Re: [PATCH 31/34] vfs: syscall: Add fspick() to select a superblock for reconfiguration [ver #12]

[Alan Jenkins]

On 13/10/2018 07:11, Al Viro wrote:
> On Fri, Oct 12, 2018 at 03:49:50PM +0100, Alan Jenkins wrote:
> > > +SYSCALL_DEFINE3(fspick, int, dfd, const char __user *, path, unsigned int, flags)
> > > +{
> > > +	struct fs_context *fc;
> > > +	struct path target;
> > > +	unsigned int lookup_flags;
> > > +	int ret;
> > > +
> > > +	if (!ns_capable(current->nsproxy->mnt_ns->user_ns, CAP_SYS_ADMIN))
> > > +		return -EPERM;
> >
> > This seems to accept basically any mount. Specifically: are you sure it's
> > OK to return a handle to a SB_NO_USER superblock?
> Umm...  As long as we don't try to do pathname resolution from its ->s_root,
> shouldn't be a problem and I don't see anything that would do that.  I might've
> missed something, but...

Sorry, I guess SB_NOUSER was the wrong word. I was trying find if 
anything stopped things like

int memfd = memfd_create("foo", 0);
int fsfd = fspick(memfd, "", FSPICK_EMPTY_PATH);

fsconfig(fsfd, FSCONFIG_SET_FLAG, "ro", NULL, 0);
fsconfig(fsfd, FSCONFIG_SET_STRING, "size", "100M", 0);
fsconfig(fsfd, FSCONFIG_CMD_RECONFIGURE, NULL, NULL, 0);

So far I'm getting -EBUSY if I try to apply the "ro", -EINVAL if I try 
to apply the "size=100M". But if I don't apply either, then 
FSCONFIG_CMD_RECONFIGURE succeeds.

It seems worrying that it might let me set options on shm_mnt. Or at 
least letting me get as far as the -EBUSY check for the "ro" superblock 
flag.

I'm not sure why I'm getting the -EINVAL setting the "size" option. But 
it would be much more reassuring if I was getting -EPERM :-).

Alan