On 03.08.2018 13:59, Kirill Tkhai wrote:
> 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.
patch *fixes* the problem
>
>> ---
>> 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;
>> }
>>
>>
