Re: riscv+KASAN does not boot

Dmitry Vyukov
2021-03-09T17:13:01Z

On Fri, Feb 19, 2021 at 11:26 PM 'Palmer Dabbelt' via syzkaller
<syzkaller@googlegroups.com> wrote:
>
> On Fri, 19 Feb 2021 10:53:43 PST (-0800), dvyukov@google.com wrote:
> > On Fri, Feb 19, 2021 at 6:01 PM Alex Ghiti <alex@ghiti.fr> wrote:
> >>
> >> Hi Dmitry,
> >>
> >> Le 2/18/21 à 6:36 AM, Dmitry Vyukov a écrit :
> >> > On Thu, Feb 18, 2021 at 8:54 AM Alex Ghiti <alex@ghiti.fr> wrote:
> >> >>
> >> >> Hi Dmitry,
> >> >>
> >> >>> On Wed, Feb 17, 2021 at 5:36 PM Alex Ghiti <alex@ghiti.fr> wrote:
> >> >>>>
> >> >>>> Le 2/16/21 à 11:42 PM, Dmitry Vyukov a écrit :
> >> >>>>> On Tue, Feb 16, 2021 at 9:42 PM Alex Ghiti <alex@ghiti.fr> wrote:
> >> >>>>>>
> >> >>>>>> Hi Dmitry,
> >> >>>>>>
> >> >>>>>> Le 2/16/21 à 6:25 AM, Dmitry Vyukov a écrit :
> >> >>>>>>> On Tue, Feb 16, 2021 at 12:17 PM Dmitry Vyukov <dvyukov@google.com> wrote:
> >> >>>>>>>>
> >> >>>>>>>> On Fri, Jan 29, 2021 at 9:11 AM Dmitry Vyukov <dvyukov@google.com> wrote:
> >> >>>>>>>>>> I was fixing KASAN support for my sv48 patchset so I took a look at your
> >> >>>>>>>>>> issue: I built a kernel on top of the branch riscv/fixes using
> >> >>>>>>>>>> https://github.com/google/syzkaller/blob/269d24e857a757d09a898086a2fa6fa5d827c3e1/dashboard/config/linux/upstream-riscv64-kasan.config
> >> >>>>>>>>>> and Buildroot 2020.11. I have the warnings regarding the use of
> >> >>>>>>>>>> __virt_to_phys on wrong addresses (but that's normal since this function
> >> >>>>>>>>>> is used in virt_addr_valid) but not the segfaults you describe.
> >> >>>>>>>>>
> >> >>>>>>>>> Hi Alex,
> >> >>>>>>>>>
> >> >>>>>>>>> Let me try to rebuild buildroot image. Maybe there was something wrong
> >> >>>>>>>>> with my build, though, I did 'make clean' before doing. But at the
> >> >>>>>>>>> same time it worked back in June...
> >> >>>>>>>>>
> >> >>>>>>>>> Re WARNINGs, they indicate kernel bugs. I am working on setting up a
> >> >>>>>>>>> syzbot instance on riscv. If there a WARNING during boot then the
> >> >>>>>>>>> kernel will be marked as broken. No further testing will happen.
> >> >>>>>>>>> Is it a mis-use of WARN_ON? If so, could anybody please remove it or
> >> >>>>>>>>> replace it with pr_err.
> >> >>>>>>>>
> >> >>>>>>>>
> >> >>>>>>>> Hi,
> >> >>>>>>>>
> >> >>>>>>>> I've localized one issue with riscv/KASAN:
> >> >>>>>>>> KASAN breaks VDSO and that's I think the root cause of weird faults I
> >> >>>>>>>> saw earlier. The following patch fixes it.
> >> >>>>>>>> Could somebody please upstream this fix? I don't know how to add/run
> >> >>>>>>>> tests for this.
> >> >>>>>>>> Thanks
> >> >>>>>>>>
> >> >>>>>>>> diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile
> >> >>>>>>>> index 0cfd6da784f84..cf3a383c1799d 100644
> >> >>>>>>>> --- a/arch/riscv/kernel/vdso/Makefile
> >> >>>>>>>> +++ b/arch/riscv/kernel/vdso/Makefile
> >> >>>>>>>> @@ -35,6 +35,7 @@ CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_FTRACE) -Os
> >> >>>>>>>> # Disable gcov profiling for VDSO code
> >> >>>>>>>> GCOV_PROFILE := n
> >> >>>>>>>> KCOV_INSTRUMENT := n
> >> >>>>>>>> +KASAN_SANITIZE := n
> >> >>>>>>>>
> >> >>>>>>>> # Force dependency
> >> >>>>>>>> $(obj)/vdso.o: $(obj)/vdso.so
> >> >>>>>>
> >> >>>>>> What's weird is that I don't have any issue without this patch with the
> >> >>>>>> following config whereas it indeed seems required for KASAN. But when
> >> >>>>>> looking at the segfaults you got earlier, the segfault address is 0xbb0
> >> >>>>>> and the cause is an instruction page fault: this address is the PLT base
> >> >>>>>> address in vdso.so and an instruction page fault would mean that someone
> >> >>>>>> tried to jump at this address, which is weird. At first sight, that does
> >> >>>>>> not seem related to your patch above, but clearly I may be wrong.
> >> >>>>>>
> >> >>>>>> Tobias, did you observe the same segfaults as Dmitry ?
> >> >>>>>
> >> >>>>>
> >> >>>>> I noticed that not all buildroot images use VDSO, it seems to be
> >> >>>>> dependent on libc settings (at least I think I changed it in the
> >> >>>>> past).
> >> >>>>
> >> >>>> Ok, I used uClibc but then when using glibc, I have the same segfaults,
> >> >>>> only when KASAN is enabled. And your patch fixes the problem. I will try
> >> >>>> to take a look later to better understand the problem.
> >> >>>>
> >> >>>>> I also booted an image completely successfully including dhcpd/sshd
> >> >>>>> start, but then my executable crashed in clock_gettime. The executable
> >> >>>>> was build on linux/amd64 host with "riscv64-linux-gnu-gcc -static"
> >> >>>>> (10.2.1).
> >> >>>>>
> >> >>>>>
> >> >>>>>>> Second issue I am seeing seems to be related to text segment size.
> >> >>>>>>> I check out v5.11 and use this config:
> >> >>>>>>> https://gist.github.com/dvyukov/6af25474d455437577a84213b0cc9178
> >> >>>>>>
> >> >>>>>> This config gave my laptop a hard time ! Finally I was able to boot
> >> >>>>>> correctly to userspace, but I realized I used my sv48 branch...Either I
> >> >>>>>> fixed your issue along the way or I can't reproduce it, I'll give it a
> >> >>>>>> try tomorrow.
> >> >>>>>
> >> >>>>> Where is your branch? I could also test in my setup on your branch.
> >> >>>>>
> >> >>>>
> >> >>>> You can find my branch int/alex/riscv_kernel_end_of_address_space_v2
> >> >>>> here: https://github.com/AlexGhiti/riscv-linux.git
> >> >>>
> >> >>> No, it does not work for me.
> >> >>>
> >> >>> Source is on b61ab6c98de021398cd7734ea5fc3655e51e70f2 (HEAD,
> >> >>> int/alex/riscv_kernel_end_of_address_space_v2)
> >> >>> Config is https://gist.githubusercontent.com/dvyukov/6af25474d455437577a84213b0cc9178/raw/55b116522c14a8a98a7626d76df740d54f648ce5/gistfile1.txt
> >> >>>
> >> >>> riscv64-linux-gnu-gcc -v
> >> >>> gcc version 10.2.1 20210110 (Debian 10.2.1-6+build1)
> >> >>>
> >> >>> qemu-system-riscv64 --version
> >> >>> QEMU emulator version 5.2.0 (Debian 1:5.2+dfsg-3)
> >> >>>
> >> >>> qemu-system-riscv64 \
> >> >>> -machine virt -smp 2 -m 2G \
> >> >>> -device virtio-blk-device,drive=hd0 \
> >> >>> -drive file=image-riscv64,if=none,format=raw,id=hd0 \
> >> >>> -kernel arch/riscv/boot/Image \
> >> >>> -nographic \
> >> >>> -device virtio-rng-device,rng=rng0 -object
> >> >>> rng-random,filename=/dev/urandom,id=rng0 \
> >> >>> -netdev user,id=net0,host=10.0.2.10,hostfwd=tcp::10022-:22 -device
> >> >>> virtio-net-device,netdev=net0 \
> >> >>> -append "root=/dev/vda earlyprintk=serial console=ttyS0 oops=panic
> >> >>> panic_on_warn=1 panic=86400 earlycon"
> >> >>
> >> >> It still works for me but I had to disable CONFIG_DEBUG_INFO_BTF (I
> >> >> don't think that changes anything at runtime). But your above command
> >> >> line does not work for me as it appears you do not load any firmware, if
> >> >> I add -bios images/fw_jump.elf, it works. But then I don't know where
> >> >> your opensbi output below comes from...
> >> >>
> >> >> And regarding your issue with calling clock_gettime 'directly' compared
> >> >> to using the syscall, I have the same consistent output from both calls.
> >> >>
> >> >> I have an older gcc (9.3.0) and the same qemu. I think what is missing
> >> >> here is your buildroot config, so that we have the exact same
> >> >> environment: could you post your buildroot config as well ?
> >> >
> >> > I don't think the image is relevant because I don't even get to kernel
> >> > code. If the kernel will complain about no init later, that's fine.
> >> > Re bios, this version of qemu already has OpenSBI bios builtin, you
> >> > can pass -bios default, but that's, well, the default :)
> >> > Here are more reproducible repro instructions that capture gcc and
> >> > qemu. I think gcc version may be potentially relevant as I suspect
> >> > code size.
> >> >
> >> >
> >> > curl https://gist.githubusercontent.com/dvyukov/6af25474d455437577a84213b0cc9178/raw/55b116522c14a8a98a7626d76df740d54f648ce5/gistfile1.txt
> >> >> $KERNEL_SRC/.config
> >> > docker pull gcr.io/syzkaller/syzbot
> >> > docker run -it -v $KERNEL_SRC:/kernel gcr.io/syzkaller/syzbot
> >> > cd /kernel
> >> > make -j72 ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu- olddefconfig
> >> > make -j72 ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu-
> >> > qemu-system-riscv64 -machine virt -smp 2 -m 4G -kernel
> >> > arch/riscv/boot/Image -nographic -append "earlycon earlyprintk=serial
> >> > console=ttyS0"
> >> > [this does not, only OpenSBI output]
> >> >
> >>
> >> Indeed the issue was code size, please find the fix below. I will send a
> >> proper patch once I made sure the fix is the right one, but I'm pretty
> >> confident, there's no reason to limit the mapping size to 128MB whereas
> >> we have a whole pgdir.
> >
> > Great you get to the bottom of this!
> > Riscv kernels are going to be YUGE!
>
> IIRC I tried that a while ago and it didn't work. It's possible I was just
> running into some other bug, but I'm just build testing allyesconfig as opposed
> to boot testing it.
>
> If you've got a setup that does boot I'm happy to take a patch, though. It'll
> at least be one step forward.

OK, it's getting better.
The next issue is called "512 bytes should be enough for everyone!" :)
https://elixir.bootlin.com/linux/v5.12-rc2/source/include/uapi/asm-generic/setup.h#L5
Most other arches redefine it to something bigger:
https://elixir.bootlin.com/linux/v5.12-rc2/source/arch/s390/include/uapi/asm/setup.h#L10
even arm32 redefines it.
I am not sure the default is even reasonable anymore. Failure mode is
also not nice (silent truncation).
We are trying to pass this:

earlyprintk=serial oops=panic nmi_watchdog=panic panic=86400
net.ifnames=0 sysctl.kernel.hung_task_all_cpu_backtrace=1
ima_policy=tcb kvm-intel.nested=1 nf-conntrack-ftp.ports=20000
nf-conntrack-tftp.ports=20000 nf-conntrack-sip.ports=20000
nf-conntrack-irc.ports=20000 nf-conntrack-sane.ports=20000
vivid.n_devs=16 vivid.multiplanar=1,2,1,2,1,2,1,2,1,2,1,2,1,2,1,2
netrom.nr_ndevs=16 rose.rose_ndevs=16 spec_store_bypass_disable=prctl
numa=fake=2 nopcid dummy_hcd.num=8 binder.debug_mask=0
rcupdate.rcu_expedited=1 watchdog_thresh=165
workqueue.watchdog_thresh=420 panic_on_warn=1

The last part gets truncated and we are getting false workqueue watchdog stalls.

Could you please increase it?