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

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

On 13/03/2021 07:20, Dmitry Vyukov wrote:
> On Fri, Mar 12, 2021 at 9:12 PM Ben Dooks <ben.dooks@codethink.co.uk> wrote:
>>
>> On 12/03/2021 16:25, Alex Ghiti wrote:
>>>
>>>
>>> Le 3/12/21 à 10:12 AM, Dmitry Vyukov a écrit :
>>>> 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:
>>>
>>> csrrc does the right thing: it cleans SR_SUM bit in status but saves the
>>> previous value that will get correctly restored.
>>>
>>> ("The CSRRC (Atomic Read and Clear Bits in CSR) instruction reads the
>>> value of the CSR, zero-extends the value to XLEN bits, and writes it to
>>> integer registerrd.  The initial value in integerregisterrs1is treated
>>> as a bit mask that specifies bit positions to be cleared in the CSR. Any
>>> bitthat is high inrs1will cause the corresponding bit to be cleared in
>>> the CSR, if that CSR bit iswritable.  Other bits in the CSR are
>>> unaffected.")
>>
>> I think there may also be an understanding issue on what the SR_SUM
>> bit does. I thought if it is set, M->U accesses would fault, which is
>> why it gets set early on. But from reading the uaccess code it looks
>> like the uaccess code sets it on entry and then clears on exit.
>>
>> I am very confused. Is there a master reference for rv64?
>>
>> https://people.eecs.berkeley.edu/~krste/papers/riscv-privileged-v1.9.pdf
>> seems to state PUM is the SR_SUM bit, and that (if set) disabled
>>
>> Quote:
>>    The PUM (Protect User Memory) bit modifies the privilege with which
>> S-mode loads, stores, and instruction fetches access virtual memory.
>> When PUM=0, translation and protection behave as normal. When PUM=1,
>> S-mode memory accesses to pages that are accessible by U-mode (U=1 in
>> Figure 4.19) will fault. PUM has no effect when executing in U-mode
>>
>>
>>>> https://elixir.bootlin.com/linux/v5.12-rc2/source/arch/riscv/kernel/entry.S#L73
>>>>
>>>
>>> Still no luck for the moment, can't reproduce it locally, my test is
>>> maybe not that good (I created threads all day long in order to trigger
>>> the put_user of schedule_tail).
>>
>> It may of course depend on memory and other stuff. I did try to see if
>> it was possible to clone() with the child_tid address being a valid but
>> not mapped page...
>>
>>> Given that the path you mention works most of the time, and that the
>>> status register in the stack trace shows the SUM bit is not set whereas
>>> it is set in put_user, I'm leaning toward some race condition (maybe an
>>> interrupt that arrives at the "wrong" time) or a qemu issue as you
>>> mentioned.
>>
>> I suppose this is possible. From what I read it should get to the
>> point of being there with the SUM flag cleared, so either something
>> went wrong in trying to fix the instruction up or there's some other
>> error we're missing.
>>
>>> To eliminate qemu issues, do you have access to some HW ? Or to
>>> different qemu versions ?
>>
>> I do have access to a Microchip Polarfire board. I just need the
>> instructions on how to setup the test-code to make it work on the
>> hardware.
> 
> For full syzkaller support, it would need to know how to reboot these
> boards and get access to the console.
> syzkaller has a stop-gap VM backend which just uses ssh to a physical
> machine and expects the kernel to reboot on its own after any crashes.
> 
> But I actually managed to reproduce it in an even simpler setup.
> Assuming you have Go 1.15 and riscv64 cross-compiler gcc installed
> 
> $ go get -u -d github.com/google/syzkaller/...
> $ cd $GOPATH/src/github.com/google/syzkaller
> $ make stress executor TARGETARCH=riscv64
> $ scp bin/linux_riscv64/syz-execprog bin/linux_riscv64/syz-executor
> your_machine:/
> 
> Then run ./syz-stress on the machine.
> On the first run it crashed it with some other bug, on the second run
> I got the crash in schedule_tail.
> With qemu tcg I also added -slowdown=10 flag to syz-stress to scale
> all timeouts, if native execution is faster, then you don't need it.

Ok, not sure what's going on. I get a lot of errors similar to:
> 
> 2021/03/15 21:35:20 transitively unsupported: ioctl$SNAPSHOT_CREATE_IMAGE: no syscalls can create resource fd_snapshot, enable some syscalls that can create it [openat$snapshot]

Followed by:

> 2021/03/15 21:35:48 executed 0 programs
> 2021/03/15 21:35:48 failed to create execution environment: failed to mmap shm file: invalid argument

The qemu is 5.2.0 and root is Debian/unstable riscv64 (same as chroot
used to build the syz tools)

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

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