Re: [REVIEW][PATCH 1/4] vfs: Don't allow overwriting mounts in the current mount namespace

[ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman)]

Al Viro <viro-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org> writes:

> On Tue, Oct 15, 2013 at 01:16:48PM -0700, Eric W. Biederman wrote:
>
>>  int vfs_rmdir(struct inode *dir, struct dentry *dentry)
>>  {
>>  	int error = may_delete(dir, dentry, 1);
>> @@ -3622,6 +3636,9 @@ retry:
>>  		error = -ENOENT;
>>  		goto exit3;
>>  	}
>> +	error = -EBUSY;
>> +	if (covered(nd.path.mnt, dentry))
>> +		goto exit3;
>
> Ugh...  And it's not racy because of...?  IOW, what's to keep the return
> value of covered() from getting obsolete just as it's being calculated,
> let alone returned?

I have been fighting a cold off and on so I have been taking much longer
to dig through all of these issues than I would like.

Aftering having thought through all of the issues I completely agree
that this is a racy bug that needs to be fixed.

The fix needs to be holding i_mutex of the parent directory in do_mount,
and pivot_root.

We need to hold i_mutex in do_mount and pivot_root not because of this
issue but to prevent mount points being renamed before we mount on them.

With todays kernel because of races between when we lookup a mount point
and when we take locks, and which locks we take.  When mount(2) returns
the mount point can be located anywhere.  Which completely defeats
returning -EBUSY mount points from a userspace semantics perspective.

I was really hoping I could think through this and say that this was a
trivial issue that would allow my patches to be good for 3.13.  But that
is clearly not the case.  kern_path_locked(...,LOOKUP_FOLLOW,...) is
non-trivial to implement, and there are issues like having to move
get_fs_type before we take any locks to prevent deadlocks.

I almost have the issues worked through, so hopefully I can send a
rebased set of patches in a few days.

Eric