[kernel-hardening] Re: [PATCH] exec: Account for argv/envp pointers

[Michal Hocko <mhocko@kernel.org>]

On Fri 23-06-17 07:05:37, Kees Cook wrote:
> On Fri, Jun 23, 2017 at 6:59 AM, Michal Hocko <mhocko@kernel.org> wrote:
[...]
> >> --- a/fs/exec.c
> >> +++ b/fs/exec.c
> >> @@ -220,8 +220,18 @@ static struct page *get_arg_page(struct linux_binprm *bprm, unsigned long pos,
> >>
> >>       if (write) {
> >>               unsigned long size = bprm->vma->vm_end - bprm->vma->vm_start;
> >> +             unsigned long ptr_size;
> >>               struct rlimit *rlim;
> >>
> >> +             /*
> >> +              * Since the stack will hold pointers to the strings, we
> >> +              * must account for them as well.
> >> +              */
> >> +             ptr_size = (bprm->argc + bprm->envc) * sizeof(void *);
> >> +             if (ptr_size > ULONG_MAX - size)
> >> +                     goto fail;
> >> +             size += ptr_size;
> >> +
> >>               acct_arg_size(bprm, size / PAGE_SIZE);
> >
> > Doesn't this over account? I mean this gets called for partial arguments
> > as they fit into a page so a single argument can get into this function
> > multiple times AFAIU. I also do not understand why would you want to
> > account bprm->argc + bprm->envc pointers for each argument.
> 
> Based on what I could understand in acct_arg_size(), this is called
> repeatedly with with the "current" size (it handles the difference
> between prior calls, see calls like acct_arg_size(bprm, 0)).
> 
> The size calculation is the entire vma while each arg page is built,
> so each time we get here it's calculating how far it is currently
> (rather than each call being just the newly added size from the arg
> page). As a result, we need to always add the entire size of the
> pointers, so that on the last call to get_arg_page() we'll actually
> have the entire correct size.

Ohh, I forgot about this tricky part. The code just looks confusing
becauser we are mixing 2 things together here. This deserves a comment I
guess.

Other than that feel free to add
Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!
-- 
Michal Hocko
SUSE Labs