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

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

On 16/03/2021 08:52, Dmitry Vyukov wrote:
> On Mon, Mar 15, 2021 at 10:38 PM Ben Dooks <ben.dooks@codethink.co.uk> wrote:
>>
>> 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]
> 
> This is not an error, just a notification that some syscalls are not
> enabled in the kernel and won't be fuzzed.
> 
>> 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)
> 
> This is an error. But I see it the first time ever.
> It comes from here:
> https://github.com/google/syzkaller/blob/fdb2bb2c23ee709880407f56307e2800ad27e9ae/pkg/osutil/osutil_unix.go#L119-L121
> There should be pretty simple logic inside of syscall.Mmap. Perhaps
> you are using some older Go toolchain with incomplete riscv support?
> I think I've used 1.14 and 1.15. But there is already 1.16. You can
> always download a toolchain here:
> https://golang.org/dl/

Hmm it would have been useful to print out what file it failed to map.

I've got go 1.15 from the debian/unstable riscv64 chroot.
I'll have a look at this in a bit to see if it throws the same issue on 
a real system.


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

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