linux-kernel - Re: [PATCH v2] mm/kfence: select random number before taking raw lock

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Date:   Thu, 9 Jun 2022 14:52:48 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kasan-dev@...glegroups.com,
        John Ogness <john.ogness@...utronix.de>,
        Alexander Potapenko <glider@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>
Subject: Re: [PATCH v2] mm/kfence: select random number before taking raw lock

On Thu 2022-06-09 14:33:19, Jason A. Donenfeld wrote:
> The RNG uses vanilla spinlocks, not raw spinlocks, so kfence should pick
> its random numbers before taking its raw spinlocks. This also has the
> nice effect of doing less work inside the lock. It should fix a splat
> that Geert saw with CONFIG_PROVE_RAW_LOCK_NESTING:
> 
>      dump_backtrace.part.0+0x98/0xc0
>      show_stack+0x14/0x28
>      dump_stack_lvl+0xac/0xec
>      dump_stack+0x14/0x2c
>      __lock_acquire+0x388/0x10a0
>      lock_acquire+0x190/0x2c0
>      _raw_spin_lock_irqsave+0x6c/0x94
>      crng_make_state+0x148/0x1e4
>      _get_random_bytes.part.0+0x4c/0xe8
>      get_random_u32+0x4c/0x140
>      __kfence_alloc+0x460/0x5c4
>      kmem_cache_alloc_trace+0x194/0x1dc
>      __kthread_create_on_node+0x5c/0x1a8
>      kthread_create_on_node+0x58/0x7c
>      printk_start_kthread.part.0+0x34/0xa8
>      printk_activate_kthreads+0x4c/0x54
>      do_one_initcall+0xec/0x278
>      kernel_init_freeable+0x11c/0x214
>      kernel_init+0x24/0x124
>      ret_from_fork+0x10/0x20
> 
> Cc: John Ogness <john.ogness@...utronix.de>
> Cc: Alexander Potapenko <glider@...gle.com>
> Cc: Marco Elver <elver@...gle.com>
> Cc: Dmitry Vyukov <dvyukov@...gle.com>
> Reported-by: Geert Uytterhoeven <geert@...ux-m68k.org>
> Tested-by: Geert Uytterhoeven <geert+renesas@...der.be>
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>

Reviewed-by: Petr Mladek <pmladek@...e.com>

Thanks a lot for fixing this. It is great to know that the printk
kthreads were not the culprit here ;-)

Best Regards,
Petr