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Date: Wed, 11 Dec 2013 14:34:50 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>,
        Thomas Gleixner <tglx@linutronix.de>,
        Darren Hart <dvhart@linux.intel.com>, linux-kernel@vger.kernel.org,
        linux-m68k@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH] futex: Switch to USER_DS for futex test
Message-Id: <20131211143450.329edd5199fd9a4d56abb6bc@linux-foundation.org>
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On Wed, 11 Dec 2013 11:43:21 +0100 Geert Uytterhoeven <geert@linux-m68k.org> wrote:

> Since commit e4f2dfbb5e92be4e46c0625f4f8eb101110f756f ("m68k: implement
> futex.h to support userspace robust futexes and PI mutexes"), the kernel
> crashes during boot up on MC68030:
> 
> Data read fault at 0x00000000 in Super Data (pc=0x3afec)
> BAD KERNEL BUSERR
> Oops: 00000000
> Modules linked in:
> PC: [<0003afec>] cmpxchg_futex_value_locked+0x14/0x4a
> SR: 2004  SP: 0082fed4  a2: 0082c000
> d0: 00000000    d1: 00000001    d2: 00000018    d3: 00000000
> d4: 00000061    d5: 00001000    a0: 00000000    a1: 0082e000
> Process swapper (pid: 1, task=0082c000)
> Frame format=B ssw=074d isc=4a80 isb=661c daddr=00000000 dobuf=00000001
> baddr=0003aff2 dibuf=00000000 ver=f
> Stack from 0082ff5c:
>         002b8cb8 0082ff70 00000000 00000000 00000000 00000000 00000000 000020ac
>         00000018 00000007 00000061 00001000 00000000 00000000 002cab50 00002008
>         002b3a56 002b8ca4 0082c3f0 00000000 0082c53c 001e316a 00000000 00000000
>         001e3172 001e316a 000025d4 00000000 00000000 00000000 00000000 00000000
>         00000000 00000000 00000000 00000000 00000000 00000000 00000000 20000000
>         00000000
> Call Trace: [<002b8cb8>] futex_init+0x14/0x54
>  [<000020ac>] do_one_initcall+0xa4/0x144
>  [<00001000>] kernel_pg_dir+0x0/0x1000
>  [<00002008>] do_one_initcall+0x0/0x144
>  [<002b3a56>] kernel_init_freeable+0xca/0x152
>  [<002b8ca4>] futex_init+0x0/0x54
>  [<001e316a>] kernel_init+0x0/0xc8
>  [<001e3172>] kernel_init+0x8/0xc8
>  [<001e316a>] kernel_init+0x0/0xc8
>  [<000025d4>] ret_from_kernel_thread+0xc/0x14
> 
> This happens because the futex test in futex_init() lacks a switch to the
> USER_DS address space, while cmpxchg_futex_value_locked() and
> futex_atomic_cmpxchg_inatomic() operate on userspace pointers (albeit NULL
> for this particular test).
> 
> Fix this by switching to USER_DS before running the test, and restoring the
> old address space afterwards.
> 
> ...
>
>  static int __init futex_init(void)
>  {
> +	mm_segment_t fs;
>  	u32 curval;
>  	int i;
>  
> @@ -2745,8 +2747,11 @@ static int __init futex_init(void)
>  	 * implementation, the non-functional ones will return
>  	 * -ENOSYS.
>  	 */
> +	fs = get_fs();
> +	set_fs(USER_DS);
>  	if (cmpxchg_futex_value_locked(&curval, NULL, 0, 0) == -EFAULT)
>  		futex_cmpxchg_enabled = 1;
> +	set_fs(fs);

hm.  Are we sure that a reference to NULL will continue to fault on all
architectures when switched to USER_DS?