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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Thu, 9 Jun 2022 14:31:18 +0200
Message-ID: <CAMuHMdXocvPDViVHjEfeWGxA+7SwOrD3pP7v8wFM80Us3xSU4Q@mail.gmail.com>
Subject: Re: [PATCH] mm/kfence: select random number before taking raw lock
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        John Ogness <john.ogness@linutronix.de>,
        Alexander Potapenko <glider@google.com>,
        Marco Elver <elver@google.com>,
        Dmitry Vyukov <dvyukov@google.com>
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Hi Jason,

On Thu, Jun 9, 2022 at 2:17 PM Jason A. Donenfeld <Jason@zx2c4.com> 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@linutronix.de>
> Cc: Alexander Potapenko <glider@google.com>
> Cc: Marco Elver <elver@google.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

Thank you, the splat is gone.

Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds