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

[Mark Rutland]

On Thu, Aug 24, 2017 at 04:02:49PM +0100, Mark Rutland wrote:
> On Wed, Aug 23, 2017 at 02:14:08PM -0700, 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.  Since the ->exe_file of the new
> > mm_struct was already set to the old ->exe_file by the memcpy() in
> > dup_mm(), it was possible for the mmput() in the error path of dup_mm()
> > to drop a reference to ->exe_file which was never taken.  This caused
> > the struct file to later be freed prematurely.
> > 
> > Fix it by updating mm_init() to NULL out the ->exe_file, in the same
> > place it clears other things like the list of mmaps.
> 
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index e075b7780421..cbbea277b3fb 100644
> > --- 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);
> >  	mmu_notifier_mm_init(mm);
> >  	init_tlb_flush_pending(mm);
> >  #if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !USE_SPLIT_PMD_PTLOCKS
> 
> I've been seeing similar issues on arm64 with use-after-free of a file
> and other memory corruption [1].
> 
> This patch seems to fix that; a test that normally fired in a few
> minutes has been happily running for hours with this applied.

Those haven't triggered after 24 hours, and in 16+ hours of fuzzing with
this applied, I haven't seen new issues. FWIW:

Tested-by: Mark Rutland <mark.rutland@xxxxxxx>

Thanks,
Mark.