Re: [PATCH] timer: Fix unsafe cpu variable access in migrate_timers

[Jan Kiszka]

On 2015-06-27 13:00, Borislav Petkov wrote:
> On Sat, Jun 27, 2015 at 11:55:49AM +0200, Jan Kiszka wrote:
>> From: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
>>
>> migrate_timers is invoked with preemption enabled. Therefore we have to
>> get/put the cpu-local variable tvec_bases like before commit 0eeda71bc3.
>>
>> This fixes
>>
>> BUG: using smp_processor_id() in preemptible [00000000] code: bash/4917
>> caller is debug_smp_processor_id+0x17/0x19
>> CPU: 0 PID: 4917 Comm: bash Not tainted 4.1.0-dbg+ #97
>> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.8.1-0-g4adadbd-20150316_085822-nilsson.home.kraxel.org 04/01/2014
>>  ffff880038674040 ffff880002e6fb98 ffffffff815356a0 0000000000000002
>>  0000000000000000 ffff880002e6fbc8 ffffffff8130c16f ffff88003fd8d500
>>  000000000000d500 0000000000000000 0000000000000003 ffff880002e6fbd8
>> Call Trace:
>>  [<ffffffff815356a0>] dump_stack+0x4f/0x7b
>>  [<ffffffff8130c16f>] check_preemption_disabled+0xdd/0xef
>>  [<ffffffff8130c198>] debug_smp_processor_id+0x17/0x19
>>  [<ffffffff810a9b37>] timer_cpu_notify+0x4f/0x112
>>  [<ffffffff8106718b>] notifier_call_chain+0x4c/0x71
>>  [<ffffffff810671be>] __raw_notifier_call_chain+0xe/0x10
>>  [<ffffffff810484b6>] __cpu_notify+0x20/0x37
>>  [<ffffffff810484e0>] cpu_notify+0x13/0x15
>>  [<ffffffff81048591>] cpu_notify_nofail+0xe/0x16
>>  [<ffffffff8152efb2>] _cpu_down+0x178/0x268
>>  [<ffffffff8108bc3a>] ? trace_hardirqs_on+0xd/0xf
>>  [<ffffffff8152f0ca>] cpu_down+0x28/0x3c
>>  [<ffffffff813cbdb9>] cpu_subsys_offline+0x14/0x16
>>  [<ffffffff813c779d>] device_offline+0x7d/0xb1
>>  [<ffffffff813c78a2>] online_store+0x48/0x68
>>  [<ffffffff813c5544>] dev_attr_store+0x18/0x22
>>  [<ffffffff811dac6c>] sysfs_kf_write+0x49/0x51
>>  [<ffffffff811da139>] kernfs_fop_write+0x105/0x158
>>  [<ffffffff8116c54f>] __vfs_write+0x28/0xbd
>>  [<ffffffff812ab014>] ? security_file_permission+0x23/0x90
>>  [<ffffffff8116ccb2>] vfs_write+0xb2/0x169
>>  [<ffffffff8116d717>] SyS_write+0x4a/0x91
>>  [<ffffffff8153d12e>] entry_SYSCALL_64_fastpath+0x12/0x76
>>
>> triggered when offlining a CPU, e.g. via sysfs.
>>
>> Signed-off-by: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
>> ---
>>  kernel/time/timer.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/time/timer.c b/kernel/time/timer.c
>> index 520499d..c826178 100644
>> --- a/kernel/time/timer.c
>> +++ b/kernel/time/timer.c
>> @@ -1566,7 +1566,7 @@ static void migrate_timers(int cpu)
>>  
>>  	BUG_ON(cpu_online(cpu));
>>  	old_base = per_cpu_ptr(&tvec_bases, cpu);
>> -	new_base = this_cpu_ptr(&tvec_bases);
>> +	new_base = &get_cpu_var(tvec_bases);
> 
> Hmm, tglx's version doesn't disable preemtion around it:
> 
> http://git.kernel.org/cgit/linux/kernel/git/tip/tip.git/commit/?h=timers/urgent&id=24bfcb100959c8641a627b5604d967243f8f240c
> 

Oh, there is a fix already. It's just the same: both implicitly disable
preemption via get_cpu_ptr/var, look at the macros.

Jan

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