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

Tue Mar 16 11:44:18 GMT 2021

On Tue, Mar 16, 2021 at 12:35 PM Ben Dooks <ben.dooks at 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 at 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 at 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 at 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.

What do you want to do with the file name? It's not one of
pre-existing files, so the name won't tell the user much. It's just a
temp file, it won't exist afterwards and it's easy to create an
equivalent file.
It was created in that function with:

    f, err = ioutil.TempFile("./", "syzkaller-shm")
    if err != nil {
        err = fmt.Errorf("failed to create temp file: %v", err)
        return
    }
    if err = f.Truncate(int64(size)); err != nil {
        err = fmt.Errorf("failed to truncate shm file: %v", err)
        f.Close()
        os.Remove(f.Name())
        return
    }
    f.Close()
    fname := f.Name()
    f, err = os.OpenFile(f.Name(), os.O_RDWR, DefaultFilePerm)
    if err != nil {
        err = fmt.Errorf("failed to open shm file: %v", err)
        os.Remove(fname)
        return
    }

> 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
>
> https://www.codethink.co.uk/privacy.html
>
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