Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd

[Fuad Tabba]

Hi

On Fri, Sep 30, 2022 at 5:23 PM Kirill A . Shutemov
<kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
>
> On Fri, Sep 30, 2022 at 05:14:00PM +0100, Fuad Tabba wrote:
> > Hi,
> >
> > <...>
> >
> > > diff --git a/mm/memfd_inaccessible.c b/mm/memfd_inaccessible.c
> > > new file mode 100644
> > > index 000000000000..2d33cbdd9282
> > > --- /dev/null
> > > +++ b/mm/memfd_inaccessible.c
> > > @@ -0,0 +1,219 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > > +#include "linux/sbitmap.h"
> > > +#include <linux/memfd.h>
> > > +#include <linux/pagemap.h>
> > > +#include <linux/pseudo_fs.h>
> > > +#include <linux/shmem_fs.h>
> > > +#include <uapi/linux/falloc.h>
> > > +#include <uapi/linux/magic.h>
> > > +
> > > +struct inaccessible_data {
> > > +       struct mutex lock;
> > > +       struct file *memfd;
> > > +       struct list_head notifiers;
> > > +};
> > > +
> > > +static void inaccessible_notifier_invalidate(struct inaccessible_data *data,
> > > +                                pgoff_t start, pgoff_t end)
> > > +{
> > > +       struct inaccessible_notifier *notifier;
> > > +
> > > +       mutex_lock(&data->lock);
> > > +       list_for_each_entry(notifier, &data->notifiers, list) {
> > > +               notifier->ops->invalidate(notifier, start, end);
> > > +       }
> > > +       mutex_unlock(&data->lock);
> > > +}
> > > +
> > > +static int inaccessible_release(struct inode *inode, struct file *file)
> > > +{
> > > +       struct inaccessible_data *data = inode->i_mapping->private_data;
> > > +
> > > +       fput(data->memfd);
> > > +       kfree(data);
> > > +       return 0;
> > > +}
> > > +
> > > +static long inaccessible_fallocate(struct file *file, int mode,
> > > +                                  loff_t offset, loff_t len)
> > > +{
> > > +       struct inaccessible_data *data = file->f_mapping->private_data;
> > > +       struct file *memfd = data->memfd;
> > > +       int ret;
> > > +
> > > +       if (mode & FALLOC_FL_PUNCH_HOLE) {
> > > +               if (!PAGE_ALIGNED(offset) || !PAGE_ALIGNED(len))
> > > +                       return -EINVAL;
> > > +       }
> > > +
> > > +       ret = memfd->f_op->fallocate(memfd, mode, offset, len);
> >
> > I think that shmem_file_operations.fallocate is only set if
> > CONFIG_TMPFS is enabled (shmem.c). Should there be a check at
> > initialization that fallocate is set, or maybe a config dependency, or
> > can we count on it always being enabled?
>
> It is already there:
>
>         config MEMFD_CREATE
>                 def_bool TMPFS || HUGETLBFS
>
> And we reject inaccessible memfd_create() for HUGETLBFS.
>
> But if we go with a separate syscall, yes, we need the dependency.

I missed that, thanks.

>
> > > +       inaccessible_notifier_invalidate(data, offset, offset + len);
> > > +       return ret;
> > > +}
> > > +
> >
> > <...>
> >
> > > +void inaccessible_register_notifier(struct file *file,
> > > +                                   struct inaccessible_notifier *notifier)
> > > +{
> > > +       struct inaccessible_data *data = file->f_mapping->private_data;
> > > +
> > > +       mutex_lock(&data->lock);
> > > +       list_add(&notifier->list, &data->notifiers);
> > > +       mutex_unlock(&data->lock);
> > > +}
> > > +EXPORT_SYMBOL_GPL(inaccessible_register_notifier);
> >
> > If the memfd wasn't marked as inaccessible, or more generally
> > speaking, if the file isn't a memfd_inaccessible file, this ends up
> > accessing an uninitialized pointer for the notifier list. Should there
> > be a check for that here, and have this function return an error if
> > that's not the case?
>
> I think it is "don't do that" category. inaccessible_register_notifier()
> caller has to know what file it operates on, no?

The thing is, you could oops the kernel from userspace. For that, all
you have to do is a memfd_create without the MFD_INACCESSIBLE,
followed by a KVM_SET_USER_MEMORY_REGION using that as the private_fd.
I ran into this using my port of this patch series to arm64.

Cheers,
/fuad


> --
>   Kiryl Shutsemau / Kirill A. Shutemov