Re: [syzbot] [io-uring?] KASAN: null-ptr-deref Write in sys_io_uring_register

[Matthew Wilcox <willy@infradead.org>]

On Wed, Dec 04, 2024 at 09:17:27AM -0700, Jens Axboe wrote:
> >   XA_STATE(xas, xa, index);
> > - return xas_result(&xas, xas_store(&xas, NULL));
> > + return xas_result(&xas, xa_zero_to_null(xas_store(&xas, NULL)));
> >  }
> >  EXPORT_SYMBOL(__xa_erase);
> > 
> > This would explain deletion of a reserved entry returning
> > `XA_ZERO_ENTRY` rather than `NULL`.
> 
> Yep this works.
> 
> > My apologies for this breakage. Should I send a new version? A new
> > "fixes" patch?
> 
> Since it seems quite drastically broken, and since it looks like Andrew
> is holding it, seems like the best course of action would be to have it
> folded with the existing patch.

... and please include an addition to the test-suite that would catch
this bug.

Wait, why doesn't this one catch it?  You did run the test-suite, right?

        /* xa_insert treats it as busy */
        XA_BUG_ON(xa, xa_reserve(xa, 12345678, GFP_KERNEL) != 0);
        XA_BUG_ON(xa, xa_insert(xa, 12345678, xa_mk_value(12345678), 0) !=
                        -EBUSY);
        XA_BUG_ON(xa, xa_empty(xa));
        XA_BUG_ON(xa, xa_erase(xa, 12345678) != NULL);
        XA_BUG_ON(xa, !xa_empty(xa));