On 03.08.2018 13:31, David Howells wrote:
> The reproducer can be reduced to:
>
> #define _GNU_SOURCE
> #include <endian.h>
> #include <stdint.h>
> #include <string.h>
> #include <stdio.h>
> #include <sys/syscall.h>
> #include <sys/stat.h>
> #include <sys/mount.h>
> #include <unistd.h>
> #include <fcntl.h>
>
> const char path[] = "./file0";
>
> int main()
> {
> mkdir(path, 0);
> mount(path, path, "cgroup2", 0, 0);
> chroot(path);
> umount2(path, 0);
> return 0;
> }
>
> and I've found two bugs (see attached patch). The issue is that
> do_remount_sb() is called with fc == NULL from umount(), but both
> cgroup_reconfigure() and do_remount_sb() dereference fc unconditionally.
>
> But! I'm not sure why the reproducer works at all because the umount2() call
> is *after* the chroot, so should fail on ENOENT before it even gets that far.
> In fact, umount2() can be called multiple times, apparently successfully, and
> doesn't actually unmount anything.
Before I also try to check why it works; just reporting you that the patch
works the problem in my environment. Thanks, David.
> ---
> diff --git a/fs/super.c b/fs/super.c
> index 3fe5d12b7697..321fbc244570 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -978,7 +978,10 @@ int do_remount_sb(struct super_block *sb, int sb_flags, void *data,
> sb->s_op->remount_fs) {
> if (sb->s_op->reconfigure) {
> retval = sb->s_op->reconfigure(sb, fc);
> - sb_flags = fc->sb_flags;
> + if (fc)
> + sb_flags = fc->sb_flags;
> + else
> + sb_flags = sb->s_flags;
> if (retval == 0)
> security_sb_reconfigure(fc);
> } else {
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index f3238f38d152..48275fdce053 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -1796,9 +1796,11 @@ static void apply_cgroup_root_flags(unsigned int root_flags)
>
> static int cgroup_reconfigure(struct kernfs_root *kf_root, struct fs_context *fc)
> {
> - struct cgroup_fs_context *ctx = cgroup_fc2context(fc);
> + if (fc) {
> + struct cgroup_fs_context *ctx = cgroup_fc2context(fc);
>
> - apply_cgroup_root_flags(ctx->flags);
> + apply_cgroup_root_flags(ctx->flags);
> + }
> return 0;
> }
>
>
