Re: [PATCH v2 4/4] powerpc: Book3S 64-bit "heavyweight" KASAN support

Daniel Axtens
2019-12-11T05:22:02Z

Hi Balbir,

>> +Discontiguous memory can occur when you have a machine with memory spread
>> +across multiple nodes. For example, on a Talos II with 64GB of RAM:
>> +
>> + - 32GB runs from 0x0 to 0x0000_0008_0000_0000,
>> + - then there's a gap,
>> + - then the final 32GB runs from 0x0000_2000_0000_0000 to 0x0000_2008_0000_0000
>> +
>> +This can create _significant_ issues:
>> +
>> + - If we try to treat the machine as having 64GB of _contiguous_ RAM, we would
>> + assume that ran from 0x0 to 0x0000_0010_0000_0000. We'd then reserve the
>> + last 1/8th - 0x0000_000e_0000_0000 to 0x0000_0010_0000_0000 as the shadow
>> + region. But when we try to access any of that, we'll try to access pages
>> + that are not physically present.
>> +
>
> If we reserved memory for KASAN from each node (discontig region), we might survive
> this no? May be we need NUMA aware KASAN? That might be a generic change, just thinking
> out loud.

The challenge is that - AIUI - in inline instrumentation, the compiler
doesn't generate calls to things like __asan_loadN and
__asan_storeN. Instead it uses -fasan-shadow-offset to compute the
checks, and only calls the __asan_report* family of functions if it
detects an issue. This also matches what I can observe with objdump
across outline and inline instrumentation settings.

This means that for this sort of thing to work we would need to either
drop back to out-of-line calls, or teach the compiler how to use a
nonlinear, NUMA aware mem-to-shadow mapping.

I'll document this a bit better in the next spin.

>> + if (IS_ENABLED(CONFIG_KASAN) && IS_ENABLED(CONFIG_PPC_BOOK3S_64)) {
>> + kasan_memory_size =
>> + ((phys_addr_t)CONFIG_PHYS_MEM_SIZE_FOR_KASAN << 20);
>> +
>> + if (top_phys_addr < kasan_memory_size) {
>> + /*
>> + * We are doomed. Attempts to call e.g. panic() are
>> + * likely to fail because they call out into
>> + * instrumented code, which will almost certainly
>> + * access memory beyond the end of physical
>> + * memory. Hang here so that at least the NIP points
>> + * somewhere that will help you debug it if you look at
>> + * it in qemu.
>> + */
>> + while (true)
>> + ;
>
> Again with the right hooks in check_memory_region_inline() these are recoverable,
> or so I think

So unless I misunderstand the circumstances in which
check_memory_region_inline is used, this isn't going to help with inline
instrumentation.

>> +void __init kasan_init(void)
>> +{
>> + int i;
>> + void *k_start = kasan_mem_to_shadow((void *)RADIX_KERN_VIRT_START);
>> + void *k_end = kasan_mem_to_shadow((void *)RADIX_VMEMMAP_END);
>> +
>> + pte_t pte = __pte(__pa(kasan_early_shadow_page) |
>> + pgprot_val(PAGE_KERNEL) | _PAGE_PTE);
>> +
>> + if (!early_radix_enabled())
>> + panic("KASAN requires radix!");
>> +
>
> I think this is avoidable, we could use a static key for disabling kasan in
> the generic code. I wonder what happens if someone tries to boot this
> image on a Power8 box and keeps panic'ing with no easy way of recovering.

Again, assuming I understand correctly that the compiler generates raw
IR->asm for these checks rather than calling out to a function, then I
don't think we get a way to intercept those checks. It's too late to do
anything at the __asan report stage because that will already have
accessed memory that's not set up properly.

If you try to boot this on a Power8 box it will panic and you'll have to
boot into another kernel from the bootloader. I don't think it's
avoidable without disabling inline instrumentation, but I'd love to be
proven wrong.

>
> NOTE: I can't test any of these, well may be with qemu, let me see if I can spin
> the series and provide more feedback

It's actually super easy to do simple boot tests with qemu, it works fine in TCG,
Michael's wiki page at
https://github.com/linuxppc/wiki/wiki/Booting-with-Qemu is very helpful.

I did this a lot in development.

My full commandline, fwiw, is:

qemu-system-ppc64 -m 8G -M pseries -cpu power9 -kernel ../out-3s-radix/vmlinux -nographic -chardev stdio,id=charserial0,mux=on -device spapr-vty,chardev=charserial0,reg=0x30000000 -initrd ./rootfs-le.cpio.xz -mon chardev=charserial0,mode=readline -nodefaults -smp 4

Regards,
Daniel