Re: [syzbot] [mm?] general protection fault in find_mergeable_anon_vma

[Lorenzo Stoakes]

On Tue, Dec 10, 2024 at 04:05:28PM +0100, Oleg Nesterov wrote:
> On 12/09, Lorenzo Stoakes wrote:
> >
> > (As discussed on IRC) how about moving up the dup_mmap_sem lock one level, we
> > can put the mm before the rmap lookup in build_map_info() is able to find it,
> > which should avoid the whole issue?
>
> Not sure I fully understand the problem, but so far I see nothing wrong in
> this idea. However,
>
> > @@ -1692,9 +1690,11 @@ static struct mm_struct *dup_mm(struct task_struct *tsk,
> >  	if (!mm_init(mm, tsk, mm->user_ns))
> >  		goto fail_nomem;
> >
> > +	uprobe_start_dup_mmap();
> >  	err = dup_mmap(mm, oldmm);
> >  	if (err)
> >  		goto free_pt;
> > +	uprobe_end_dup_mmap();
>
> If try_module_get(mm->binfmt->module)) fails after that, dup_mm() does
> "goto free_pt;" and in this case ...
>
> > @@ -1709,6 +1709,7 @@ static struct mm_struct *dup_mm(struct task_struct *tsk,
> >  	mm->binfmt = NULL;
> >  	mm_init_owner(mm, NULL);
> >  	mmput(mm);
> > +	uprobe_end_dup_mmap();
>
> ... we have the unbalanced uprobe_end_dup_mmap().

Right yeah we can obviously fix that, good spot though.

>
> Also. Perhaps we can change dup_mmap() to set MMF_XXX before uprobe_end_dup_mmap(),
>
> 	fail_uprobe_end:
> +		if (retval)
> +			set_bit(mm->flags, MMF_XXX);
> 		uprobe_end_dup_mmap();
> 		return retval;
>
> Then build_map_info() can check this flag. I guess we can reuse some of
> MMF_OOM_ bits? May be MMF_UNSTABLE...

Well this was my initial suggestion :) There is some concern as to how this will
interact with OOM however.

But I think my original suggestion, modified to fix the issue you rightly raise,
is a good proximate solution to keep thing simple.

>
> Oleg.
>