Re: [PATCH 1/2] fs/sysctl: fix potential page fault while unregistering sysctl table

[Danilo Krummrich]

On 2018-02-28 00:02, Kees Cook wrote:
> On Tue, Feb 27, 2018 at 2:43 PM, Danilo Krummrich
> <danilokrummrich@xxxxxxxxxxxxx> wrote:
> > proc_sys_link_fill_cache() does not take currently unregistering
> > sysctl tables into account, which might result into a page fault in
> > sysctl_follow_link() - add a check to fix it.
> >
> > Signed-off-by: Danilo Krummrich <danilokrummrich@xxxxxxxxxxxxx>
> > ---
> >  fs/proc/proc_sysctl.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> > index c5cbbdff3c3d..a0b6c647835e 100644
> > --- a/fs/proc/proc_sysctl.c
> > +++ b/fs/proc/proc_sysctl.c
> > @@ -709,6 +709,9 @@ static bool proc_sys_link_fill_cache(struct file 
> > *file,
> >         bool ret = true;
>
> Nothing appears to actually change "ret" in this function. It should
> likely be dropped too.
proc_sys_fill_cache() potentially changes "ret".
> >         head = sysctl_head_grab(head);
> >
> > +       if (IS_ERR(head))
> > +               return false;
> > +
>
> This looks sensible. I'd drop the blank line between sysctl_head_grab
> and the IS_ERR, though.
I'll do that.
> How are you testing this change?
Honestly, not at all. Actually, I never run in such a page fault.
I spotted it by accident while reading the code.
> Thanks!
>
> -Kees
>
> >         if (S_ISLNK(table->mode)) {
> >                 /* It is not an error if we can not follow the link 
> > ignore it */
> >                 int err = sysctl_follow_link(&head, &table);
> > --
> > 2.14.1