Re: [syzbot] BUG: unable to handle kernel access to user memory in schedule_tail

[Ben Dooks <ben.dooks@codethink.co.uk>]

On 12/03/2021 16:34, Ben Dooks wrote:
> On 12/03/2021 16:30, Ben Dooks wrote:
>> On 12/03/2021 15:12, Dmitry Vyukov wrote:
>>> On Fri, Mar 12, 2021 at 2:50 PM Ben Dooks <ben.dooks@codethink.co.uk> 
>>> wrote:
>>>>
>>>> On 10/03/2021 17:16, Dmitry Vyukov wrote:
>>>>> On Wed, Mar 10, 2021 at 5:46 PM syzbot
>>>>> <syzbot+e74b94fe601ab9552d69@syzkaller.appspotmail.com> wrote:
>>>>>>
>>>>>> Hello,
>>>>>>
>>>>>> syzbot found the following issue on:
>>>>>>
>>>>>> HEAD commit:    0d7588ab riscv: process: Fix no prototype for 
>>>>>> arch_dup_tas..
>>>>>> git tree: 
>>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/riscv/linux.git fixes
>>>>>> console output: 
>>>>>> https://syzkaller.appspot.com/x/log.txt?x=1212c6e6d00000
>>>>>> kernel config: 
>>>>>> https://syzkaller.appspot.com/x/.config?x=e3c595255fb2d136
>>>>>> dashboard link: 
>>>>>> https://syzkaller.appspot.com/bug?extid=e74b94fe601ab9552d69
>>>>>> userspace arch: riscv64
>>>>>>
>>>>>> Unfortunately, I don't have any reproducer for this issue yet.
>>>>>>
>>>>>> IMPORTANT: if you fix the issue, please add the following tag to 
>>>>>> the commit:
>>>>>> Reported-by: syzbot+e74b94fe601ab9552d69@syzkaller.appspotmail.com
>>>>>
>>>>> +riscv maintainers
>>>>>
>>>>> This is riscv64-specific.
>>>>> I've seen similar crashes in put_user in other places. It looks like
>>>>> put_user crashes in the user address is not mapped/protected (?).
>>>>
>>>> I've been having a look, and this seems to be down to access of the
>>>> tsk->set_child_tid variable. I assume the fuzzing here is to pass a
>>>> bad address to clone?
>>>>
>>>>   From looking at the code, the put_user() code should have set the
>>>> relevant SR_SUM bit (the value for this, which is 1<<18 is in the
>>>> s2 register in the crash report) and from looking at the compiler
>>>> output from my gcc-10, the code looks to be dong the relevant csrs
>>>> and then csrc around the put_user
>>>>
>>>> So currently I do not understand how the above could have happened
>>>> over than something re-tried the code seqeunce and ended up retrying
>>>> the faulting instruction without the SR_SUM bit set.
>>>
>>> I would maybe blame qemu for randomly resetting SR_SUM, but it's
>>> strange that 99% of these crashes are in schedule_tail. If it would be
>>> qemu, then they would be more evenly distributed...
>>>
>>> Another observation: looking at a dozen of crash logs, in none of
>>> these cases fuzzer was actually trying to fuzz clone with some insane
>>> arguments. So it looks like completely normal clone's (e..g coming
>>> from pthread_create) result in this crash.
>>>
>>> I also wonder why there is ret_from_exception, is it normal? I see
>>> handle_exception disables SR_SUM:
>>> https://elixir.bootlin.com/linux/v5.12-rc2/source/arch/riscv/kernel/entry.S#L73 
>>>
>>
>> So I think if SR_SUM is set, then it faults the access to user memory
>> which the _user() routines clear to allow them access.
>>
>> I'm thinking there is at least one issue here:
>>
>> - the test in fault is the wrong way around for die kernel
>> - the handler only catches this if the page has yet to be mapped.
>>
>> So I think the test should be:
>>
>>          if (!user_mode(regs) && addr < TASK_SIZE &&
>>                          unlikely(regs->status & SR_SUM)
>>
>> This then should continue on and allow the rest of the handler to
>> complete mapping the page if it is not there.
>>
>> I have been trying to create a very simple clone test, but so far it
>> has yet to actually trigger anything.
> 
> I should have added there doesn't seem to be a good way to use mmap()
> to allocate memory but not insert a vm-mapping post the mmap().
> 
> 

How difficult is it to try building a branch with the above test
modified?

-- 
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius

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