Re: [PATCH] binder_alloc: Add missing mmap_lock calls when using the VMA

[Liam Howlett]

* Carlos Llamas <cmllamas@xxxxxxxxxx> [220809 14:49]:
> On Tue, Aug 09, 2022 at 04:06:31PM +0000, Liam Howlett wrote:
> > Take the mmap_read_lock() when using the VMA in
> > binder_alloc_print_pages() and when checking for a VMA in
> > binder_alloc_new_buf_locked().
> > 
> > It is worth noting binder_alloc_new_buf_locked() drops the VMA read lock
> > after it verifies a VMA exists, but may be taken again deeper in the
> > call stack, if necessary.
> > 
> > Reported-by: Ondrej Mosnacek <omosnace@xxxxxxxxxx>
> > Reported-by: syzbot+a7b60a176ec13cafb793@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Fixes: a43cfc87caaf (android: binder: stop saving a pointer to the VMA)
> > Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> > ---
> >  drivers/android/binder_alloc.c | 29 +++++++++++++++++++----------
> >  1 file changed, 19 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
> > index f555eebceef6..c23cad7fe313 100644
> > --- a/drivers/android/binder_alloc.c
> > +++ b/drivers/android/binder_alloc.c
> > @@ -395,12 +395,15 @@ static struct binder_buffer *binder_alloc_new_buf_locked(
> >  	size_t size, data_offsets_size;
> >  	int ret;
> >  
> > +	mmap_read_lock(alloc->vma_vm_mm);
> >  	if (!binder_alloc_get_vma(alloc)) {
> > +		mmap_read_unlock(alloc->vma_vm_mm);
> >  		binder_alloc_debug(BINDER_DEBUG_USER_ERROR,
> >  				   "%d: binder_alloc_buf, no vma\n",
> >  				   alloc->pid);
> >  		return ERR_PTR(-ESRCH);
> >  	}
> > +	mmap_read_unlock(alloc->vma_vm_mm);
> >  
> >  	data_offsets_size = ALIGN(data_size, sizeof(void *)) +
> >  		ALIGN(offsets_size, sizeof(void *));
> > @@ -922,17 +925,23 @@ void binder_alloc_print_pages(struct seq_file *m,
> >  	 * Make sure the binder_alloc is fully initialized, otherwise we might
> >  	 * read inconsistent state.
> >  	 */
> > -	if (binder_alloc_get_vma(alloc) != NULL) {
> > -		for (i = 0; i < alloc->buffer_size / PAGE_SIZE; i++) {
> > -			page = &alloc->pages[i];
> > -			if (!page->page_ptr)
> > -				free++;
> > -			else if (list_empty(&page->lru))
> > -				active++;
> > -			else
> > -				lru++;
> > -		}
> > +
> > +	mmap_read_lock(alloc->vma_vm_mm);
> > +	if (binder_alloc_get_vma(alloc) == NULL)
> > +		goto uninitialized;
> > +
> > +	for (i = 0; i < alloc->buffer_size / PAGE_SIZE; i++) {
> 
> do we need to hold on to the lock while we loop through the pages here?

I think we do?  Holding this lock will ensure the pages don't go away, I
believe (looking at mm/rmap.c comments on locking at the top)?

In any case, this function is called from print_binder_proc_stats()
which looks to be a debugfs/debugging call so I thought safer would be
better than faster and with a potential race.

> 
> > +		page = &alloc->pages[i];
> > +		if (!page->page_ptr)
> > +			free++;
> > +		else if (list_empty(&page->lru))
> > +			active++;
> > +		else
> > +			lru++;
> >  	}
> > +
> > +uninitialized:
> > +	mmap_read_unlock(alloc->vma_vm_mm);
> >  	mutex_unlock(&alloc->mutex);
> >  	seq_printf(m, "  pages: %d:%d:%d\n", active, lru, free);
> >  	seq_printf(m, "  pages high watermark: %zu\n", alloc->pages_high);
> > -- 
> > 2.35.1