Re: [PATCH v2 1/2] mm: Add an F_SEAL_FS_WRITE seal to memfd

From: Christoph Hellwig
Date: Wed Oct 17 2018 - 08:08:33 EST

On Wed, Oct 17, 2018 at 03:39:58AM -0700, Joel Fernandes wrote:
> > > This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
> > > To support the usecase, this patch adds a new F_SEAL_FS_WRITE seal which
> > > prevents any future mmap and write syscalls from succeeding while
> > > keeping the existing mmap active. The following program shows the seal
> > > working in action:
> >
> > Where does the FS come from? I'd rather expect this to be implemented
> > as a 'force' style flag that applies the seal even if the otherwise
> > required precondition is not met.
> The "FS" was meant to convey that the seal is preventing writes at the VFS
> layer itself, for example vfs_write checks FMODE_WRITE and does not proceed,
> it instead returns an error if the flag is not set. I could not find a better
> name for it, I could call it F_SEAL_VFS_WRITE if you prefer?

I don't think there is anything VFS or FS about that - at best that
is an implementation detail.

Either do something like the force flag I suggested in the last mail,
or give it a name that matches the intention, e.g F_SEAL_FUTURE_WRITE.

> I could make it such that this seal would not be allowed unless F_SEAL_SHRINK
> and F_SEAL_GROW are either previously set, or they are passed along with this
> seal. Would that make more sense to you?


> > > static int memfd_add_seals(struct file *file, unsigned int seals)
> > > {
> > > @@ -219,6 +220,9 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
> > > }
> > > }
> > >
> > > + if ((seals & F_SEAL_FS_WRITE) && !(*file_seals & F_SEAL_FS_WRITE))
> > > + file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
> > > +
> >
> > This seems to lack any synchronization for f_mode.
> The f_mode is set when the struct file is first created and then memfd sets
> additional flags in memfd_create. Then later we are changing it here at the
> time of setting the seal. I donot see any possiblity of a race since it is
> impossible to set the seal before memfd_create returns. Could you provide
> more details about what kind of synchronization is needed and what is the
> race condition scenario you were thinking off?

Even if no one changes these specific flags we still need a lock due
to rmw cycles on the field. For example fadvise can set or clear
FMODE_RANDOM. It seems to use file->f_lock for synchronization.