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

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

On 18/03/2021 10:05, Dmitry Vyukov wrote:
> On Thu, Mar 18, 2021 at 10:41 AM Ben Dooks <ben.dooks@codethink.co.uk> wrote:
>>
>> On 12/03/2021 17:38, Dmitry Vyukov wrote:
>>> On Fri, Mar 12, 2021 at 6:34 PM Dmitry Vyukov <dvyukov@google.com> wrote:
>>>>
>>>> On Fri, Mar 12, 2021 at 5:36 PM Ben Dooks <ben.dooks@codethink.co.uk> wrote:
>>>>>
>>>>> 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?
>>>>
>>>> I don't have access to hardware, I don't have other qemu versions ready to use.
>>>> But I can teach you how to run syzkaller locally :)
>>>> I am not sure anybody run it on real riscv hardware at all. When
>>>> Tobias ported syzkaller, Tobias also used qemu I think.
>>>>
>>>> I am now building with an inverted check to test locally.
>>>>
>>>> I don't fully understand but this code, but does handle_exception
>>>> reset SR_SUM around do_page_fault? If so, then looking at SR_SUM in
>>>> do_page_fault won't work with positive nor negative check.
>>>
>>>
>>> The inverted check crashes during boot:
>>>
>>> --- a/arch/riscv/mm/fault.c
>>> +++ b/arch/riscv/mm/fault.c
>>> @@ -249,7 +249,7 @@ asmlinkage void do_page_fault(struct pt_regs *regs)
>>>                   flags |= FAULT_FLAG_USER;
>>>
>>>           if (!user_mode(regs) && addr < TASK_SIZE &&
>>> -                       unlikely(!(regs->status & SR_SUM)))
>>> +                       unlikely(regs->status & SR_SUM))
>>>                   die_kernel_fault("access to user memory without
>>> uaccess routines",
>>>                                   addr, regs);
>>>
>>>
>>> [   77.349329][    T1] Run /sbin/init as init process
>>> [   77.868371][    T1] Unable to handle kernel access to user memory
>>> without uaccess routines at virtual address 00000000000e8e39
>>> [   77.870355][    T1] Oops [#1]
>>> [   77.870766][    T1] Modules linked in:
>>> [   77.871326][    T1] CPU: 0 PID: 1 Comm: init Not tainted
>>> 5.12.0-rc2-00010-g0d7588ab9ef9-dirty #42
>>> [   77.872057][    T1] Hardware name: riscv-virtio,qemu (DT)
>>> [   77.872620][    T1] epc : __clear_user+0x36/0x4e
>>> [   77.873285][    T1]  ra : padzero+0x9c/0xb0
>>> [   77.873849][    T1] epc : ffffffe000bb7136 ra : ffffffe0004f42a0 sp
>>> : ffffffe006f8fbc0
>>> [   77.874438][    T1]  gp : ffffffe005d25718 tp : ffffffe006f98000 t0
>>> : 00000000000e8e40
>>> [   77.875031][    T1]  t1 : 00000000000e9000 t2 : 000000000001c49c s0
>>> : ffffffe006f8fbf0
>>> [   77.875618][    T1]  s1 : 00000000000001c7 a0 : 00000000000e8e39 a1
>>> : 00000000000001c7
>>> [   77.876204][    T1]  a2 : 0000000000000002 a3 : 00000000000e9000 a4
>>> : ffffffe006f99000
>>> [   77.876787][    T1]  a5 : 0000000000000000 a6 : 0000000000f00000 a7
>>> : ffffffe00031c088
>>> [   77.877367][    T1]  s2 : 00000000000e8e39 s3 : 0000000000001000 s4
>>> : 0000003ffffffe39
>>> [   77.877952][    T1]  s5 : 00000000000e8e39 s6 : 00000000000e9570 s7
>>> : 00000000000e8e39
>>> [   77.878535][    T1]  s8 : 0000000000000001 s9 : 00000000000e8e39
>>> s10: ffffffe00c65f608
>>> [   77.879126][    T1]  s11: ffffffe00816e8d8 t3 : ea3af0fa372b8300 t4
>>> : 0000000000000003
>>> [   77.879711][    T1]  t5 : ffffffc401dc45d8 t6 : 0000000000040000
>>> [   77.880209][    T1] status: 0000000000040120 badaddr:
>>> 00000000000e8e39 cause: 000000000000000f
>>> [   77.880846][    T1] Call Trace:
>>> [   77.881213][    T1] [<ffffffe000bb7136>] __clear_user+0x36/0x4e
>>> [   77.881912][    T1] [<ffffffe0004f523e>] load_elf_binary+0xf8a/0x2400
>>> [   77.882562][    T1] [<ffffffe0003e1802>] bprm_execve+0x5b0/0x1080
>>> [   77.883145][    T1] [<ffffffe0003e38bc>] kernel_execve+0x204/0x288
>>> [   77.883727][    T1] [<ffffffe003b70e94>] run_init_process+0x1fe/0x212
>>> [   77.884337][    T1] [<ffffffe003b70ec6>] try_to_run_init_process+0x1e/0x66
>>> [   77.884956][    T1] [<ffffffe003bc0864>] kernel_init+0x14a/0x200
>>> [   77.885541][    T1] [<ffffffe000005570>] ret_from_exception+0x0/0x14
>>> [   77.886955][    T1] ---[ end trace 1e934d07b8a4bed8 ]---
>>> [   77.887705][    T1] Kernel panic - not syncing: Fatal exception
>>> [   77.888333][    T1] SMP: stopping secondary CPUs
>>> [   77.889357][    T1] Rebooting in 86400 seconds..
>>
>> I have reproduced this on qemu, not managed to get the real hardwre
>> working with this branch yet.
>>
>> I have a working hypothesis now, having added debug to check the
>> sstatus.SR_SUM flag and reviewed the assembly, I think this is
>> what is happening:
>>
>> C code of "put_user(func(), address)" is generating code to do:
>>
>> 1:      __enable_user_access();
>> 2:      cpu_reg = func();
>> 3:      assembly for *address = cpu_reg;
>> 4:      __disable_user_access();
>>
>> I think the call to func() with all the sanitisers enabled allow
>> the func() to possibly schedule out. The __swtich_to() code does
>> not restore the original status registers which means that if
>> there is IO during the sleep SR_SUM may end up being cleared and
>> never re-set. We get back to 3 and fault as 2 cleared the result of 1.
>>
>> It is very possible no-one has seen this before as generally the
>> functions involved in feeding put_user() are fairly small and thus
>> this system is both under load and has some reason to schedule then
>> this bug has probably been rare to unseen.
>>
>> I think the correct solution is to store the SR_SUM bit status in
>> the thread_struct and make __switch_to() save/restore this when
>> changing between tasks/threads. Trying to re-order the code to
>> force swapping of 1 and 2 may reduce the bug's window.
>>
>> Further thinking of the order of 1 and 2 is that we should probably
>> fix that order so that func() is not run with the user-space access
>> protection disabled.
>>
>> I'll try and make some sort of of small test case to avoid having
>> to run syz-stress to provoke this.
> 
> Ouch!
> Can't preempt kernel schedule at almost any instruction where
> preemption is not disabled explicitly?  But if it's disabled, then the
> instrumented code won't schedule as well, right? I suspect this may be
> quite a bad issue for preempt kernels.
> 
> Shouldn't __put_user materialize the expression in a local var using
> __typeof__ magic before __enable_user_access? I suspect it may
> potentially lead to quite bad security implications.
> 
> It can also make sense to add checks to schedule to check that it's
> not called from unexpected contexts.

I wrote a kernel thread that does:

#define rd_sstatus() ({ unsigned long result; asm("  csrr %0, sstatus" : 
"=r"(result) :: "memory"); result; })


static int test_thread1(void *data)
{
	unsigned int cpu = (unsigned int)data;
	unsigned long status;

	pr_info("%s: thread starting on cpu %d\n", __func__, cpu);

	while (!kthread_should_stop()) {
		status = rd_sstatus();
		if (status & SR_SUM)
			printk_ratelimited("%s: found sstaus=0x%08lx\n",
					   __func__, status);
		msleep(1);
	}

	pr_info("%s: thread exiting\n", __func__);
	return 0;
}

And under the syz-stress I have the following

[ 1192.124018] test_thread1: found sstaus=0x00040022

this thread does not do any IO operations yet during a stress run
it got entered with SR_SUM set (the 0x00040000) in the sstatus
field.

I think this is proof that #1 this is /rare/ and #2 we need to
make __switch_user save at-least the SR_SUM field.


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

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