Re: [PATCH] xen/privcmd: fix error exit of privcmd_ioctl_dm_op()

[Juergen Gross <jgross@xxxxxxxx>]

On 25.08.22 09:38, Jan Beulich wrote:
> On 24.08.2022 16:26, Juergen Gross wrote:
> > The error exit of privcmd_ioctl_dm_op() is calling unlock_pages()
> > potentially with pages being NULL, leading to a NULL dereference.
> >
> > Fix that by calling unlock_pages only if lock_pages() was at least
> > partially successful.
> >
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Fixes: ab520be8cd5d ("xen/privcmd: Add IOCTL_PRIVCMD_DM_OP")
> > Reported-by: Rustam Subkhankulov <subkhankulov@xxxxxxxxx>
> > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> albeit I wonder whether you did consider the variant actually
> reducing code size (and avoiding the need for yet another label),
> ...
>
> > --- a/drivers/xen/privcmd.c
> > +++ b/drivers/xen/privcmd.c
> > @@ -679,7 +679,7 @@ static long privcmd_ioctl_dm_op(struct file *file, void 
> > __user *udata)
> >         rc = lock_pages(kbufs, kdata.num, pages, nr_pages, &pinned);
> >         if (rc < 0) {
> >                 nr_pages = pinned;
> ... dropping this line and ...
>
> > -               goto out;
> > +               goto unlock;
> >         }
> >   
> >   	for (i = 0; i < kdata.num; i++) {
> > @@ -691,8 +691,9 @@ static long privcmd_ioctl_dm_op(struct file *file, void 
> > __user *udata)
> >         rc = HYPERVISOR_dm_op(kdata.dom, kdata.num, xbufs);
> >         xen_preemptible_hcall_end();
> >   
> > -out:
> > + unlock:
> >         unlock_pages(pages, nr_pages);
> ... passing "pinned" here.
Looking into this I found another problem: NOT using pinned is wrong, as
lock_pages() doesn't guarantee that all pages were really locked. I think
lock_pages() should return an error, in case pin_user_pages_fast() didn't
lock as many pages es expected.


Juergen

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