by: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
> ---
> v5: Use : rather than . to separate uid from gid
> v4: Changed to reuse option -runas
> v3: Error messages fixed.  Thanks to Peter Maydell and Ross Lagerwall.
> v2: Coding style fixes.
> 
> squash! os-posix: Provide new -runas <uid>.<gid> facility
> ---
>  os-posix.c      | 64 
> +++++++++++++++++++++++++++++++++++++++++++++++----------
>  qemu-options.hx |  3 ++-
>  2 files changed, 55 insertions(+), 12 deletions(-)
> 
> diff --git a/os-posix.c b/os-posix.c
> index 92e9d85..f95b7bf 100644
> --- a/os-posix.c
> +++ b/os-posix.c
> @@ -43,6 +43,8 @@
>  #endif
>  
>  static struct passwd *user_pwd;
> +static uid_t user_uid = (uid_t)-1;
> +static gid_t user_gid = (gid_t)-1;
>  static const char *chroot_dir;
>  static int daemonize;
>  static int daemon_pipe;
> @@ -128,6 +130,34 @@ void os_set_proc_name(const char *s)
>  #endif
>  }
>  
> +
> +static bool os_parse_runas_uid_gid(const char *optarg)
> +{
> +    unsigned long lv;
> +    char *ep;
> +    uid_t got_uid;
> +    gid_t got_gid;
> +    int rc;
> +
> +    errno = 0;
> +    lv = strtoul(optarg, &ep, 0); /* can't qemu_strtoul, want *ep==':' */
> +    got_uid = lv; /* overflow here is ID in C99 */
> +    if (errno || *ep != ':' || got_uid != lv || got_uid == (uid_t)-1) {
> +        return false;
> +    }
> +
> +    lv = 0;
> +    rc = qemu_strtoul(ep + 1, 0, 0, &lv);
> +    got_gid = lv; /* overflow here is ID in C99 */
> +    if (rc || got_gid != lv || got_gid == (gid_t)-1) {
> +        return false;
> +    }
> +
> +    user_uid = got_uid;
> +    user_gid = got_gid;
> +    return true;
> +}
> +
>  /*
>   * Parse OS specific command line options.
>   * return 0 if option handled, -1 otherwise
> @@ -145,8 +175,10 @@ void os_parse_cmd_args(int index, const char *optarg)
>  #endif
>      case QEMU_OPTION_runas:
>          user_pwd = getpwnam(optarg);
> -        if (!user_pwd) {
> -            fprintf(stderr, "User \"%s\" doesn't exist\n", optarg);
> +        if (!user_pwd && !os_parse_runas_uid_gid(optarg)) {
> +            fprintf(stderr,
> +                    "User \"%s\" doesn't exist (and is not <uid>.<gid>)\n",
> +                    optarg);
>              exit(1);
>          }
>          break;
> @@ -166,18 +198,28 @@ void os_parse_cmd_args(int index, const char *optarg)
>  
>  static void change_process_uid(void)
>  {
> -    if (user_pwd) {
> -        if (setgid(user_pwd->pw_gid) < 0) {
> -            fprintf(stderr, "Failed to setgid(%d)\n", user_pwd->pw_gid);
> +    if (user_pwd || user_uid != (uid_t)-1) {
> +        gid_t intended_gid = user_pwd ? user_pwd->pw_gid : user_gid;
> +        uid_t intended_uid = user_pwd ? user_pwd->pw_uid : user_uid;
> +        if (setgid(intended_gid) < 0) {
> +            fprintf(stderr, "Failed to setgid(%d)\n", intended_gid);
>              exit(1);
>          }
> -        if (initgroups(user_pwd->pw_name, user_pwd->pw_gid) < 0) {
> -            fprintf(stderr, "Failed to initgroups(\"%s\", %d)\n",
> -                    user_pwd->pw_name, user_pwd->pw_gid);
> -            exit(1);
> +        if (user_pwd) {
> +            if (initgroups(user_pwd->pw_name, user_pwd->pw_gid) < 0) {
> +                fprintf(stderr, "Failed to initgroups(\"%s\", %d)\n",
> +                        user_pwd->pw_name, user_pwd->pw_gid);
> +                exit(1);
> +            }
> +        } else {
> +            if (setgroups(1, &user_gid) < 0) {
> +                fprintf(stderr, "Failed to setgroups(1, [%d])",
> +                        user_gid);
> +                exit(1);
> +            }
>          }
> -        if (setuid(user_pwd->pw_uid) < 0) {
> -            fprintf(stderr, "Failed to setuid(%d)\n", user_pwd->pw_uid);
> +        if (setuid(intended_uid) < 0) {
> +            fprintf(stderr, "Failed to setuid(%d)\n", intended_uid);
>              exit(1);
>          }
>          if (setuid(0) != -1) {
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 9f6e2ad..f707197e 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -3958,7 +3958,8 @@ ETEXI
>  
>  #ifndef _WIN32
>  DEF("runas", HAS_ARG, QEMU_OPTION_runas, \
> -    "-runas user     change to user id user just before starting the VM\n",
> +    "-runas user     change to user id user just before starting the VM\n" \
> +    "                user can be numeric uid:gid instead\n",
>      QEMU_ARCH_ALL)
>  #endif
>  STEXI
> -- 
> 2.1.4
> 

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