On Tue, Apr 12, 2022 at 07:33:43PM +0800, Yang Xu wrote:
> Since mknodat can create file, we should also check whether strip S_ISGID.
> Also add new helper caps_down_fsetid to drop CAP_FSETID because strip S_ISGID
> depend on this cap and keep other cap(ie CAP_MKNOD) because create character
> device needs it when using mknod.
>
> Only test mknodat with character device in setgid_create function and the another
> two functions test mknodat with whiteout device.
>
> Since kernel commit a3c751a50 ("vfs: allow unprivileged whiteout creation") in
> v5.8-rc1, we can create whiteout device in userns test. Since kernel 5.12, mount_setattr
> and MOUNT_ATTR_IDMAP was supported, we don't need to detect kernel whether allow
> unprivileged whiteout creation. Using fs_allow_idmap as a proxy is safe.
>
> Tested-by: Christian Brauner (Microsoft)<brauner@xxxxxxxxxx>
> Reviewed-by: Christian Brauner (Microsoft)<brauner@xxxxxxxxxx>
> Signed-off-by: Yang Xu <xuyang2018.jy@xxxxxxxxxxx>
> ---
> src/idmapped-mounts/idmapped-mounts.c | 219 +++++++++++++++++++++++++-
> 1 file changed, 213 insertions(+), 6 deletions(-)
>
> diff --git a/src/idmapped-mounts/idmapped-mounts.c b/src/idmapped-mounts/idmapped-mounts.c
> index 8e6405c5..617f56e0 100644
> --- a/src/idmapped-mounts/idmapped-mounts.c
> +++ b/src/idmapped-mounts/idmapped-mounts.c
> @@ -241,6 +241,34 @@ static inline bool caps_supported(void)
> return ret;
> }
>
> +static int caps_down_fsetid(void)
> +{
> + bool fret = false;
> +#ifdef HAVE_SYS_CAPABILITY_H
> + cap_t caps = NULL;
> + cap_value_t cap = CAP_FSETID;
> + int ret = -1;
> +
> + caps = cap_get_proc();
> + if (!caps)
> + goto out;
> +
> + ret = cap_set_flag(caps, CAP_EFFECTIVE, 1, &cap, 0);
> + if (ret)
> + goto out;
> +
> + ret = cap_set_proc(caps);
> + if (ret)
> + goto out;
> +
> + fret = true;
> +
> +out:
> + cap_free(caps);
> +#endif
> + return fret;
> +}
> +
> /* caps_down - lower all effective caps */
> static int caps_down(void)
> {
> @@ -7805,8 +7833,8 @@ out_unmap:
> #endif /* HAVE_LIBURING_H */
>
> /* The following tests are concerned with setgid inheritance. These can be
> - * filesystem type specific. For xfs, if a new file or directory is created
> - * within a setgid directory and irix_sgid_inhiert is set then inherit the
> + * filesystem type specific. For xfs, if a new file or directory or node is
> + * created within a setgid directory and irix_sgid_inhiert is set then inherit the
> * setgid bit if the caller is in the group of the directory.
> */
> static int setgid_create(void)
> @@ -7863,18 +7891,44 @@ static int setgid_create(void)
> if (!is_setgid(t_dir1_fd, DIR1, 0))
> die("failure: is_setgid");
>
> + /* create a special file via mknodat() vfs_create */
> + if (mknodat(t_dir1_fd, FILE2, S_IFREG | S_ISGID | S_IXGRP, 0))
> + die("failure: mknodat");
> +
> + if (!is_setgid(t_dir1_fd, FILE2, 0))
> + die("failure: is_setgid");
> +
> + /* create a character device via mknodat() vfs_mknod */
> + if (mknodat(t_dir1_fd, CHRDEV1, S_IFCHR | S_ISGID | S_IXGRP, makedev(5, 1)))
> + die("failure: mknodat");
> +
> + if (!is_setgid(t_dir1_fd, CHRDEV1, 0))
> + die("failure: is_setgid");
> +
> if (!expected_uid_gid(t_dir1_fd, FILE1, 0, 0, 0))
> die("failure: check ownership");
>
> if (!expected_uid_gid(t_dir1_fd, DIR1, 0, 0, 0))
> die("failure: check ownership");
>
> + if (!expected_uid_gid(t_dir1_fd, FILE2, 0, 0, 0))
> + die("failure: check ownership");
> +
> + if (!expected_uid_gid(t_dir1_fd, CHRDEV1, 0, 0, 0))
> + die("failure: check ownership");
> +
> if (unlinkat(t_dir1_fd, FILE1, 0))
> die("failure: delete");
>
> if (unlinkat(t_dir1_fd, DIR1, AT_REMOVEDIR))
> die("failure: delete");
>
> + if (unlinkat(t_dir1_fd, FILE2, 0))
> + die("failure: delete");
> +
> + if (unlinkat(t_dir1_fd, CHRDEV1, 0))
> + die("failure: delete");
> +
> exit(EXIT_SUCCESS);
> }
> if (wait_for_pid(pid))
> @@ -7889,8 +7943,8 @@ static int setgid_create(void)
> if (!switch_ids(0, 10000))
> die("failure: switch_ids");
>
> - if (!caps_down())
> - die("failure: caps_down");
> + if (!caps_down_fsetid())
> + die("failure: caps_down_fsetid");
>
> /* create regular file via open() */
> file1_fd = openat(t_dir1_fd, FILE1, O_CREAT | O_EXCL | O_CLOEXEC, S_IXGRP | S_ISGID);
> @@ -7917,6 +7971,19 @@ static int setgid_create(void)
> die("failure: is_setgid");
> }
Please see my comment on the earlier thread:
https://lore.kernel.org/linux-fsdevel/20220407134009.s4shhomfxjz5cf5r@wittgenstein
The same issue still exists afaict, i.e. you're not handling the irix
case.
