Arm + KASAN + syzbot

[Mark Rutland]

Hi Dmitry,

On Mon, Jan 18, 2021 at 05:31:36PM +0100, 'Dmitry Vyukov' via syzkaller wrote:
> 2. I see KASAN has just become supported for Arm, which is very
> useful, but I can't boot a kernel with KASAN enabled. I am using
> v5.11-rc4 and this config without KASAN boots fine:
> https://gist.githubusercontent.com/dvyukov/12de2905f9479ba2ebdcc603c2fec79b/raw/c8fd3f5e8328259fe760ce9a57f3e6c6f5a95c8f/gistfile1.txt
> using the following qemu command line:
> qemu-system-arm \
>   -machine vexpress-a15 -cpu max -smp 2 -m 2G \

It might be best to use `-machine virt` here instead; that way QEMU
won't need to emulate any of the real vexpress HW, and the kernel won't
need to waste any time poking it.

IIUC with that, you also wouldn't need to provide a DTB explicitly as
QEMU will generate one...

>   -device virtio-blk-device,drive=hd0 \
>   -drive if=none,format=raw,id=hd0,file=image-arm -snapshot \
>   -kernel arch/arm/boot/zImage \
>   -dtb arch/arm/boot/dts/vexpress-v2p-ca15-tc1.dtb \

... so this line could go, too.

>   -nographic \
>   -netdev user,host=10.0.2.10,hostfwd=tcp::10022-:22,id=net0 -device
> virtio-net-device,netdev=net0 \
>   -append "root=/dev/vda earlycon earlyprintk=serial console=ttyAMA0
> oops=panic panic_on_warn=1 panic=86400 vmalloc=512M"

[...]

> 3. CONFIG_KCOV does not seem to fully work.
> It seems to work except for when the kernel crashes, and that's the
> most interesting scenario for us. When the kernel crashes for other
> reasons, crash handlers re-crashe in KCOV making all crashes
> unactionable and indistinguishable.
> Here are some samples (search for __sanitizer_cov_trace):
> https://gist.githubusercontent.com/dvyukov/c8a7ff1c00a5223c5143fd90073f5bc4/raw/c0f4ac7fd7faad7253843584fed8620ac6006338/gistfile1.txt

Most of those are all small offsets from 0, which suggests an offset is
being added to a NULL pointer somewhere, which I suspect means
task_struct::kcov_area is NULL. We could hack-in a check for that, and
see if that's the case (though I can't see how from a quick scan of the
kcov code).

Thanks,
Mark.