Re: riscv+KASAN does not boot

[Dmitry Vyukov]

On Tue, Feb 16, 2021 at 9:42 PM Alex Ghiti <alex@xxxxxxxx> 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@xxxxxxxxxx> wrote:
> >>
> >> On Fri, Jan 29, 2021 at 9:11 AM Dmitry Vyukov <dvyukov@xxxxxxxxxx> 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).
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.


> > Then trying to boot it using:
> > QEMU emulator version 5.2.0 (Debian 1:5.2+dfsg-3)
> > $ qemu-system-riscv64 -machine virt -smp 2 -m 4G ...
> >
> > It shows no output from the kernel whatsoever, even though I have
> > earlycon and output shows very early with other configs.
> > Kernel boots fine with defconfig and other smaller configs.
> >
> > If I enable KASAN_OUTLINE and CC_OPTIMIZE_FOR_SIZE, then this config
> > also boots fine. Both of these options significantly reduce kernel
> > size. However, I can also boot the kernel without these 2 configs, if
> > I disable a whole lot of subsystem configs. This makes me think that
> > there is an issue related to kernel size somewhere in
> > qemu/bootloader/kernel bootstrap code.
> > Does it make sense to you? Can somebody reproduce what I am seeing? >
>
> I did not bring any answer to your question, but at least you know I'm
> working on it, I'll keep you posted.
>
> Thanks for taking the time to setup syzkaller.
>
> Alex
>
> > Thanks
> >
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> >