Re: [PATCH] fork: fix incorrect fput of ->exe_file causing use-after-free

[Oleg Nesterov]

On 08/24, Eric Biggers wrote:
>
> On Thu, Aug 24, 2017 at 03:20:41PM +0200, Oleg Nesterov wrote:
> > On 08/23, Eric Biggers wrote:
> > >
> > > From: Eric Biggers <ebiggers@xxxxxxxxxx>
> > >
> > > Commit 7c051267931a ("mm, fork: make dup_mmap wait for mmap_sem for
> > > write killable") made it possible to kill a forking task while it is
> > > waiting to acquire its ->mmap_sem for write, in dup_mmap().  However, it
> > > was overlooked that this introduced an new error path before a reference
> > > is taken on the mm_struct's ->exe_file.
> >
> > Hmm. Unless I am totally confused, the same problem with mm->exol_area?
> > I'll recheck....
>
> I'm not sure what you mean by ->exol_area.

I meant mm->uprobes_state.xol_area, sorry

> > > --- a/kernel/fork.c
> > > +++ b/kernel/fork.c
> > > @@ -806,6 +806,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
> > >  	mm_init_cpumask(mm);
> > >  	mm_init_aio(mm);
> > >  	mm_init_owner(mm, p);
> > > +	RCU_INIT_POINTER(mm->exe_file, NULL);
> > 
> > Can't we simply move
> > 
> > 	RCU_INIT_POINTER(mm->exe_file, get_mm_exe_file(oldmm));
> > 
> > from dup_mmap() here? Afaics this doesn't need mmap_sem.
> > 
> 
> Two problems, even assuming that get_mm_exe_file() doesn't require mmap_sem:
> 
> - If mm_alloc_pgd() or init_new_context() in mm_init() fails, mm_init() doesn't
>   do the full mmput(), so the file reference would not be dropped.  So it would
>   need to be changed to drop the file reference too.

Ah yes, I forgot that mm_init() can fail after that, thanks for correcting me.

Oleg.